Gallium
<p>Gallium is a soft, silvery metallic element with an atomic number of 31 and the chemical symbol Ga. Gallium is used in a wide variety of products that…
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Gallium Chapter H of Critical Mineral Resources of the United States—Economic and Environmental Geology and Prospects for Future Supply U.S. Department of the Interior U.S. Geological Survey Professional Paper 1802-HPhotograph Samples of Bauxite Ore and Sphalerite Ore, Sources of Gallium
Periodic Table of Elements Ta bl e El em en ts Modified from Los Alamos National Laboratory Chemistry Division; available at ://periodic.lanl.gov/images/periodictable.pdf. Cover. Photographs showing samples of bauxite ore and sphalerite ore, which are the primary mineralogical sources of gallium. Gallium is used in a variety of products from smartphones to experimental cars, also shown. Photograph of experimental car courtesy of the University of Michigan; used with permission.
Gallium By Nora K. Foley, Brian W. Jaskula, Bryn E. Kimball, and Ruth F. Schulte Chapter H of Critical Mineral Resources of the United States—Economic and Environmental Geology and Prospects for Future Supply Edited by Klaus J. Schulz, John H. DeYoung, Jr., Robert R. Seal II, and Dwight C. Bradley Professional Paper 1802- H U.S. Department of the Interior U.S. Geological Survey
U.S. Department of the Interior RYAN K. ZINKE, Secretary U.S. Geological Survey William H. Werkheiser, Acting Director U.S. Geological Survey, Reston, Virginia: 2017 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit ://www.usgs.gov or call 1-888-ASK-USGS. For an overview of USGS information products, including maps, imagery, and publications, visit ://store.usgs.gov/. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Foley, N.K., Jaskula, B.W., Kimball, B.E., and Schulte, R.F., 2017, Gallium, chap. H of Schulz, K.J., DeYoung, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., Critical mineral resources of the United States—Economic and environ mental geology and prospects for future supply: U.S. Geological Survey Professional Paper 1802, p. H1- H35, ://doi.org/10.3133/pp1802H. ISSN 2330-7102 (online)
Contents Abstract H1 Introduction H1 Strategic and Critical Resource Issues H2 Uses and Applications H2 Geology H6 Geochemistry H6 Mineralogy H12 Gibbsite-Diaspore-Boehmite H14 Sphalerite H15 Deposit Types H15 Bauxite Deposits H15 Sediment-Hosted Lead-Zinc Deposits H18 Clastic-Dominated Lead-Zinc Type H19 Mississippi Valley Lead-Zinc Type H19 Kipushi Polymetallic Type H20 Resources and Production H21 Identified Resources H21 Undiscovered Resources H21 Production H21 Exploration for New Deposits H22 Environmental Considerations H22 Sources and Fate in the Environment H22 Mine Waste Characteristics H23 Human Health Concerns H24 Ecological Health Concerns H24 Carbon Footprint H25 Mine Closure H25 Problems and Future Research H26 References Cited H26
Tables H1. Selected properties of gallium, a Group 13 post-transition metal H8 H2. Gallium concentrations in rocks, ore, coal, soils, and natural waters H9 H3. Selected gallium-bearing minerals—Formula, content, and occurrence H13 H4. Significant global and domestic deposit types from which gallium is obtained or is potentially extractable H16 Figures H1. Photograph of gallium metal with an inset showing its position on the periodic table of elements H1 H2. Photographs showing examples of some current uses for gallium H3 H3. Charts showing U.S. and world gallium production and consumption from 2007 to 2012 H4 H4. World map showing locations of selected mineral deposits, by type; gallium has been produced from these types of deposits H5 H5. Cross sections showing the general geologic environments for the types of mineral deposits with which gallium is most commonly associated and from which gallium is typically extracted H6 H6. McDermitt caldera, Nevada H12 H7. Photographs showing samples of bauxite ore and sphalerite ore, which are the primary mineralogical sources of gallium H14
Conversion Factors International System of Units to Inch/Pound Multiply By To obtain Length angstrom (Å) (0.1 nanometer) microinch angstrom (Å) (0.1 nanometer) 0.000003937 mil micrometer (µm) [or micron] mil millimeter (mm) inch (in.) centimeter (cm) inch (in.) meter (m) foot (ft) meter (m) yard (yd) kilometer (km) mile (mi) Area hectare (ha) acre square kilometer (km2) acre square meter (m2) square foot (ft2) square centimeter (cm2) square inch (ft2) square kilometer (km2) square mile (mi2) Volume milliliter (mL) ounce, fluid (fl. oz) liter (L) ounce, fluid (fl. oz) liter (L) quart (qt) liter (L) gallon (gal) cubic meter (m3) gallon (gal) cubic centimeter (cm3) cubic inch (in3) cubic meter (m3) cubic yard (yd3) cubic kilometer (km3) cubic mile (mi3) Mass microgram (μg) 0.00000003527 ounce, avoirdupois (oz) milligram (mg) 0.00003527 ounce, avoirdupois (oz) gram (g) ounce, avoirdupois (oz) gram (g) 0.03215075 ounce, troy kilogram (kg) ounce, troy kilogram (kg) pound avoirdupois (lb) ton, metric (t) ton, short [2,000 lb] ton, metric (t) ton, long [2,240 lb] Deposit grade gram per metric ton (g/t) 0.0291667 ounce per short ton (2,000 lb) (oz/T) Pressure megapascal (MPa) bar gigapascal (GPa) 10,000 bar Density gram per cubic centimeter (g/cm3) pound per cubic foot (lb/ft3) milligram per cubic meter (mg/m3) 0.00000006243 pound per cubic foot (lb/ft3) Energy joule (J) 0.0000002 kilowatthour (kWh) joule (J) 6.241 × 1018 electronvolt (eV) joule (J) calorie (cal) kilojoule (kJ) 0.0002388 kilocalorie (kcal)
International System of Units to Inch/Pound—Continued Multiply By To obtain Radioactivity becquerel (Bq) 0.00002703 microcurie (μCi) kilobecquerel (kBq) microcurie (μCi) Electrical resistivity ohm meter (Ω-m) ohm inch (Ω-in.) ohm-centimeter (Ω-cm) ohm inch (Ω-in.) Thermal conductivity watt per centimeter per degree Celsius (watt/cm °C) International British thermal unit inch per hour per square foot per degree Fahrenheit (Btu in/h ft2 °F) watt per meter kelvin (W/m-K) International British thermal unit inch per hour per square foot per degree Fahrenheit (Btu in/h ft2 °F) Inch/Pound to International System of Units Length mil micrometer (µm) [or micron] inch (in.) centimeter (cm) inch (in.) millimeter (mm) foot (ft) meter (m) mile (mi) kilometer (km) Volume ounce, fluid (fl. oz) milliliter (mL) ounce, fluid (fl. oz) liter (L) Mass ounce, avoirdupois (oz) 28,350,000 microgram ounce, avoirdupois (oz) 28,350 milligram ounce, avoirdupois (oz) gram (g) ounce, troy 31.10 348 gram (g) ounce, troy 0.03110348 kilogram (kg) pound, avoirdupois (lb) kilogram (kg) ton, short (2,000 lb) ton, metric (t) ton, long (2,240 lb) ton, metric (t) Deposit grade ounce per short ton (2,000 lb) (oz/T) 34.285714 gram per metric ton (g/t) Energy kilowatthour (kWh) 3,600,000 joule (J) electronvolt (eV) 1.602 × 10-19 joule (J) Radioactivity microcurie (μCi) 37,000 becquerel (Bq) microcurie (μCi) kilobecquerel (kBq) Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:
°F (1.8 × °C) + 32 Temperature in degrees Celsius (°C) may be converted to kelvin (K) as follows:
K °C + 273.15 Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:
°C (°F - 32) / 1.8
Datum Unless otherwise stated, vertical and horizontal coordinate information is referenced to the World Geodetic System of 1984 (WGS 84). Altitude, as used in this report, refers to distance above the vertical datum. Supplemental Information Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 °C). Concentrations of chemical constituents in soils and (or) sediment are given in milligrams per kilogram (mg/kg), parts per million (ppm), or parts per billion (ppb). Concentrations of chemical constituents in water are given in milligrams per liter (mg/L), micrograms per liter (µg/L), nanogams per liter (ng/L), nanomoles per kilogram (nmol/kg), parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt). Concentrations of suspended particulates in water are given in micrograms per gram (µg/g), milligrams per kilogram (mg/kg), or femtograms per gram (fg/g). Concentrations of chemicals in air are given in units of the mass of the chemical (milligrams, micrograms, nanograms, or picograms) per volume of air (cubic meter). Activities for radioactive constituents in air are given in microcuries per milliliter (μCi/mL). Deposit grades are commonly given in percent, grams per metric ton (g/t)—which is equivalent to parts per million (ppm)—or troy ounces per short ton (oz/T). Geologic ages are expressed in mega-annum (Ma, million years before present, or 10 6 years ago) or giga-annum (Ga, billion years before present, or 10 9 years ago). For ranges of years, "to" and (or) the en dash ("-") mean "up to and including." Concentration unit Equals milligram per kilogram (mg/kg) part per million microgram per gram (µg/g) part per million microgram per kilogram (μg/kg) part per billion (109) Equivalencies part per million (ppm): 1 ppm 1,000 ppb 1,000,000 ppt 0.0001 percent part per billion (ppb): 0.001 ppm 1 ppb 1,000 ppt 0.0000001 percent part per trillion (ppt): 0.000001 ppm 0.001 ppb 1 ppt 0.0000000001 percent Metric system prefixes tera- (T-) 1 trillion giga- (G-) 1 billion mega- (M-) 1 million kilo- (k-) 1 thousand hecto- (h-) 1 hundred deka- (da-) 1 ten deci- (d-) 10-1 1 tenth centi- (c-) 10-2 1 hundredth milli- (m-) 10-3 1 thousandth micro- (µ-) 10-6 1 millionth nano- (n-) 10-9 1 billionth pico- (p-) 10-12 1 trillionth femto- (f-) 10-15 1 quadrillionth atto- (a-) 10-18 1 quintillionth
Abbreviations and Symbols °C degree Celsius µg/L microgram per liter µg Al/L microgram of aluminum per liter µg Mn/m3 microgram of manganese per cubic meter µg Mn/L microgram of manganese per liter Å angstrom AMD acid mine drainage ATSDR Agency for Toxic Substances and Disease Registry CIGS
g CO2-equiv/kWh gram of CO2 equivalent per kilowatthour lethal concentration 50 (concentration that kills 50 percent of test population within a given timeframe) LED light-emitting diode Ma mega-annum mg Al/kg milligram of aluminum per kilogram mg/kg milligram per kilogram mg/L milligram per liter mm millimeter ng/m3 nanogram per cubic meter OSHA Occupational Safety and Health Administration ppm part per million RF radio-frequency SAF submerged-arc furnace USGS U.S. Geological Survey
Gallium By Nora K. Foley, Brian W. Jaskula, Bryn E. Kimball, and Ruth F. Schulte Abstract Gallium is a soft, silvery metallic element with an atomic number of 31 and the chemical symbol Ga. Gallium is used in a wide variety of products that have microelectronic components containing either gallium arsenide (GaAs) or gallium nitride (GaN). GaAs is able to change electricity directly into laser light and is used in the manufacture of optoelectronic devices (laser diodes, light-emitting diodes [LEDs], photo detectors, and solar cells), which are important for aerospace and telecommunications applications and industrial and medical equipment. GaAs is also used in the production of highly specialized integrated circuits, semiconductors, and transistors; these are necessary for defense applications and high-performance computers. For example, cell phones with advanced personal computer-like functionality (smartphones) use GaAs-rich semiconductor components. GaN is used principally in the manufacture of LEDs and laser diodes, power electronics, and radio-frequency electronics. Because GaN power transistors operate at higher voltages and with a higher power density than GaAs devices, the uses for advanced GaN-based products are expected to increase in the future. Gallium technologies also have large power-handling capabilities and are used for cable television transmission, commercial wireless infrastructure, power electronics, and satellites. Gallium is also used for such familiar applications as screen backlighting for computer notebooks, flat-screen televisions, and desktop computer monitors. Gallium is dispersed in small amounts in many minerals and rocks where it substitutes for elements of similar size and charge, such as aluminum and zinc. For example, gallium is found in small amounts (about 50 parts per million) in such aluminum-bearing minerals as diaspore-boehmite and gibbsite, which form bauxite deposits, and in the zinc-sulfide mineral sphalerite, which is found in many mineral deposits. At the present time, gallium metal is derived mainly as a byproduct of the processing of bauxite ore for aluminum; lesser amounts of gallium metal are produced from the processing of sphalerite ore from three types of deposits (sediment-hosted, Mississippi Valley-type, and volcanogenic massive sulfide) for zinc. The United States is expected to meet its current and expected future needs for gallium through imports of primary, recycled, and refined gallium, as well as through domestic production of recycled and refined gallium. The U.S. Geological Survey estimates that world resources of gallium in bauxite exceed 1 billion kilograms, and a considerable quantity of gallium could be present in world zinc reserves. Introduction The existence of gallium was first predicted in 1871 by Dmitri Mendeleev, a Russian chemist who published the first periodic table of elements (Mendeleev, 1871). Mendeleev noted a gap in his table and named the missing element "eka-aluminum" because he determined that its location was one place away from aluminum in the table (fig. H1). Mendeleev thought that the missing element (gallium) would be P hotog ra ph of G al um Me ta and T ab le Elem en ts Figure H1. Photograph of gallium metal with an inset showing its position on the periodic table of elements. The gallium crystals shown in the photograph grew in a tank of liquid gallium. The drop in the foreground fell off the crystal as the liquid slowly cooled below a temperature of about 30 degrees Celsius. Photograph courtesy of Afshin Yaghmaei, Simulated Planetary Interiors Lab, Department of Earth and Space Sciences, University of California, Los Angeles.
H2 Critical Mineral Resources of the United States— Gallium very much like aluminum in its chemical properties, and he was right. The French chemist Paul-Emile Lecoq de Boisbaudran discovered gallium in sphalerite in 1875 using spectroscopy. Lecoq de Boisbaudran isolated gallium metal by electrolysis of gallium hydroxide in a potassium hydroxide solution. He named the element "gallia" after the Latin name for his native land of France (Gallia, or Gaul). Solid gallium has a low melting temperature (about 29 degrees Celsius [°C]) and an unusually high boiling point (about 2,204 °C) (table H1), so that gallium is a liquid across a wider range of temperatures than any other element. Because of these properties, the earliest uses of gallium were in high-temperature thermometry and in metal alloys that melt easily. The development of gallium arsenide as a direct band-gap semiconductor in the 1960s led to what are now some of the most well-known uses of gallium—in feature-rich, application-intensive, third- and fourth-generation smart phones and in data-centric networks. Strategic and Critical Resource Issues Gallium first became recognized as a strategic and critical metal during World War II because it was a necessary compo nent of a stable gallium-plutonium alloy that was developed as part of the Manhattan Project and used to make a reliable atomic bomb. Gallium is currently important for U.S. manu facturing because of its expanding uses for novel thin-film photovoltaics, especially those of the copper-indium-gallium- (CIGS) semiconductor type, and it is particularly important for clean energy technologies. Gallium is currently produced as a byproduct of the mining of other mineral commodities—mainly zinc, copper, and aluminum—and the mining technologies used are optimized for the extraction of those primary mineral commodities. The small market for gallium as a specialty metal creates little incentive for refiners of zinc, copper, and aluminum ores to invest in improvements to increase byproduct recovery of gallium. Because of the likelihood of rapid growth in the areas of photovoltaics and clean energy technologies, a potential exists for bottlenecks in the gallium supply pipeline. Production of gallium is limited by market factors that influence the production of the principal mineral commodity, whether it is zinc or aluminum. Uses and Applications A wide variety of products have microelectronic compo nents that contain either gallium arsenide (GaAs) or gallium nitride (GaN). GaAs is able to change electricity directly into laser light and is used in the manufacture of optoelectronic devices (laser diodes, light-emitting diodes [LEDs], photo detectors, and solar cells), which are important for aerospace and telecommunications applications and industrial and medical equipment. GaAs is also used in the production of highly specialized integrated circuits, semiconductors, and transistors; these are necessary for defense applications and high-performance computers. For example, smartphones (cell phones with advanced computer-like functionality) use GaAsrich semiconductor components (fig. H2A). GaN is employed principally in the manufacture of LEDs and laser diodes, power electronics, and radio-frequency (RF) electronics. Because GaN power transistors operate at higher voltages and with a higher power density than GaAs devices, the applications for advanced GaN-based products are expected to increase in the future. The CIGS direct band-gap semiconductor was developed to make lightweight, flexible, and durable thin-film photovoltaics that have a high absorption coefficient; these attributes have potential applications in the manufacture of efficient solar cells (fig. H2B). Because of its high boiling temperature, gallium is used to make thermometers designed to measure very high temperatures, and because of its silvery color and ability to form metal alloys, gallium is used to make brilliant mirrors. The fundamental properties of thermal convection in liquid gallium metal are employed to study aspects of planetary and astrophysical magnetohydrodynamics. Gallium can be replaced by other materials in some applications, but for others, it has no effective substitutes. Liquid crystals made from organic compounds are used in visual displays as substitutes for LEDs. Researchers also are working to develop organic-based LEDs that may compete with GaAs in the future. Indium phosphide components can be substituted for GaAs-based infrared laser diodes in some specific-wavelength applications, and helium-neon lasers can be used in place of GaAs lasers in visible laser diode applications. Silicon is the principal competitor with GaAs in solar-cell applications. GaAs-based integrated circuits are used in many defense-related applications because of their unique properties, and there are no effective substitutes for GaAs in these applications. GaAs in heterojunction bipolar transistors is being substituted by silicon-germanium in some applications. In 2012, imports of gallium and GaAs wafers, which were valued at about $32 million, continued to satisfy almost all U.S. demand for gallium. GaAs and GaN electronic components represented about 99 percent of domestic gallium consumption (fig. H3). About 70 percent of the gallium was used in integrated circuits (both analog and digital), including those in defense applications, high-performance computers, and telecommunications. Optoelectronic devices for use in aerospace and telecommunications applications, consumer goods, industrial equipment, medical equipment, and so forth, as well as a small amount for research and development, represented the remaining 30 percent of gallium consumption. Gallium prices decreased throughout 2012 when significant increases in gallium production from 2007 to 2012 (fig. H4) exceeded the declining demand from LED producers. Gallium production capacity in China
Introduction H3 expanded tremendously from 2008 to 2012 on the expectation of a strong LED-based backlighting market, which failed to materialize. In January 2012, the price for low-grade gallium metal (with a purity of 99.099 percent compared with a purity of 99.99999 for the higher grade) averaged $580 per kilogram. By July, the average price for low-grade gallium metal had decreased to $320 per kilogram, and by early October, to $280 per kilogram. Markets continued to expand for GaAs- and GaN-based products in 2012. GaAs demand mainly results from cell phones and other high-speed wireless applications. In 2012, GaAs demand increased owing to the rapid increase in the use of feature-rich, application-intensive, third- and fourthgeneration smartphones, which contain up to 10 times the amount of GaAs compared with standard cellular handsets. Smartphones accounted for 37 percent of all handsets sold in 2012. Owing to the increased amounts of GaAs in smart phones and the increased use of GaAs-based LEDs in general lighting and automotive applications, the value of sales in the GaAs market is forecast to reach $650 million by 2017 at an annual growth rate of nearly 11 percent (Jaskula, 2013b). Owing to the large power-handling capabilities, highswitching frequencies, and higher voltage capabilities of GaN technology, GaN-based products, which historically have been used in defense and military applications, have begun to gain acceptance in the cable television transmission, commercial wireless infrastructure, power electronics, and satellite markets. The value of sales for the GaN power device market was expected to reach $178 million by 2015 at an annual growth rate of nearly 29 percent (Jaskula, 2013b). In 2012, the worldwide LED market, which was a significant driver for GaN-based technologies, increased by only 1.5 percent in revenues from that of 2011 owing to slower-than-expected growth in the LED backlighting market and to lower LED prices, as production was outpacing demand. Although televisions were increasingly being built with LED backlighting in 2012, improvements in technology required up to 50 percent fewer LEDs. LED-backlit televisions accounted for 63 percent of the television market in 2012 and were forecast to account for 93 percent of market sales in 2013. By 2014, the strongest market segments for sales of LEDs were expected to be in general lighting, followed by signs and automotive applications. Sustained high prices for energy continued to spark interest in solar energy in 2012. Sales of CIGS semiconductors for use in the production of high-efficiency solar cells was slower than expected, however, owing to a complicated manufacturing process that has impeded commercial mass production of CIGS panels. Decreased prices of silicon-based solar cells also affected demand for the more expensive CIGS technology. These two factors resulted in a large oversupply of CIGS modules that caused prices to decrease by 20 percent in 2011 and to remain low throughout 2012. In an effort to keep CIGS technology viable and competitive, CIGS manufacturers began in 2011 to trim production costs, improve module conversion efficiencies, and increase the use of CIGS-based photovoltaics in commercial rooftops. The following sections of this chapter provide an over view of the principal types and global distribution of domestic and global resources of gallium (fig. H4). This information updates the report by Weeks (1973). P h Figure H2. Photographs showing examples of some current uses for gallium. A, Cell phone that has advanced computer-like functionality (that is, a smartphone), which contains galliumbased semiconductor components and, consequently, up to 10 times the amount of gallium arsenide (GaAs) than is used in standard cell phones. B, Experimental three-wheel car displaying high-efficiency, triple-junction GaAs solar cells. Such solar cells have many experimental applications; this three-wheel car, the Continuum, which was built at the University of Michigan, weighs 220 kilograms and is covered with about 2,500 aerospace-grade GaAs-type solar cells. Photograph of experimental car courtesy of the University of Michigan.
H4 Critical Mineral Resources of the United States— Gallium Figure H3 - Charts Showing U.S. and World Gallium Produ ction and Consump tion Figure H3. Charts showing U.S. and world gallium production and consumption from 2007 to 2012. A, Estimated world primary gallium production and refined gallium production from 2007 to 2012. B, Estimated world primary gallium production capacity for China and the rest of the world from 2008 to 2012. C, Gallium consumption in the United States from 2007 to 2012, and major end uses of gallium as a percentage of U.S. consumption in 2012. Percentages in the pie chart have been rounded and consequently do not add to 100. LEDs, light-emitting diodes
Introduction H5 Figu re H Wo rl d M ap Show ing Loc ati ons of Se lec te d Gal liu m M ine ral De pos its Not all the Kipushi-type osits. The selected bauxite deposits are both karst and lateritic types. The sediment-hosted zinc deposits and districts named in the text (Austinville, World map showing locations of selected mineral deposits, by type; gallium has been produced from these types of dep (See Taylor and others [2009] for names and details regarding other zinc deposits.) s major bauxite provinces and include rend) are labeled. iburnum T
Figure H4. located within the world' Nanisivik, Red Dog, Ruby Creek, and V deposits mentioned in the text appear on this map.
H6 Critical Mineral Resources of the United States— Gallium Geology Gallium occurs in low amounts in a diverse group of geologic environments and is found most commonly in asso ciation with deposits of aluminum and zinc (fig. H5), which are its immediate neighbors in the periodic table of elements (fig. H1). The average abundance of gallium in Earth's crust is generally less than 19 parts per million (ppm) (table H2). Because of the rarity of gallium in most geologic environ ments, minerals containing gallium as an essential structural component are also rare (table H3). Because gallium occurs as a minor or trace element in many minerals, there are a large number of rock types and tectonic settings for mineral deposits that are known to contain gallium in potentially extractable amounts (table H4). At the present time, the majority of the world's supply of newly mined gallium metal comes from bauxite and sediment-hosted lead-zinc deposits (fig. H4). Gallium is currently derived as a byproduct of the processing of bauxite ore for aluminum, with lesser amounts produced from residues resulting from the processing of sphalerite ore for zinc. Geochemistry Gallium is a soft, silvery post-transition metal with an atomic number of 31 (table H1; fig. H1) that is very much like aluminum in its chemical properties. The chemical similarities between gallium and aluminum, which include a trivalent oxidation state, a similar atomic radius, tetrahedral or octahedral coordination in minerals, and amphoterism (that is, they can act as both acid and base in reactions), result in their generally synchronous geochemical behavior. Gallium is found primarily in the trivalent oxidation state; mixed-oxidation state compounds containing both Ga1+ and Ga3+ (ionic radius of about 0.62 angstrom [Å]) occur but are not common (Eagleson, 1994, p. 438). In compounds, gallium exhibits a chemical similarity to Al3+ (ionic radius of about 0.54 Å), Fe3+ (ionic radius of about 0.69 Å) and Zn2+ (ionic radius of about 0.72 Å) (Shannon, 1976) and can substitute for these elements in the common rock-forming minerals (Burton and Culkin, 1978). Other elements typically found in geochemical association with gallium include cadmium, indium, germanium, silicon, and tin (fig. H1). Under certain geochemical conditions, these elements can be separated in near-surface environments—for example, by changes in pH or redox conditions—because their acid/base and electronic properties are sufficiently different (table H1). The aqueous geochemistry of gallium was studied by Wood and Samson (2006, and references therein) based on a number of earlier works. Trivalent gallium is also the most common oxidation state of gallium metal in aqueous solution and, owing to the high charge, the hydrated ions of gallium are Pearson hard acids (Pearson, 1963), which favor the formation of complexes with hard ligands, such as fluoride, hydroxide, phosphate, and sulfate. In aqueous solutions, gallium is predicted to occur mainly in fluoride and hydroxide complexes (Wood and Samson, 2006). Solubility products of most solid phases containing gallium are generally not available; however, the limited available solubility data constrain the maximum concentrations of gallium to be expected in aqueous fluids to the subparts-per-million to subparts-per-billion (or micrograms per liter) range (Wood and Samson, 2006), and this is evident in the values seen for hydrothermal fluids, river waters, and seawater (table H2). The hydrolysis of (solid gallium oxyhydroxide) occurs at low pH, and its speciation is strongly dominated by - in the pH range of natural solutions at 25 °C. At increasing temperature, the amount of + and becomes significant, although at 300 °C, - remains the predominant species. A comparison of the geochemical behavior of gallium and aluminum in natural fluids indicates that - exhibits a chemical behavior very similar to that of − (Wood and Samson, 2006). The low solubility of (in the absence of fluoride or strong organic ligands at low temperatures) is consistent with the immobility of gallium relative to most other elements, except aluminum, during weathering and alteration processes. This immobility leads to the accumulation of gallium in some bauxites and kaolins, and in the high-alumina hydrothermal alteration halos that form around many mineral deposits (for example, epithermal, porphyry, and volcanogenic deposits). Figure H5. (page H7) Cross sections showing the general geologic environments for the types of mineral deposits with which gallium is most commonly associated and from which gallium is typically extracted. A, Karst bauxite deposits, which form by deep weathering and transport of material in carbonate rock-dominated sequences; unlabeled patterns show detritus. B, Lateritic bauxite deposits, which form mainly by deep weathering of material in silicate rock-dominated sequences. A and B modified from Retallack (2010). C, Sediment-hosted lead-zinc deposits, which form in passive margin environments. D, Clastic-dominated-type lead-zinc deposits, which may form by brine reflux in shales and deep-water carbonates. E, Mississippi Valley-type lead-zinc deposits, which form by replacement in platform carbonate sequences. C, D, and E modified from Kelley and others (1995); Bradley and Leach (2003); Leach, Sangster, and others (2005); and Leach, Taylor, and others (2010).
Geology H7 Figure H5 - Cross Sectio ns Showing Genera l Geologic Enviro nments
H8 Critical Mineral Resources of the United States— Gallium Table H1. Selected properties of gallium, a Group 13 posttransition metal. [Sources: Instytutu Problem Materialoznavstva (1968) and Lide (2005, p. 14-39). Å, angstrom; °C, degree Celsius; cm, centimeter; eV, electron volt; g/cm3, gram per cubic centimeter; GPa, gigapascal; J/mol-K, joule per mole kelvin; K, kelvin; kJ/mol, kilojoule per mole; nΩ-m, nano ohm-meter; μm/m-K, micrometer per meter kelvin; W/m-K, watt per meter kelvin] Property Description Symbol Ga Atomic number Atomic weight Isotopes 24 radioisotopes; 69Ga, 71Ga are stable isotopes; 67Ga has a half-life of about 3.3 days Density at 293 K (g/cm3) Melting point (°C) Boiling point (°C) 2,204 Heat of fusion (kJ/mol) Heat of vaporization (kJ/mol) Molar heat capacity (J/mol-K) Hardness (Mohs scale) Brinell hardness (GPa) Electrical resistivity at 20 °C (nΩ-m) Thermal conductivity (W/m-K) Thermal expansion at 25 °C (µm/m-K) Young's modulus (GPa) Crystal structure Orthorhombic Magnetic ordering Diamagnetic Electron configuration [Ar] 4s2 3d10 4p1 Ionic radius (Å) First ionization potential (eV) Second ionization potential (eV) Third ionization potential (eV) Common valance states Primarily +3, also +2, +1 Gallium is the 34th most abundant element found in Earth's crust (Emsley, 2001), and it is widely distributed in low amounts in many rock types (table H2). The average values for gallium in igneous rocks vary considerably, from a low of about 5 ppm gallium or less in ultramafic rocks to a range of 10 to 22 ppm for most mafic to intermediate rocks, and to about 16 to 35 ppm gallium for granitic rocks (Burton and Culkin, 1978). Alkaline rocks, particularly nepheline syenites, granites, and related pegmatites, have generally higher ranges, from 20 to 70 ppm gallium (Burton and Culkin, 1978). Values of gallium in shale, sandstone, and carbonates are 19 ppm, 12 ppm, and 4 ppm, respectively (Mielke, 1979), and gallium in loess averages about 14 ppm (McLennan and Murray, 1999). Gallium contents of rocks may be increased by greiseni zation, albitization, hydrothermal alteration, and weathering processes, including saprolitization and lateritization. In general, the bulk content of gallium in a given rock type corresponds to the composition and bulk content of feldspar and mica in the rock. Alteration processes that selectively remove mobile elements from aluminosilicate minerals result in relative increases in the concentrations of gallium, aluminum, and other less mobile elements in all rock types. The chalcophile behavior of gallium contributes to its concentration in sulfide ores, which can contain hundreds of parts per million gallium (Burton and Culkin, 1978). In hydrothermal systems related to sulfide mineralization and in residual deposits formed by surficial weathering, the lithophile and chalcophile tendencies of gallium work in opposition in terms of enrichment, as sulfide and aluminosilicate minerals compete for gallium. The value of the ratio of aluminum to gallium (the Al:Ga ratio) has been used as an indicator of gallium enrich ment because it is thought to be relatively constant in most types of crustal rocks (Burton and Culkin, 1978). The value of the Al:Ga ratio in various common rock types can range from 5,000 to 40,000. It is lower, in the range of 1,000 to 10,000, in alkaline igneous rocks and in sedimentary rocks produced by intense weathering (Burton and Culkin, 1978; Tervek and Fay, 1986). Rytuba and others (2003) evaluated the geochemical behavior of gallium in igneous rocks using the worldwide GEOROC (Geochemistry of Rocks of the Oceans and Continents) database (Max Planck Institute, 2002). They found that volcanic rocks from convergent-margin environ ments with about 8 to 30 ppm gallium had values of Al:Ga that increased from about 3,000 to 8,000 with increasing aluminum content. They concluded that gallium is not enriched by petrologic processes in more evolved silicic volcanic rocks, such as peralkaline rocks, because the Al:Ga ratio did not correlate with increasing quartz (SiO2 ) content or alkalinity. A comparison of the data for volcanic rocks worldwide, hydro thermally altered rocks of the McDermitt caldera near the McDermitt Mine in Humboldt County, Nevada (from Rytuba and others, 2003), and globally distributed bauxite deposits shows that gallium concentration is higher in rocks that have lower ratios of Al:Ga owing to hydrothermal alteration and (or) intense weathering (fig. H6). Because the Al:Ga ratio can be quite variable in natural waters (table H2) owing to differ ences in the mobility of gallium and aluminum in aqueous solutions, the Al:Ga ratio has been used effectively to model the behavior of trace metals as a result of surficial weathering processes (Hieronymus and others, 1990; Shiller and Frilot, 1996) and in ocean waters (Orians and Bruland, 1988; Shiller, 1998) and river waters (Shiller, 1998; Shiller and Frilot, 1996).
Geology H9 Table H2. Gallium concentrations in rocks, ore, coal, soils, and natural waters.— Continued [FOREGS, Forum of European Geological Surveys, now EuroGeoSurveys; Ga, gallium; ICP-MS, inductively coupled plasma-mass spectrometry; XRF, X-ray fluorescence. Units of measure: cm, centimeter; m, meter; mm, millimeter; ppb, part per billion; ppm, part per million; µg/L, microgram per liter; µm, micrometer. —, no data] Environment and (or) location Gallium concentration (value or range) Unit Type Notes
Typical Average Rocks Upper continental crust — 17, 17.5 ppm Continental crust Average Taylor and McLennan (1995); Rudnick and Gao (2003) Bulk continental crust — ppm Continental crust Average Taylor and McLennan (1995) Lower continental crust — ppm Continental crust Average Taylor and McLennan (1995) Volcanic rocks — About 8 to 30 ppm Convergent margins 2,929 analyses Rytuba and others (2003) High-sulfidation epithermal — Up to 320 ppm Alteration None Melfos and Voudouris (2012) High-sulfidation epithermal
ppm Alteration None Khashgerel and others (2008) All types 8 to 812 ppm Bauxite None Schulte and Foley (2014) Arkansas (lateritic bauxite) — ppm Bauxite None Burton and Culkin (1978) Bosnia — ppm Bauxite None Schulte and Foley (2014) France — ppm Bauxite None Schulte and Foley (2014) India 5 to 122 ppm Bauxite None Burton and Culkin (1978) Karst 10 to 812 ppm Bauxite None Schulte and Foley (2014) Lateritic 8 to 136 ppm Bauxite None Schulte and Foley (2014) Malaysia 22 to 47 ppm Bauxite None Burton and Culkin (1978) Russia 10 to 170 ppm Bauxite None Burton and Culkin (1978) Turkey — ppm Bauxite None Schulte and Foley (2014) Indiana—Danville member 1.7 to 8.9 ppm Coal Pennsylvanian Mastalerz and Drobniak (2012) Indiana—Springfield member 1.4 to 12.3 ppm Coal Pennsylvanian Mastalerz and Drobniak (2012) USGS COALQUAL database About 3 to 7 — ppm Selected domestic coal-bearing regions, averages Averages Finkelman (1993) General
ppm Kaolin None Hieronymus and others (1990) Georgia 27 to 100 — ppm Kaolin None This study; Bárdossy (1982); Bárdossy and Aleva (1990) Haile kaolin deposit, South Carolina ppm Kaolin None This study Malaysia 20 to 25 ppm Kaolin None Burton and Culkin (1978) Geothermal fields 8 to 144 — ppm Muds and sinter None Crump (1994)
H10 Critical Mineral Resources of the United States— Gallium Table H2. Gallium concentrations in rocks, ore, coal, soils, and natural waters.— Continued [FOREGS, Forum of European Geological Surveys, now EuroGeoSurveys; Ga, gallium; ICP-MS, inductively coupled plasma-mass spectrometry; XRF, X-ray fluorescence. Units of measure: cm, centimeter; m, meter; mm, millimeter; ppb, part per billion; ppm, part per million; µg/L, microgram per liter; µm, micrometer. —, no data] Environment and (or) location Gallium concentration (value or range) Unit Type Notes
Typical Average Rocks—Continued Alkaline Carbonate rock Granitic Loess Mafic to intermediate Peralkaline Shale Sandstone Ultramafic Apex Mine, Utah Ruby Creek Mine, Alaska Cadmium-lead-zinc ore, general Kipushi ore, general Mississippi Valleytype ore, general Sulfide ore, bulk Callahan Mine, Maine Red Dog Mine, Alaska — — — — — — — — — 320 to 1,480 — to 1,600 to 2,500 to 500 20 to 1,000 — — About 20 to 70 About 16 to 35 About 10 to 22 About 28 to 39 About 5 —
10s to 100s 100? ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm Rock Rock Rock Rock Rock Rock Rock Rock Rock Sulfide Sulfide Sulfide Sulfide Sulfide Sulfide Volcanogenic massive sulfide, bulk sulfide Clastic-dominated, ore concentrate None Average None Average None None Average Average None None None None None None None None None Burton and Culkin (1978) Mielke (1979) Burton and Culkin (1978) McLennan and Murray (1999) Burton and Culkin (1978) MacDonald and Bailey (1973) Mielke (1979) Mielke (1979) Burton and Culkin (1978) Bernstein (1986) Bernstein and Cox (1986) Schulte and Foley (2014) Schulte and Foley (2014) Schulte and Foley (2014) Burton and Culkin (1978) Ayuso and others (2013) Alaska Department of Environmental Conservation (2005) Soils World Western United States Eastern United States Poland Poland Poland Poland Poland Sweden — — — — — — — — — 100 ± 5 219 ± 5 38 ± 4 93 ± 7 268 ± 19 7 to 13 ppm ppm ppm ppm ppm ppm ppm ppm ppm Soil, bulk Soil Soil Soil Soil Soil Soil, proximal to a former Zn-Pb mine Soil, proximal to Zn-Pb ore and a bauxite mine Soil developed on quartzite and gneiss None Mean for 20 cm depth Mean for 20 cm depth Agricultural Agricultural Agricultural Proximal to a former Zn-Pb mine Proximal to Zn-Pb ore and a bauxite mine Profile range Koljonen (1992) Shacklette and Boerngen (1984) Shacklette and Boerngen (1984) Połedniok and others (2012) Połedniok and others (2012) Połedniok and others (2012) Połedniok and others (2012) Połedniok and others (2012) Tyler (2004)
Geology H11 Table H2. Gallium concentrations in rocks, ore, coal, soils, and natural waters.— Continued [FOREGS, Forum of European Geological Surveys, now EuroGeoSurveys; Ga, gallium; ICP-MS, inductively coupled plasma-mass spectrometry; XRF, X-ray fluorescence. Units of measure: cm, centimeter; m, meter; mm, millimeter; ppb, part per billion; ppm, part per million; µg/L, microgram per liter; µm, micrometer. —, no data] Environment and (or) location Gallium concentration (value or range) Unit Type Notes
Typical Average Soils—Continued FOREGS samples, median concentration FOREGS samples, median concentration FOREGS samples, median concentration — — — ppm ppm ppm Humus, median concentration Topsoil Subsoil
2.0 mm fraction, n=367, Total (ICP-MS) 2.0 mm fraction, n=843, Total (ICP-MS) 2.0 mm fraction, n=790, Total (ICP-MS) Salminen (2015) Salminen (2015) Salminen (2015) Waters Atlantic Ocean African rivers North American rivers South American rivers California streams Idel River Japan Wairakei, New Zealand Broadlands, New Zealand, Bath, United Kingdom Taiwan World rivers FOREGS samples, median concentration FOREGS samples, median concentration FOREGS samples, median concentration — — — — — — — — — — — — — — 0.0003 to 0.004 0.02 to 0.11 0.001 to 0.012 0.003 to 0.12 0.0001 to 0.006 About 0.2 About 0.2 0.01 to >10.00 — µg/L µg/L µg/L µg/L µg/L µg/L ppb ppb ppb ppb µg/L ppm µg/L ppm ppm Seawater River River River River River Geothermal Geothermal Geothermal Geothermal Groundwater Suspended sediment Water Stream sediment Flood plain sediment Dissolved and colloidal (<0.4 µm); surface to 5,000 m deep Dissolved load (<0.2 µm) Dissolved load (<0.2 µm) Dissolved load (<0.2 µm) Dissolved and colloidal load (<0.4 µm) Dissolved load (<0.2 µm) None None None None None Average Filtered 0.45 micrometer fraction 2.0 mm fraction, n=852, Total (XRF) 2.0 mm fraction, n=747, Total (XRF) Shiller (1998) Gaillardet and others (2003) Gaillardet and others (2003) Gaillardet and others (2003) Shiller and Frilot (1996) Gaillardet and others (2003) Uzumasa and Nasu (1960) Goguel (1988) Goguel (1988) Riley (1961) Chen (2006) Viers and others (2009) Salminen (2015) Salminen (2015) Salminen, 2015
H12 Critical Mineral Resources of the United States— Gallium Mineralogy Gallium is an essential structural component of only a few minerals that form in nature. Gallium is dispersed, however, in low amounts in many minerals and rocks, where it substitutes for elements of similar size and charge (table H3). Because gallium minerals are too rare in nature to serve as a primary source of the element or its compounds, most gallium is produced from minerals containing less than 100 ppm gallium (table H3), where the gallium may occur as inclusions, as stoichiometric or nonstoichiometric substitu tions, or as adsorbed ions or molecules. Four gallium-rich minerals have been recognized (approved) by the International Mineralogical Association, Commission on New Minerals, Nomenclature and Classification (table H3). These include the sulfide gallite, which has about 35 weight percent gallium; gallobeudantite, which is an alunite of variable gallium content; and the hydroxide minerals sohngeite and tsumgallite, both of which contain about 60 weight percent gallium. These minerals have been identified in association with carbonate-hosted copperlead-zinc deposits at the Tsumeb Mine in the Oshikoto Region, Namibia (Frimmel and others, 1996, and references therein). Gallite occurs as a primary sulfide mineral and is essentially a common copper sulfide, chalcopyrite (CuFeS2), in which gallium replaces iron in the crystal structure. Typical occur rences of gallite are as inclusions in sphalerite (ZnS). Because gallite has a chalcopyrite structure, it may occur as either epitaxial overgrowths or inclusions as "chalcopyrite disease" (Barton and Bethke, 1987) in sphalerite. Gallobeudantite, sohngeite, and tsumgallite are supergene minerals resulting from surficial oxidation of other gallium-bearing minerals. Gallium also substitutes in many minerals, including silicates, sulfides, and hydroxides, in concentrations ranging from parts per million to weight percent (table H3). For example, significant amounts occur in krieselite (about 20 weight percent gallium), which is the germanium analog of topaz, and in gallium-rich plumbogummite, where it may fill one-third of the aluminum sites (about 25 weight percent gallium). Major rock-forming minerals, however, contain significantly less than 1 percent gallium, and most minerals have less than 100 ppm gallium. Feldspars and micas are the dominant host minerals for gallium in most igneous and sedimentary rocks. For example, gallium has been reported at relatively Fig ure H 6 - Pl ot of the Ra t io o f A luminum to Gallium Figure H6. Plot of the ratio of aluminum to gallium (Al:Ga, log scale) versus gallium content (in parts per million [ppm], log scale) for bauxite deposits, volcanic rocks, and hydrothermally altered rocks of the McDermitt caldera, Nevada. Results from the McDermitt Mine, Nev., are included for comparison; see text for details. Equations define slope of lines drawn through datasets for comparison; R2 value is a measure of goodness of the fit of the line to the data. Modified from Rytuba and others (2003) with the addition of data for various globally distributed bauxite deposits from Schulte and Foley (2014).
Geology H13 Table H3. Selected gallium-bearing minerals—Formula, content, and occurrence. [Compiled from Burton and Culkin (1978), Bernstein (1986), Dutrizac and others (1986), Tervek and Fay (1986), Frimmel and others (1996), üter and others (2003), Ye and others (2011), and Schulte and Foley (2014). b.d.l., below detection limit; ppm, part per million; approximately. Elements: Al, aluminum; Cu, copper; Fe, iron; Ga, gallium; Pb, lead; Zn, zinc] Mineral name Chemical formula Ga content Group Deposit model, occurrence type Minerals with essential gallium1 (weight percent) Gallite CuGaS2 Sulfide Carbonate-hosted Cu-Pb-Zn; Pb-Zn-Cu Tsumeb Mine in Namibia Gallobeudantite
14.55, variable Alunite Carbonate-hosted Cu-Pb-Zn; supergene, Tsumeb Mine in Namibia Sohngeite
Hydroxide Carbonate-hosted Cu-Pb-Zn; supergene, Tsumeb Mine in Namibia. Tsumgallite
Hydroxide Carbonate-hosted Cu-Pb-Zn; supergene, Tsumeb Mine in Namibia Minerals with nonessential gallium Krieselite
20.3 weight percent Topaz group, germanate Ge analog of topaz Sphalerite ZnS b.d.l. to 2,500 ppm Sulfide Volcanogenic massive sulfide, sedimentary exhalative, and Mississippi Valley-type base-metal deposits Eyselite
6,900 ppm Hydroxide Carbonate-hosted Cu-Pb-Zn; Pb-Zn-Cu Tsumeb Mine in Namibia Ovamboite
4,600 ppm Sulfide, germanite group Massive sulfide base-metal ores Calvertite Cu5Ge0.5S4 3,500 ppm Sulfide Carbonate-hosted Cu-Pb-Zn; Pb-Zn-Cu Tsumeb Mine in Namibia Ferrohogbomite2N2S
1,600 ppm Oxide High grade Fe-oxide deposits Maikainite
1,500 ppm Sulfide, germanite group Volcanogenic massive sulfide, sedimentary exhalative, and Mississippi Valley-type base-metal deposits. (1) Maikain deposit, Kazakhstan, and (2) Tsumeb deposit, Namibia Plumbogummite, Ga-rich
~25 weight percent Crandallite sub group, alunite supergroup Carbonate-hosted Cu-Pb-Zn; Pb-Zn-Cu Tsumeb Mine in Namibia Gibbsite
About 50 ppm Oxide, hydroxide Bauxite-silicate; bauxite-karst Diaspore
About 50 ppm Oxide, hydroxide Bauxite-silicate; bauxite-karst Boehmite
About 50 ppm Oxide, hydroxide Bauxite-silicate; bauxite-karst Kaolinite
About 50 to 200 ppm Silicate Kaolins; bauxite-silicate; bauxite-karst Halloysite
About 50 to 200 ppm Silicate Kaolins; bauxite-silicate; bauxite-karst Jarosite
0.7 weight percent Sulfite Supergene Limonite nH2O 2 weight percent Hydroxide Supergene Goethite
200 ppm Hydroxide Supergene Renierite
0.04 to 400 ppm Sulfide, germanite group Massive sulfide base-metal ores Germanite Cu26Fe4Ge4S32 0.09 to 900 ppm Sulfide, germanite group Massive sulfide base-metal ores Feldspar group NaAlSi3O8, CaAl2Si2O8, KAlSi3O8 b.d.l. to 100 ppm, rarely up to 4,000 ppm Silicate Felsic igneous rocks 1Approved by the International Mineralogical Association, Commission on New Minerals, Nomenclature and Classification.
H14 Critical Mineral Resources of the United States— Gallium high concentrations (0.05 weight percent) in feldspars from the Nechalacho layered nepheline-aegirine syenite suite at Thor Lake in the Northwest Territories, Canada (Tervek and Fay, 1986); however, most feldspar contains much less than 200 ppm gallium (table H3). Gallium is retained in clay minerals, such as kaolinite and smectite, and in aluminum hydroxide minerals during weathering, which accounts for the higher gallium contents in many clay-bearing rocks and bauxites (table H2). At the present time, gallium is derived mainly from aluminum hydroxide minerals, including boehmite, diaspore, and gibbsite which form in deeply weathered deposits of bauxite (fig. H7A) (Dutrizac and others, 1986; Bernstein and Waychunas, 1987; Moskalyk, 2003). Significant amounts of gallium are also produced from zinc sulfide deposits where sphalerite (fig. H7B) is the main carrier (Moskalyk, 2003). Gallium (and germanium) can also be enriched in other sulfide minerals, such as bornite, enargite, galena, and luzonite (Hörmann, 1978). Weathering of gallium-bearing sulfide minerals can lead to gallium-enriched sulfate and hydroxide minerals, and in oxidized environ ments, gallium commonly occurs in plumbojarosite and hydrogoethite (Burton and Culkin, 1978). At the Apex Mine in Summit County, Utah, jarosite (a potassium-iron-sulfate mineral) reportedly contains concentrations of up to 1 weight percent gallium (Bernstein, 1986). Gibbsite-Diaspore-Boehmite Gibbsite forms in weathering environments by a series of reactions generated during the hydrolysis of feldspar and other aluminosilicate minerals in rocks undergoing saproliti zation (Foley and Ayuso, 2013, and references therein). The proportion of gibbsite-diaspore-boehmite increases at the expense of kaolinite in more intensely weathered rocks, such as bauxites and laterites (Soler and Lasaga, 1996). In these environments, as a result of burial diagenesis or low-grade regional metamorphism, gibbsite transforms to boehmite by dehydration, and turns into diaspore after compaction. Gallium in these minerals may be found as substitutions for trivalent aluminum or it may form as a separate phase (table H3). Sohngeite, which is the gallium analog of gibbsite, has a similar stability, and it transforms readily into tsumgallite in aqueous solution (table H3). The unit cell parameters of tsumgallite mostly resemble diaspore compared with gibbsite or boehmite (üter and others, 2003). Gallium hydroxide minerals rarely form as discrete phases; rather, they generally occur in supergene parts of carbonate-hosted polymetallic replacement deposits, such as at the Tsumeb Mine in Namibia (Frimmel and others, 1996). Diaspore and tsumgallite are isostructural, have similar crystal chemical characteristics, and are the most F igure H7 - Photographs Showing Samples of Bauxite Ore and Sphalerite Ore - Sources of Gallium Figure H7. Photographs showing samples of A, bauxite ore, and B, sphalerite ore, which are the primary mineralogical sources of gallium. The three samples of bauxite are from U.S. Geological Survey (USGS) collections and the powdered bauxite ore sample was provided by E.L. Bray, USGS. The sphalerite ore is from the Creede district, San Juan County, Colorado.
Geology H15 stable hydroxides of aluminum and gallium, respectively. Thus, gallium, in the form of is more likely to be enriched in diaspore than in the other aluminum hydroxides. Sphalerite Sphalerite (fig. H7B) is an important source of gallium in spite of the generally low concentrations in natural samples (table H3) because it is abundant in a wide variety of mineral deposits (table H4). Gallium enrichment in sphalerite under hydrothermal conditions results from its weak chalcophile behavior, which is similar to that of zinc. Gallium is thought to be incorporated in sphalerite by a coupled substitution of two Zn for Ga plus either Ag, Cu, Ge, In, or Sn to maintain a balanced charge (Johan and others, 1983; Johan, 1988; Cook and others, 2009). The highest known concentrations of gallium in sphalerite (2.1 to 3.7 weight percent gallium occur in sulfide nodules in the Qingzhen (EH3) chondrite (Rambaldi and others, 1986), which is a range that matches experimental determinations (Krämer and others, 1987; Ueno and Scott, 2002). Sphalerite from gallium-rich low temperature carbonate-hosted lead-zinc deposits of Namibia (Khusib Spring Mine in the Otjozondjupa Region and the Tsumeb Mine) show a range of from hundreds to more than 3,000 ppm gallium (Melcher and others, 2006). In general, however, concentrations in most sphalerites rarely exceed 100 ppm gallium. For example, sphalerite from clasticdominated lead-zinc deposits of the Red Dog Mine in the Northwest Arctic Borough of Alaska contains an average of 26 ppm gallium (Alaska Department of Environmental Conservation, 2005), whereas zinc ores from U.S. Mississippi Valley-type deposits average about 50 ppm gallium (Moskalyk, 2003). Gallium contents can vary widely in volcanogenic massive sulfide deposits, even within a single district or within individual sphalerite grains from a deposit. For example, some individual deposits within the Bonnifield mining district in Alaska (such as the Anderson Mountain deposit in Denali Borough and the Fosters Creek zone) can contain anywhere from 20 to 275 ppm gallium in sphalerite, whereas others (the West Tundra Flats deposit in Southeast Fairbanks Borough) have less than 1 ppm gallium in sphalerite (Foley and others, 2008; Dusel-Bacon and others, 2011; Dusel-Bacon and others, 2012). Sphalerite from the Callahan Mine in Hancock County, Maine (Ayuso and others, 2013), shows a range of from less than 3 ppm to up to 105 ppm gallium in single, 1- to 5-millimeter (mm)-wide crystals (table H3). Deposit Types The principal deposit types (table H4) and significant global and domestic deposits (fig. H4) from which gallium metal is currently being obtained are emphasized here; however, other deposit types that have gallium in potentially extractable amounts are included in table H4. Although gallium is found in a diverse group of mineral deposit types, current sources are limited to a small number of deposit types because gallium is produced mainly as a byproduct. More than 80 percent of the world's refined gallium is obtained from bauxite deposits (Jaskula, 2013b). The balance is derived from operations that collect zinc residues from a variety of zinc deposit types; the residues are combined prior to the gallium extraction cycle. Among zinc deposits, gallium concentrations tend to be high in carbonate-hosted replacement deposits and Mississippi Valley-type deposits, and variably enriched in clastic-dominated systems and volcanogenic massive sulfide deposits, whereas skarns are universally low in gallium (Cook and others, 2009). Gallium and other metals produced from zinc-refining residues are derived largely from such deposits as the Red Dog zinc mine in Alaska (Tervek and Fay, 1986). Other sources include volcanogenic massive sulfide deposits; for example, the Tizapa Mine in Mexico (Dowa Holdings Co., Ltd., 2013). Carbonate-hosted lead-zinc deposits of the Kipushi-type—for example, the Apex Mine in Utah—have the potential to produce gallium (and germanium) as a primary product. The two major types of gallium sources— bauxite deposits and sediment-hosted lead-zinc deposits— are described below. Bauxite Deposits Bauxite is mined for a number of products that have commercial application in abrasive, cement, chemical, metallurgical, and refractory industries. Approximately 70 to 80 percent of the world's dry bauxite production is processed into aluminum metal (Freyssinet and others, 2005), and these are the ores from which most gallium metal is currently derived. Bauxite is a rock that consists mostly of fine-grained aluminum minerals, notably boehmite, diaspore, and gibbsite, in mixtures with lesser amounts of iron oxides, typically goethite and hematite, kaolinite, and small amounts of anatase (Bárdossy and Aleva, 1990). Some bauxites form as accumulations of clayey remnants within paleokarst discon formities in limestone sequences; these are referred to as karst bauxites (fig. H5A). Karst bauxite is typically black to gray in color because of the presence of admixed organic matter and, in some instances, minerals that form in chemically reduced environments, such as pyrite (Bárdossy, 1982). Another type of bauxite, lateritic bauxite, occurs in parts of thick altered profiles formed by lateritization of aluminosilicate rocks (fig. H5B) (Retallack, 2010). Lateritization is a complex integration of surficial biological, chemical, and physical processes that result in thick sequences of intensely weathered saprolite and soil. Laterites are a form of ferricrete that can occur as horizons within a soil profile. By definition, periods of lateritization record exceptionally intense, regionally widespread weathering under tropical temperature regimes with ample water (Bárdossy and Aleva, 1990); thus, lateritic bauxites occur in thick profiles formed by deep tropical weathering. A combination of climate, tectonics, geomor phology, and hydrogeological factors are required to produce deposits that make up productive bauxite ore districts and provinces (fig. H4; Bogatyrev, Zhukov, and Tsekhovsky, 2009).
H16 Critical Mineral Resources of the United States— Gallium Table H4. Significant global and domestic deposit types from which gallium is obtained or is potentially extractable.— Continued [VMS, volcanogenic massive sulfide; Elements: Au, gold; Ag, silver; Al, aluminum; As, arsenic; Cd, cadmium; Co, cobalt; Cu, copper; Ga, gallium; Ge, germanium; Hg, mercury; In, indium; Pb, lead; Zn, zinc] Petrologic association Principal commodity (byproduct) Tectonic/deposit character Description Primary references Example deposits/occurrences Gallium host mineralogy Volcanic-hosted, hydrothermal Hydrothermal, hot springs Au Active epithermal gold-depositing environments Taupo Volcanic Zone, Rotokawa geothermal system, New Zealand Muds and sinters of geothermal fields; alunite, other aluminous phosphate minerals Crump (1994) High-sulfidation epithermal gold Au Aluminum-phosphate-sulfate minerals occurring in the advanced argillic zone of alteration Hugo Dummett porphyry copper-gold deposit, Mongolia; Agios Philippos Mine, Kirki, Greece; Lavrion intrusion-related deposit, Plaka deposit, Greece Aluminosilicate minerals, alunite, other aluminous phosphate minerals, aluminum hydroxide Khashgerel and others (2008); Melfos and Voudouris (2012) Epithermal mercury Au, Ag, Hg Tertiary age andesite and rhyolite volcanic flows Gold Canyon's Cordero Gallium Project, McDermitt, Nevada, United States Alunite, other aluminous phosphate minerals Rytuba and others (2003) Volcanogenic massive sulfide Zn, Cu, Pb (Au, Ag) Submarine volcanoes of backarc basins Tizapa zinc mine, Mexico; Bonnifield district, Alaska; Bald Mountain, Maine, United States Sphalerite Shanks and Thurston (2012); DuselBacon and others (2012) Sediment-hosted Clasticdominated lead-zinc Zn, Pb, Cu, Ag (Ga, In, Ge) Clastic-dominated lead-zinc ores, which are hosted in shale, sandstone, siltstone, or mixed clastic rocks, or occur as carbonate replacement, within a clastic-dominated sedimentary rock sequence Red Dog zinc mine, Howards Pass, Su-Lik, and Anarraaq, Alaska, United States; Sullivan, British Columbia, Canada; Broken Hill, Australia; Balmat-Edwards, New York, United States; Rammelsberg, Meggen, Germany Sphalerite Leach, Bradley, and others (2010); Leach, Taylor, and others (2010); Taylor and others (2009) Mississippi Valley-type Pb, Zn, Cu, (Ge, Ag, Ga, As, Cd) Carbonate-hosted lead-zinc deposits, which are typically found in platform carbonate sequences in passive-margin tectonic settings Tri-State area in the United States (Southeast ern Missouri, Eastern and Central Tennessee, Northern Arkansas); Pine Point, Canada; Polaris, Nanisivik, Alaska, United States; Navan, Ireland Sphalerite Leach, Bradley, and others (2010); Leach, Taylor, and others (2010); Taylor and others (2009) Carbonatehosted Cu±Pb±Zn (alt. Tsumeb or Kipushi) Cu, Pb, Zn (Co, Ge, Ag, Ga, As, Cd) Carbonate-hosted (karstic) polymetallic deposits found in intracratonic platform and rifted continental margin sedimentary sequences; typically folded and locally faulted; often overlain disconformably by oxidized sandstone-siltstone-shale units. Largest deposits are epigenitic replace ments within thick sedimentary sequences Grinnell and Kanuyak Island, Northwest Territories, Canada; Gortdrum, Ireland; M'Passa, Republic of the Congo; Timna, Israel; Nifty, Australia; portions of the Dongchuan deposits, China; Ruby Creek and Omar deposits, Alaska, United States; Tsumeb and Kombat, Namibia; Kipushi, Democratic Republic of the Congo; Apex Mine, Utah, United States Sphalerite Söhnge (1964); Intiomale and Oosterbosch (1974); Bernstein (1986)
Geology H17 Table H4. Significant global and domestic deposit types from which gallium is obtained or is potentially extractable.— Continued [VMS, volcanogenic massive sulfide; Elements: Au, gold; Ag, silver; Al, aluminum; As, arsenic; Cd, cadmium; Co, cobalt; Cu, copper; Ga, gallium; Ge, germanium; Hg, mercury; In, indium; Pb, lead; Zn, zinc] Petrologic association Principal commodity (byproduct) Tectonic/deposit character Description Primary references Example deposits/occurrences Gallium host mineralogy Weathering-related Bauxite, karstic Al (Ga) Formed by deep, in situ weathering, transport, and accumulation of intercalated clays or by clay dissolution residues of limestone or other carbonate rocks, karst setting, accu mulation of aluminum hydroxide minerals Europe (France, Croatia, Italy); Jamaica; bauxite mine in Arkansas, United States; China Gibbsite, diaspore, boehmite Freyssinet and others (2005); Retallack (2010); Bárdossy (1982), Bogatyrev, Zhukov, and Tsek hovsky (2009) Bauxite, lateritic Al (Ga) Formed by lateritization of various predomi nately silicate rocks such as granite, gneiss, basalt, syenite, and shale. Intense mineral hydrolysis resulting in kaolinite dissolution and precipitation of aluminum hydroxide minerals Major bauxite provinces; mostly in the countries of the tropics (current and paleo): (1) the South American province. includes the Guiana shield (Guyana, French Guiana, Suriname) and the central Brazilian shield; (2) the West African province, includes Guinea, Ivory Coast, Burkina Faso, Sierra Leone, Ghana, and Mali; (3) the Indian province, includes both the western and eastern coasts of India; (4) the South East Asian province, includes Vietnam, Laos, and Cambodia; and (5) the Austalian province. Minor provinces include the southern United States and the south ern Ural Mountains (Russia and Kazakhstan) Gibbsite, diaspore, boehmite Freyssinet and others (2005); Retallack (2010); Bárdossy and Aleva (1990); Bogatyrev, Zhukov, and Tsekhovsky (2009) Supergene, gossan Ge, Ga Gossans and other oxidized zones of copperrich, arsenic-bearing sulfide deposits, particularly those of the carbonate hosted polymetallic Kipushi-type rocks Apex Mine, Utah, United States; see Carbonate-hosted Cu ± Pb ± Zn, Kipushi-type Jarosite, Fe-hydroxide, Ga-hydroxides Bernstein (1986); Dutrizac and others (1986) Coal Coal (Ga?) Coal seams and waste products from coalfired powerplants; gallium is enriched in fly ash, sometimes 6 to 10 times more than in the feed coal Jungar Coalfield, Inner Mongolia; West Virginia, United States; Canada Coal, fly ash, kaolinite, boehmite Finkelman (1993)
H18 Critical Mineral Resources of the United States— Gallium The formation of large and superlarge deposits is thought to require more than 10 million years or the superposition of several intense weathering epochs. Various models for the origin of laterites and lateritic bauxites (for example, McFarlane, 1976; Brimhall and others, 1991; Retallack, 2010) have included both (a) elemental removal by hydrolysis and chemical leaching, and (b) elemental addition by mineral precipitation or fluvial or atmospheric deposition (for example, eolian dust) to explain the chemical compositions and volume and mass losses in bauxites. Bauxites usually form in the pallid soil zone (fig. H5B) but can occur in the B or C horizons of exceptionally thick profiles (Retallack, 2010). Lateritic bauxites typically have distinctive pisolitic textures and red colors (fig. H7A). Karst bauxites that form by transport and redeposition of lateritic bauxite clays and rubble in paleokarst depressions may retain the laterite characteristics of color and texture (Retallack, 2010). The extreme weathering conditions that contribute to bauxite formation remove virtually all the alkalis and alkaline earths, leaving only the relatively immobile component of the original rock; as a result, paleosols within bauxites and soils that form on bauxite are classified as oxisols (U.S. Natural Resources Conservation Service, Soil Survey Staff, 2000). Bauxite deposits form in a diverse array of geologic and tectonic environments over a wide range of geologic time (for example, Bárdossy, 1982; Bárdossy and Aleva, 1990; Retallack, 2010). An extensive study of well-characterized and dated bauxites (Retallack, 2010) showed that although the oldest known bauxites formed on the Arabian Shield at about 3,500 mega-annum (Ma, or million years ago), bauxites are most abundant in rocks formed since about 100 Ma owing to better preservation of the rock record (Wilkinson and others, 2009). Bauxites show an uneven distribution through time that is consistent with evidence supporting their correlation with paleoclimatic events of intense weathering, possibly related to greenhouse gas emissions, which lasted for geologic time intervals of less than about 100,000 years (Valeton, 1983; Retallack, 2010). Bauxite formation during these events had an expanded distribution from the Quaternary (between the Tropics of Cancer and Capricorn) to encompass areas near the Arctic and Antarctic Circles (Retallack, 2010). As a consequence, greenhouse gas events of the past, which are shown to have influenced the distribution of aluminum ores (Retallack, 2010), are also associated with gallium ores. Bauxite, lateritic bauxite, and karst bauxite are rocks that have concentrations of aluminum exceeding about 45 percent by weight, by definition (Valeton, 1972). This reflects a high proportion of hydroxide minerals with aluminum contents on the order of about 60 or more percent alumina by weight. Bauxite generally is defined as rock with 20 percent or less total iron, whereas lateritic bauxite, in addition to a high percentage of aluminum, can have greater than 20 percent total iron, which is generally ferric owing to oxidation unless reducing conditions prevail locally. The limited mobility of aluminum and gallium in near-surface environments accounts for gallium enrichment in bauxite and related materials (soils, kaolins, and ball clays) derived from deep weathering of crustal rocks. Overall, bauxite contains concentrations in the range of 8 to 800 ppm gallium, with most values in the range 40 to 80 ppm gallium (table H2). The world average concentration of gallium in bauxite was initially estimated to be 52 ppm (Burton and Culkin, 1978), and additional data have not changed that value significantly. For example, carbonate-hosted bauxites from central and southern Europe display average concentrations of about 50 ppm gallium (Schroll, 1999); bauxites from Arkansas range from 50 to 100 ppm gallium (average 86 ppm); and bauxite from India has a similar range of 5 to 122 ppm gallium (table H2). High gallium values are reported in bauxite originating from alteration of alkali rocks (for example, nepheline syenite of Russia ranges up to 170 ppm gallium) and in some unusual karst deposits (table H2). In general, however, there appear to be no significant differences in gallium concentrations of karst- or lateritic-type bauxites (Schulte and Foley, 2014). Sediment-Hosted Lead-Zinc Deposits Sediment-hosted lead-zinc deposits are historically the most significant sources of zinc, and these deposits are mined throughout the United States and the world (Leach, Bradley, and others, 2010). The largest and most economically important sediment-hosted deposits for zinc production are the clastic-rock-dominated and carbonate-rock-dominated (Mississippi Valley-type) subtypes (Leach, Marsh, and others, 2004; Leach, Bradley, and others, 2010; Leach, Taylor, and others, 2010); these are the main types of sulfide deposits from which gallium metal is obtained at present because they are currently the predominant sources of zinc ore. Carbonatehosted polymetallic deposits of the Kipushi (or Tsumeb) type are of minor importance for zinc production; however, they are considered separately here because they have high gallium content relative to other deposit types of the sediment-hosted class and are historic gallium producers for the United States. Much of the following discussion is summarized from a number of seminal review studies of sediment-hosted leadzinc deposits, including Leach and Sangster (1993), Leach, Viets, and others (1995); Leach, Marsh, and others (2005); Leach, Sangster, and others, (2005); Groves and Bierlein (2007); Leach, Bradley, and others (2010); and Leach, Taylor, and others (2010). Sediment-hosted lead-zinc deposits form in rocks of widely varying geologic age and tectonic environment (figs. H5C through H5E). In general, these deposits are hosted by large-scale sedimentary systems consisting of a variety of siliciclastic and carbonate rocks and have no direct genetic association with igneous activity. Many deposits in the sediment-hosted class display transitional features, and the distinctions among some of the deposit types are subjective. Most sediment-hosted deposits occur in sedimentary rock sequences that were originally deposited in extensional rift or passive-margin settings. There is an apparent uneven distribution of sediment-hosted deposits through time that is
Geology H19 attributed to their genesis in large-scale sedimentary systems and to continued recycling of the sedimentary rock record within the evolving tectonic and geochemical systems of Earth through geologic time. Tectonic setting, depositional environ ment, and host-rock lithology influence the mineral chemistry of the contained sulfide minerals, including their trace element characteristics. The ores consist mainly of galena, sphalerite, and generally lesser amounts of iron sulfides, which may be marcasite or pyrite. In addition to lead and zinc, gallium, germanium, and silver can be important constituents; copper is generally quite low, except in the Kipushi-type deposits, but can be economically important in all these deposits. Laminated ore textures and a layered morphology are generally characteristic of clastic-dominated deposits, whereas Mississippi Valley-type deposits generally exhibit abundant evidence for epigenetic and replacement ores. Nonetheless, some clastic-dominated ores show replaced sediments in early or burial diagenetic environments and some Mississippi Valley-type deposits display laminated ore textures. Clastic-Dominated Lead-Zinc Type The most definitive characteristic of clastic-dominated deposits as described by Leach, Marsh, and others (2005) is their occurrence in clastic rock-dominated sedimentary sequences formed mainly in passive margins, continental rifts, continental sag basins, and back-arc basins (figs. H5C and H5D). The orebodies are typically hosted in (in order of predominance) shale, sandstone, siltstone, carbonate, or mixed clastic rocks and their metamorphosed equivalents. They may also occur as carbonate replacement ores within a clastic-rock-dominated sedimentary sequence. The vast majority of clastic-dominated deposits globally occur in Proterozoic basins that formed between 2.02 billion years ago—the age of the earliest known deposit of these ores—and about 1.85 to 1.58 billion years ago, which reflects a major period of clastic-dominated lead-zinc deposition in Australia and India. The appearance of clasticdominated deposits in the Precambrian period corresponds to a time of formation of large and long-lived basins on stable cratons that followed the Great Oxidation Event at about 2.4 to 1.8 billion years ago. The formation of clastic-dominated deposits is related to enhanced oxidation of sulfides in the crust (which released sulfate to the hydrosphere and lead and zinc to sediments); the development of major redox and compositional gradients in the oceans; the first formation of significant sulfatebearing evaporates, red beds, and oxidized aquifers; and the evolution of sulfate-reducing bacteria (Leach, Bradley, and others, 2010, p. 593). A number of clastic-dominated lead-zinc deposits also formed during the Carboniferous Period along passive margins in sedimentary-evaporite belts of Pangea and were deposited after the second oxidation event. Mineralogical and geochemical evidence suggests that clastic-dominated ore fluids were principally hot metalli ferous basinal brines (for example, Badham, 1981; Lydon, 1983; Cooke and others, 2000). For example, the brines that deposited sphalerite at the Red Dog deposit in Alaska had temperatures in the range of 100 to 200 °C and salinities from 14 to 19 weight percent NaCl equivalent (Leach, Marsh, and others, 2004). The dominant sulfide minerals present are sphalerite (which is generally the most common sulfide mineral) and lesser amounts of galena and the iron sulfides, marcasite, pyrite, or pyrrhotite; chalcopyrite and sulfosalt minerals may be present in minor amounts (Large, 1981, 1983; Lydon, 1983; Large and others, 2005). The sulfide minerals form lens-like bodies that are interbedded with fine-grained dark clastic and chemical sedimentary rocks. Carbonate minerals (ankerite, calcite, dolomite, and siderite), barite and quartz, siliceous shale, and low-grade silica-replaced rocks are common and can be gradational into massive ore. Ore from these deposits (table H4) generally contains less than 100 ppm gallium, with a typical range of 5 to 50 ppm gallium (table H2). For example, the Red Dog deposit (fig. H4) is hosted by organic-rich mudstone and shale of the Mississippian Kuna Formation (Kelley and others, 2004), and zinc concen trate from the deposit has 26 ppm gallium (Alaska Department of Environmental Conservation, 2005). Other elements in association with clastic-dominated lead-zinc deposits include Ag, Al, As, Au, B, Ba, Cd, Co, Cu, Fe, Ge, Hg, Mn, Mo, Ni, and Sb (Briskey, 1986; Kelley and others, 1995). Mississippi Valley Lead-Zinc Type Carbonate-hosted lead-zinc deposits are found throughout the world, and large famous examples occur in central North America; hence, they are generally referred to as Mississippi Valley-type deposits. The most important characteristics of Mississippi Valley-type deposits (Sangster, 1990; Leach and Sangster, 1993; Leach, Sangster, and others, 2005) are that they occur globally in dolostone and limestone that formed in platform carbonate sequences. They are commonly related to extensional domains inboard of contractional tectonic belts or continental extensional basins transitional to passive margin basins (figs. H5C and H5E). The ores are usually located at flanks of basins, orogenic forelands, or foreland thrust belts inboard of the clastic-rock-dominated passive margin sequences. Numerous lines of evidence have established that the metals were carried in fluids derived mainly from evapo rated seawater and driven within the carbonate sequences by large-scale tectonic events. More than 80 percent of Mississippi Valley-type deposits are hosted by rocks formed during the Phanerozoic Eon; less than 20 percent occur in Precambrian rocks (Sangster, 1990; Leach, Bradley, and others, 2001; Leach, Sangster, and others, 2005; Kesler and Reich, 2006). Increased oxygenation of the oceans following the second oxidation event of the atmosphere in the late Neoproterozoic Era led to an abundance of evaporites, oxidized brines, and a dramatic increase in the volume of coarse-grained and permeable carbonates in Paleozoic carbonate platforms. Most Phanerozoic-hosted Mississippi Valley-type deposits formed in the carbonate platform sequences during Devonian to Permian time as a
H20 Critical Mineral Resources of the United States— Gallium result of tectonic events generated by the assimilation of Pangea. A lesser number formed during Cretaceous to Tertiary time when microplate assimilation affected the western margin of North America and Africa-Eurasia. These ores consist mainly of galena, sphalerite, and generally lesser amounts of iron sulfides. Copper and silver are important constituents in some of these deposits (for example, deposits of the Irish subtype and some deposits of the Viburnum Trend). Gangue minerals are dominantly carbonates (ankerite, calcite, dolomite, and siderite), with typically minor amounts of barite. Silicification of the host rocks (or quartz gangue) is generally minor, but may be abundant in a few deposits. Minor minerals associated with Mississippi Valleytype mineralization differ widely between individual districts, and minor and trace metals also vary (Lavery and others, 1994). Minor elements and element associations can include Ag, As, Au, Ba, Bi, Cd, Co, Cu, Ga, Ge, In, Fe, Hg, Mn, Mo, Ni, Sb, Sn, and Tl (Hall and Heyl, 1968; Hagni, 1983; Viets and others, 1992; Leach, Taylor, and others, 2010). In Mississippi Valley-type deposits, gallium is primarily in sphalerite, which typically can contain from 0.02 to more than 500 ppm gallium (table H4). The overall content of gallium in studied deposits averages about 58 ppm; however, averages for individual deposits can vary widely, from 6 to 130 ppm gallium. Kipushi Polymetallic Type Kipushi-type deposits are relatively rare, epigenetic, discordant deposits of Cu-Zn-Pb -Ag-As-Sb-Ge-Ga-bearing ores that occur in carbonate platform rock sequences and are typically associated with karst, solution collapse breccias, and related features, such as salt diapirs (fig. H5F; Hitzman and others, 2005). The most important characteristics of these deposits are their geologic setting and distinctive polymetallic mineralogy. Considerable controversy remains concerning whether these deposits have an igneous affinity; however, most are classified as sediment-hosted deposits. Deposits of the Kipushi type (fig. H4) include world-class polymetallic orebodies at Tsumeb and Khusib Springs in Namibia (Söhnge, 1964), and at Kipushi (in the Democratic Republic of the Congo) in the Zaire-Zambia copper belt (Intiomale and Oosterbosch, 1974). Domestic deposits of this type include orebodies at the Ruby Creek Mine in Alaska (Hitzman, 1986; Bernstein and Cox, 1986; Selby and others, 2009) and at the Apex Mine in Utah (Bernstein, 1986). Other large deposits included in this group in the past are, in Australia, the Mount Isa Mine in the State of Queensland, and the Cooley and Ridge deposits of the McArthur River Mine in the Northern Territory (Bernstein and Cox, 1986; Hitzman, 1986; Selby and others, 2009); more recent studies, however, suggest that these deposits are better classified as clastic-dominated-type deposits because they are mainly lead-zinc orebodies in dolomitic siltstones, mudstones, and sandstones formed in a continental sag basin (Cooke and others, 2000; Leach, Marsh, and others, 2005; Leach, Bradley, and others, 2010; Taylor and others, 2009). Features of Kipushi-type deposits that are similar to aspects of clastic-dominated-type and Mississippi Valley-type deposits include the general platform carbonate setting, a lack of spatially associated igneous intrusions, regional-scale ore fluid migration (as evidenced by the widespread occurrence of mineralization over thousands of square kilometers in the largest deposits), and ore localization in solution collapse breccias. The orebody at Kipushi-type deposits forms a highly irregular subvertical pipe at the faulted boundary of dolomitic shale and marine dolomite, commonly within breccia zones, and prospecting for copper-rich ores is focused on paleokarst structures that are possibly related to salt diapirs (Intiomale and Oosterbosch, 1974). The Khusib Springs deposit, which is a lens-shaped sulfide orebody up to 10 meters in thickness and elongated parallel to the bedding, replaces locally brecciated limestone of the lower Tsumeb subgroup. The origin of the breccia pipes is probably related to the erosion and karstification of the Hüttenberg Formation (Hughes and others, 1984; Hughes, 1987); however, there is some evidence that hydrothermal brecciation and silicification occurred during mineralization (Chetty and Frimmel, 2000; Kamona and Günzel, 2007). Fluid inclusion studies have also established that mineralizing fluids for Kipushi-type deposits are comparable to or higher than those of typical Mississippi Valley-type deposits and clastic-dominated lead-zinc deposits (see above). Ore-stage minerals contain highly saline fluids (17 to 43 weight percent NaCl equivalent) in inclusions with homogenization temperatures that vary from about 100 to 380 °C; the fluids were generated in postorogenic, extensional settings (Misiewicz, 1988; Chetty and Frimmel, 2000; Selby and others, 2009; Haest and others, 2009; van Wilderode and others, 2012). Kipushi-type deposits are distinguished from Mississippi Valley-type deposits by having significantly higher trace element concentrations and more diverse mineralogy (Frimmel and others, 1996). Primary minerals include enargite, galena, and pyrite, and a zinc-rich tennantite; locally abundant zones of bornite, chalcocite-digenite, and chalcopyrite; galena, pyrite (cobaltiferous), pyrrhotite, and iron-poor sphalerite. Late-stage phases include carrollite and copper-thiogermanate minerals, pearceite-polybasite, native silver, tetrahedrite, and silver-tennantite. Additional minerals identified in some deposits include covellite, a tungsten-bearing germanite, germanium-bearing stannoidite, and stromeyerite. Gangue minerals include barite and bariumsilicate minerals, calcite, chlorite, dolomite, fluorite, sparse quartz, and siderite. Ore minerals in Kipushi-type deposits can have gallium contents that range from hundreds of parts per million gallium to about 60 weight percent gallium (tables H2 and H3). Compared with other sediment-hosted ore systems, Kipushi-type deposits are characterized by marked enrichments in germanium and rhenium (Schneider and others, 2007). Many weathered deposits of this type contain gallium-bearing secondary minerals formed largely through supergene processes (table H3). For example, at the Apex Mine in Utah, the primary ore is inferred to be similar
Resources and Production H21 to bornite-chalcopyrite-tennantite-galena-sphalerite ore of a Kipushi type, although the Apex orebodies are almost completely oxidized to germanium-bearing hydrogoethite and gallium-bearing (up to about 1 percent gallium) jarosite. Resources and Production Identified Resources The U.S. Geological Survey (USGS) estimates that world resources of gallium in bauxite exceed 1 billion kilograms and that a considerable quantity of gallium could be present in world zinc resources (Bray, 2013). Using USGS information coupled with data on global bauxite provinces (Bogatyrev and Zhukov, 2009), comparable estimates of geologically available gallium in bauxite reserves and resources (of nearly 1.4 million metric tons and 4 million metric tons, respectively) have been suggested (Dittrich and others, 2011). Analogous estimates of world resources of gallium in zinc resources have not been made owing to insufficient data. The United States is expected to meet its current and expected future needs for gallium through imports of primary gallium from these resources, from imports of recycled and refined gallium, and from domestic production of recycled and refined gallium. Identified bauxite resources of the world are about 55 to 75 billion metric tons and occur in Africa (32 percent), Oceania (23 percent), South America and the Caribbean (21 percent), Asia (18 percent), and elsewhere (6 percent) (Bray, 2012). Lateritic bauxite deposits make up approximately 90 percent of global bauxite resources, and aluminum produc tion is mainly from these deposits (Freyssinet and others, 2005). Karst bauxite deposits are more scattered in distribution and are mined mainly in eastern Europe and the Caribbean islands (Bárdossy, 1982). Domestic bauxite deposits consist mainly of subeconomic resources that are not generally suit able for alumina production owing to their high silica content; thus, recovery of gallium from these deposits is unlikely. Although domestic resources of bauxite are inadequate to meet either short- or long-term U.S. demand, the United States has essentially inexhaustible subeconomic resources of aluminum in materials other than bauxite. These are primarily clay deposits, which also contain gallium in amounts comparable to those of bauxite (30 to 50 ppm gallium) (table H2). Identified zinc resources of the world are about 1.9 billion metric tons (Tolcin, 2013). Mississippi Valley-type deposits, which are widespread in North America, account for about 38 percent of the global resources of lead and zinc in sediment-hosted ore deposits; significant zinc resources are also contained in clastic-dominated deposits of Alaska (fig. H4; Leach, Marsh, and others, 2005). The estimates given here apply to total gallium content; only a small percentage of gallium metal in bauxite and zinc ores is recoverable using current methods because processing techniques to separate aluminum, zinc, and other byproduct metals (for example, germanium and indium) compete for gallium in source materials. Factors controlling gallium recovery are typically proprietary, and scant literature is avail able on the general operating practices of companies involved in gallium production. The potential remains for additional gallium production given more efficient and improved extrac tion technologies and market-driven economic incentives. Undiscovered Resources The USGS has conducted no quantitative mineral resource assessments focused on undiscovered domestic or global resources of bauxite since the work of Patterson and others (1986). Because the major bauxite provinces of the world are well established and have been intensely explored, most of the significant bauxite deposits globally are thought to be known (Freyssinet and others, 2005). The major bauxite provinces (fig. H4) occur in the following five major regions: (a) the South American region—the Guiana shield (French Guiana, Guyana, Suriname) and the central Brazilian shield; (b) the West African region—Burkina Faso, Ghana, Guinea, Ivory Coast, Mali, and Sierra Leone; (c) the Indian region—the western and eastern coasts; (d) Southeast Asia— Cambodia, Laos, and Vietnam; and (e) the Australian region. Other bauxite provinces occur in the southern United States (Arkansas region), the Caribbean, the southern Ural Mountains (Russia and Kazakhstan), and China. The most recent USGS National Mineral Resource Assessment identified areas of the United States that have significant potential for undiscovered resources of zinc (Schruben, 2002). The estimate of the amount of zinc in undiscovered U.S. deposits ranged from a 90-percent probability of at least 130 million metric tons to a 10-percent probability of at least 290 million metric tons. The mean esti mate of zinc in undiscovered deposits was 210 million metric tons. Nearly 40 percent of the zinc was thought to be contained in undiscovered Mississippi Valley-type deposits. Other major deposit types were sedimentary exhalative (31 percent), volcanogenic massive sulfide (15 percent), and polymetallic replacement (8 percent) deposit types. At that time, the total identified zinc resources in the United States were estimated to be 99 million metric tons. No comparable estimates of undiscovered global resources of zinc have been made. Production In 2012, world primary gallium production was estimated to be 273 metric tons (fig. H4A). China, Germany, Kazakhstan, and Ukraine were the leading producers; coun tries with lesser output were Hungary, Japan, the Republic of Korea, and Russia. Refined gallium production was estimated to be about 354 metric tons; this estimate includes primary gallium production and some scrap refining. China, Japan, the United Kingdom, and the United States were the principal producers of refined gallium. Gallium was recycled from new scrap in Canada, Germany, Japan, the United Kingdom, and
H22 Critical Mineral Resources of the United States— Gallium the United States. World primary gallium production capacity in 2012 was estimated to be 474 metric tons; refinery capacity, 270 metric tons; and recycling capacity, 198 metric tons (Jaskula, 2013a). Imports of gallium into Japan and the United States (the two leading consuming countries) were used as the basis for estimating world gallium production. In addition, Metal Bulletin (2013) provided an updated Chinese gallium production estimate, expanding the estimate of China's production considerably owing to the substantial capacity increases of China's gallium operations (fig. H4B). No domestic production of primary gallium was reported in 2012. Molycorp Inc. recovered gallium from scrap materials, predominantly those generated during the production of GaAs. Molycorp's facility in Blanding, Utah, has the capability to produce about 50 metric tons per year of high-purity gallium. The company recovered gallium from its customers' scrap on a fee basis and purchased scrap and low-purity gallium for processing into high-purity material. Exploration for New Deposits Most domestic exploration for gallium is done as a second-order effort related to new discoveries of zinc resources because U.S. bauxite resources are currently subeconomic. Exploration is active in the United States and globally for all types of large-tonnage zinc deposits, which constitute a major source for gallium (Emsbo, 2009; Taylor and others, 2009; Leach, Bradley, and others, 2010). There is some evidence that partitioning of gallium may be influenced by the amount of sphalerite in bulk ore, leading to a dilution effect, such that the largest zinc deposits of a given type may be expected to have relatively lower gallium contents (Cook and others, 2009). This, however, may have little effect on resource estimation because economic factors that support gallium extraction are tilted in favor of large tonnage values for the principal commodity over small changes in grade for a byproduct element. The most gallium-enriched sedimenthosted ores are the Kipushi-type deposits, which are mined for copper, lead, silver, and zinc and byproduct antimony, arsenic, germanium, and gallium; some deposits of this group may contain gallium (and germanium) in amounts that justify extraction as a primary product. Active exploration efforts are underway in the major Kipushi-type provinces of the Central African copperbelt—Kipushi and Tsumeb. Important domestic examples of Kipushi-type deposits include the Ruby Creek deposit in Alaska and the Apex deposit in Utah. Additional domestic exploration efforts are focused on gallium resources associated with epithermal mineralized and hydrothermally altered volcanic rock suites. Ruby Creek deposit, Alaska.—This deposit is located along the northern flank of the Cosmos Hills near the boundary between a carbonate platform and a shale basin within the Ambler district. The 1-kilometer-diameter body of hydrothermal dolostone is one of several similarly mineralized occur rences in the Cosmos Hills. The Ruby Creek deposit contains more than 90 million metric tons of 1.2 percent copper, plus elevated contents of silver, cobalt, germanium, gallium, and zinc (Hitzman, 1986). The metacarbonate sequence consists primarily of thin-bedded to massive metadolostone and marble that is locally graphitic or phyllitic and contains thin micaceous layers interpreted as airfall tuffs (Hitzman, 1986; Hitzman and others, 1986). The presence of coeval volcano genic massive sulfide deposits just north of the Ruby Creek deposit is possible evidence for regionally extensive hydro thermal activity, perhaps related to igneous activity associated with continental extension (Hitzman and others, 1986; Selby and others, 2009). Apex deposit, southwestern Utah.—This deposit has served as the only primary source in the world for the production of gallium and germanium. The Apex Mine operated intermittently from 1884 to 1962, and more than 7,000 metric tons of copper, 5,600 kilograms of silver, and small amounts of gold and lead were produced. The mine was reopened in 1985 with plans to produce gallium and germanium (Washington County Historical Society, 2013). At that time, the mine was estimated to contain 220,000 metric tons of ore grading 0.064 percent germanium, 0.032 percent gallium, 1.63 percent copper, 0.77 percent lead, 1.58 percent zinc, 0.50 percent arsenic, 16.73 percent iron, and 38 grams per metric ton silver (Bernstein, 1986). Surface dumps from the old mine contained an additional 45,000 metric tons of material grading 0.037 percent germanium, 0.019 percent gallium, and 1.55 percent copper. Similar deposits may occur in association with copper in fault-hosted breccias in Pennsylvanian limestone near Hansonburg, New Mexico (Eveleth and Lueth, 2009). Cordero Gallium Project, Humboldt County, Nevada.—This project is reportedly North America's largest known primary gallium occurrence (Rêserva International LLC, 2008). Anomalous gallium mineralization occurs over a wide area in and around the historic Cordero and McDermitt mercury mines. There are indications that a number of high-grade "feeder zones" occur through the deposit, which also hosts significant occurrences of rare-earth elements. The resources for the project above a cutoff grade of 30 ppm gallium are esti mated to exceed 13 million metric tons (Rêserva International LLC, 2008). Environmental Considerations Sources and Fate in the Environment In weathering environments, gallium is generally immobile and is only slightly less reactive than aluminum (Shiller and Frilot, 1996). The tendency for gallium to be dissolved and transported depends upon its chemical form, which in turn depends largely upon the pH and temperature of weathering solutions. Gallium associated with bauxite at a pH near 4 and a weathering solution temperature of 25 ºC would be expected to remain as a solid (for example,
Environmental Considerations H23 or but can theoretically dissolve to form aqueous hydroxide complexes or at pH values 4 and and at pH values 4; Bénézeth and others, 1997; Diakonov and others, 1997). Gallium is expected to be transported in soils, streams, and rivers in the solid phase as small particulates because of the low solubility of gallium minerals in natural waters. Gallium associated with sulfide minerals is more mobile based on the instability of sulfides at Earth's surface. Dissolution of sulfide minerals releases metals and sulfuric acid, and the acidic pH values allow higher concentrations of metals to be dissolved in solution—potentially causing the environmental problem known as acid mine drainage (AMD). Under acidic conditions, gallium would most likely form dissolved complexes with sulfate and (or) chloride (for example, GaSO4 + or GaCl−) (Wood and Samson, 2006). Metals dissolved in AMD can be naturally attenuated through precipitation, sorption to oxyhydroxide minerals, or dilution by mixing with water at circumneutral pH; otherwise, metals are transported downstream. Examples of natural concentrations of gallium in soil, water, and air are given in table H2. Gallium contents in soils range from 3 to 70 milligrams per kilogram (mg/kg) (or 3 to 70 ppm) (Kabata-Pendias and Pendias, 2001), and concentrations as high as 220 mg/kg have been observed in agricultural soil (Połedniok and others, 2012). Gallium generally behaves like aluminum in the environment, but, under certain pH and redox conditions, the two elements may be separated. As a result, values of the ratio of gallium to aluminum (Ga:Al ratio) can aid in detecting gallium enrichment in various materials. Shiller and Frilot (1996) showed that the Ga:Al ratio of parent rocks (and residual soils) and streambed solids were similar, whereas the Ga:Al ratio in nearby streams was higher than those in parent rocks. The concentration of gallium in these streams ranged from 0.0001 to 0.006 micrograms per liter (µg/L) (or 0.0001 to 0.006 parts per billion). These concentrations are generally lower than the 0.001 to 0.12 µg/L range of dissolved gallium found in large rivers around the world and are compa rable to gallium concentrations measured in the Atlantic Ocean (Shiller, 1998). Compared with dissolved species, gallium is highly concentrated in small particulates in river water, with a global average of 18.1 mg/kg in suspended sediment (Viers and others, 2009). Gallium occurs naturally in the atmosphere as part of mineral dust particles and has been measured to be less than 0.14 nanograms per cubic meter (ng/m3 ) at the South Pole (Kabata-Pendias and Pendias, 2001). Mining and ore processing can hasten the weathering process and can lead to above-background concentrations of gallium in soil and water (table H2). For example, gallium concentrations ranging from 93 to 270 mg/kg have been observed in soils near bauxite and lead-zinc mine sites in Poland (Połedniok and others, 2012). In addition, anthro pogenic release of gallium to the atmosphere occurs during coal combustion, ore processing, and semiconductor manufac turing, leading to concentrations as high as 1 to 12 ng/m3 near industrial areas (Chen, 2007; Kabata-Pendias and Mukherjee, 2007). Likewise, dissolved and colloidal concentrations of gallium in groundwater near semiconductor manufacturing areas in China have been measured to be greater than 10 µg/L (Chen, 2006). The recycled content, or proportion of scrap, used in gallium production is between 10 and 25 percent, but the fraction of gallium in discarded products that get recycled is less than 1 percent (Graedel and others, 2011). One of the main challenges of gallium recycling is that it is commonly comingled with other "specialty metals" in high performance alloys, making recovery technologically and economically unfeasible (Reck and Graedel, 2012). Given the importance of gallium in thin-film photovoltaic cells and integrated circuits, however, gallium recycling is likely to increase in the future (Graedel, 2011). Mine Waste Characteristics Mine waste is generally considered to be the material that originates and accumulates at a mine site that has no current economic value (Lottermoser, 2010), and it includes both solid and liquid waste. Gallium associated with bauxite is usually mined by open pit or underground methods and then refined through the Bayer process, which can lead to large volumes of processing waste. Based on its appearance, bauxite (or alumina) processing waste is commonly referred to as red mud. Red mud depositories are connected with alumina processing plants and are not necessarily located at the mine site. For example, four active plants in the United States predominantly process alumina ore from overseas (Bray, 2012). On October 4, 2010, a breached red mud depository released between 600,000 to 700,000 cubic meters of material from the Ajka Timfoldgyar Zrt alumina plant in Hungary (Reeves and others, 2011). This spill was unprecedented both in scale (the spill covered an estimated 800 hectares of land [Ruyters and others, 2011]) and in its delivery of trace elements across such a vast area (Mayes and others, 2011). Bauxite ore results from extreme chemical weathering, and many elements that were originally present in the parent rock have been leached away. The exceptions are minor elements that can sorb to or coprecipitate with aluminum and iron minerals that make up the ore and waste. For example, in addition to Al and Fe, bauxite processing waste has been shown to contain elevated concentrations of As, Cr, Ga, Mo, Ni, and V (Mayes and others, 2011). Bauxite mine and processing waste generally lacks acid-generating minerals, so these elements are expected to remain immobile under oxidizing and near-neutral (5 pH 8) weathering conditions as long as abundant iron-rich particles are present (Smith and Huyck, 1999). Waste products that result from mining sediment-hosted deposits usually consist of waste rock, tailings piles, and possibly pit lakes. For example, the cumulative volume of tailings generated at the Red Dog Mine, which is a clasticdominated lead-zinc deposit, totaled 27.4 million metric tons in 2006, and it is projected to total 88 million metric tons
H24 Critical Mineral Resources of the United States— Gallium by 2031 (SRK Consulting [Canada] Inc., 2007). The largest U.S. Environmental Protection Agency Superfund site occurs in the Tri-State zinc mining district and contains an estimated 91 million metric tons of waste rock (U.S. Environmental Protection Agency, 2006). The district contains a large number of Mississippi Valley-type lead-zinc deposits and covers approximately 1,800 square kilometers of Kansas, Oklahoma, and Missouri (Leach, Taylor, and others, 2010). The mineralogy of the mine waste associated with these deposits reflects the mineralogy of the unmined deposit, except that the proportion of lead-zinc-copper sulfides, such as chalcopyrite, galena, and sphalerite, is reduced relative to the gangue minerals (barite, calcite, dolomite, pyrite, and quartz) (Kelley and others, 1995; Leach, Viets, and others, 1995). Because of their association with sulfides and secondary minerals, such elements as Ag, Al, As, Au, B, Ba, Cd, Co, Fe, Ga, Ge, Hg, Mn, Mo, Ni, and Sb can also be found in clastic-dominated deposits (Briskey, 1986; Kelley and others, 1995). Likewise, Mississippi Valley-type deposits can contain minor to trace amounts of Ag, As, Au, Ba, Bi, Cd, Co, Cu, F, Fe, Ga, Ge, In, Mo, Ni, Pb, Sb, Sn, and Tl (Leach, Viets, and others, 1995; Foley, 2002a). Under oxidizing and acidic (pH 3) weathering conditions, which would be expected in clastic-dominated mine waste having little to no acid-neutralizing capacity and significant pyrite content, many of these elements are mobile (Smith and Huyck, 1999). The acid-neutralizing capacity of carbonate-hosted mine waste is greater, however, and the oxidizing and circumneutral (5 pH 8) conditions that prevail during weathering would cause most of the elements listed above to be less mobile or immobile (Smith and Huyck, 1999). An important exception is zinc, which can make up a large percentage of the total dissolved metals draining from Mississippi Valley-type deposits (Plumlee and others, 1999). A review of the environmental geochemistry of platform carbonate-hosted sulfide deposits identifies surface disposal of tailings as a potential environmental concern, especially if As, Cd, Tl, and Zn are present, because airborne dust may contain metal particulates (Foley, 2002a). A study of the Austinville lead-zinc mine in Wythe County, Virginia (Foley, 2002b), shows that residual soils near a Mississippi Valleytype deposit can be acidic enough to influence the mobility of metals in surface environments and that this is especially true for Mississippi Valley-type deposits that have high jasperoid and pyrite content. Human Health Concerns Study of the health effects of gallium on humans largely focuses on exposure to synthetic gallium arsenide (GaAs) by workers in the semiconductor industry, where small GaAs particles can be inhaled or ingested (Flora, 2000). The toxicity of GaAs may result from the arsenic (because arsenic trioxide may be released from GaAs after absorption); gallium toxicity, however, though not well studied, has not been ruled out (Chitambar, 2010). Humans can tolerate intravenous infusions of gallium nitrate during medical treatments for diseases such as cancer (Chitambar, 2010), but the long-term effects of human exposure to gallium in drinking water and soil resources remain unknown. Gallium does not appear in the U.S. Clean Water Act, and the Occupational Safety and Health Administration (OSHA) does not consider gallium metal to be hazardous, although some State governments include gallium in their right-to-know lists (Ziegler and others, 2004). Given their similar chemical characteristics, the health effects of gallium may be similar to those of aluminum. The Agency for Toxic Substances and Disease Registry (ATSDR) provides a useful summary of the human toxicology of aluminum (Agency for Toxic Substances and Disease Registry, 2008). Exposure to aluminum by the general public is most likely to occur through consumption of food, water, and aluminum-bearing medicines (such as antacids or antidiar real agents). Occupational exposure to aluminum is most likely to occur during ore processing and product fabrication. Most ingested or inhaled aluminum is not absorbed by the body, although absorption can vary depending upon the chemical form and particle size. Overexposure to aluminum can affect the nervous system, causing impaired performance in motor, sensory, and cognitive function. In addition, aluminum workers may suffer respiratory effects, such as impaired lung function and fibrosis. The United States national secondary drinking water regulation for aluminum, which is a nonenforceable guideline, ranges from 50 to 200 µg/L (U.S. Environmental Protection Agency, 2013b). Most surface waters contain aluminum concentrations below 100 µg/L (Agency for Toxic Substances and Disease Registry, 2008). Mining of deposits where gallium is a byproduct can potentially mobilize elements that are known human toxins; these elements have the potential to affect human health when present above threshold concentrations in air, drinking water, and soils. These concentrations can be exceeded near mine sites. The best known examples are the neurological effects of lead on children (Holecy and Mousavi, 2012) and the carcinogenic effects of arsenic in drinking water (Gupta and others, 2012). The current U.S. National Ambient Air Quality Standard for lead is 0.15 micrograms per cubic meter (U.S. Environmental Protection Agency, 2013a), and current U.S. primary and secondary drinking water standards for lead, zinc, and copper are 0, 5, and 1 milligrams per liter (mg/L), respectively (U.S. Environmental Protection Agency, 2013b). Canadian agricultural soil quality guidelines for lead, zinc, and copper are 70, 200, and 63 mg/kg, respectively (Canadian Council of Ministers of the Environment, 2007). Ecological Health Concerns Relatively few studies focus on the ecological effects of gallium mobility in the environment. Fish tend to be sensitive to low concentrations of dissolved metals, and they are therefore useful indicators of contamination in aquatic systems. One of several useful endpoints used in toxicity tests is the lethal concentration that leads to 50 percent mortality (LC50 ) after exposure to a substance for a certain amount of time.
Environmental Considerations H25 A study on the acute toxicity of dissolved gallium to carp (Cyprinus carpio Linnaeus) revealed a mean LC50 value of 95.6 ± 14.3 mg/L after 96 hours of exposure (Betoulle and others, 2002). Chronic toxicity tests with dissolved gallium and developing rainbow trout (Oncorhynchus mykiss) revealed a mean LC50 value of 3.5 mg/L after 28 days of exposure (Birge and others, 1980). In acute toxicity tests with saltwater organisms, LC50 values ranged from 12 to 55 mg/L of gallium after 24, 48, or 96 hours of exposure (Onikura and others, 2005). Gallium is typically not included in the suite of elemental concentrations reported in studies of mine waste water chemistry, so these LC50 values cannot be compared with likely gallium concentrations in mining environments. The ecological effects of gallium may be similar to those of aluminum given their similar behavior in the environment. When brown trout (Salmo trutta Linnaeus) were exposed to stream water samples of various pH levels and aluminum concentrations, the healthiest populations were observed in waters with pH above 5.0 and aluminum concentrations below 20 µg/L; significantly higher aluminum concentrations in gills were observed in moribund fish, suggesting that complications with respiration contributed to their decline (Andrén and Rydin, 2012). In toxicity tests with tropical freshwater algal and zooplankton species, LC50 values for aluminum increased with increasing dissolved organic carbon concentrations (Trenfield and others, 2012), suggesting that the affinity of aluminum for complexing with organic carbon renders it less toxic. The National Ambient Water Quality Criteria recom mended for the protection of aquatic life and adopted by some regions in the United States are 750 micrograms of aluminum per liter (µg Al/L) of water (acute) and 87 µg Al/L of water (chronic) (Suter and Tsao, 1996). Because aluminum is more mobile under acidic conditions, plants growing in acidic soils may suffer from aluminum toxicity, which is known to inhibit root growth and crop yields (Kabata-Pendias and Mukherjee, 2007). As a result, some regions of the United States have adopted a soil screening benchmark of 50 ppm of soil for terrestrial plants, as recommended by Oak Ridge National Laboratory (Efroymson and others, 1997). The ecological effects of other elements that gallium is associated with are relatively well documented. For example, in toxicity tests with aquatic invertebrates incubated with mine tailings from Mississippi Valley-type deposits, Besser and Rabeni (1987) showed that increased concentrations of dissolved cadmium, lead, and zinc correlated with decreased survival and growth (Besser and Rabeni, 1987). Likewise, various fish species experienced rapid mortality when exposed to increasing dissolved zinc concentrations, but this effect decreased with increasing calcium, magnesium, and sodium concentrations (De Schamphelaere and Janssen, 2004). High metal concentrations in soils can also cause phytotoxicity. Zinc is a micronutrient for many plant species, but when highly concentrated in soils, it can inhibit a plant's metabolic functions and cause deficiencies in other essential nutrients, such as copper, iron, manganese, and phosphorus (Nagajyoti and others, 2010, and references therein). Carbon Footprint Gallium has potential application in the effort to reduce the carbon footprint from energy production because of its use in efficient thin-film photovoltaic cells (solar panels) (Miles and others, 2007). The (CIGS) alloy, among others, is an attractive thin-film product because of its high efficiency and relatively low material and manufacturing costs (Kaneshiro and others, 2010). In a comparative study of lifecycle greenhouse gas emissions for various technologies, Kim and others (2012) found that estimates for the carbon footprint of CIGS technologies under test conditions are about 26 grams of CO2 equivalent per kilowatthour (g CO2-equiv/kWh), which compares well with older technologies that have greenhouse gas emissions in the range of 39 to 110 g CO2-equiv/kWh (Fthenakis and others, 2008). The CIGS photovoltaic cells are in the initial stages of commercialization, but they are expected eventually to compete with other forms of energy production as economies of scale permit future cost reductions (Miles and others, 2007; Rockett, 2010). With the rapid evolution of the thin-film photovoltaic cell industry, the recyclability of generated products and the potential for leaching of toxic elements from improperly disposed products would need to be addressed. Mine Closure Most recent and new mining operations include closure plans that address issues related to the mine footprint, which includes waste left onsite and locally affected soil and water, as well as ecological effects, such as habitat destruction and loss of biodiversity. Bauxite mines can be particularly prone to soil and habitat loss given that many are located in tropical, biodiverse locations. A study of a 10-year-old reforested area at a closed bauxite mine in Brazil showed that, although the reforestation program created adequate recovery conditions, recovery of native flora was influenced by the uneven return of important seed-dispersing animals (Parrotta and others, 1997). At one of the largest bauxite mine operations in the State of Western Australia, Australia, the Alcoa World Alumina Australia company clears the Jarrah (Eucalyptus marginata) forest before developing an open pit mine; after extracting bauxite, the company returns the topsoil and spreads Jarrah seeds at a rate of approximately 550 hectares per year (Gardner and Bell, 2007). With respect to mining in zinc deposits, mine closure issues often relate to the stability of large waste piles and tailings dams, and the potential for acid mine drainage from the site. Mine waste pile stability is commonly addressed through grading and covering dewatered piles. Acid mine drainage can be addressed with active water treatment facilities, passive limestone-lined channels, or constructed wetlands (Plumlee and Logsdon, 1999). The end result of both active and passive approaches is precipitation of dissolved metals. Precipitated metals in reconstructed wetland systems tend to be more stable under the prevailing anoxic conditions,
H26 Critical Mineral Resources of the United States— Gallium whereas the metal-rich precipitates that result from active treatment facilities form a sludge that can be similar to the metal-rich bauxite processing residues. At large mines, mine waste is typically consolidated into pits and submerged under water, forming a tailings pond. Acid-generating minerals are less reactive under water, but seepage is typically monitored and, if necessary, treated. At such sites as the Red Dog Mine in Alaska, seepage from the tailings ponds may need to be treated in perpetuity (U.S. Environmental Protection Agency, 2010). Failure of tailings pond impoundments can cause catastrophic flooding (McDermott and Sibley, 2000). Thus, plans for securing waste piles and prevention and treatment of acid mine drainage are important aspects of long-term active and proposed metal mining projects. Problems and Future Research Future research efforts related to gallium resources are likely to be directed at understanding the factors that may affect future supplies. Such research could include developing improved assessment models, conducting experimental studies, and seeking new and more efficient extraction and recycling technologies. Assessment models.—Improved models could help in locating new and unconventional resources. Efforts focused on incorporating additional geochemical and mineralogical data into existing mineral deposit and grade tonnage models (for zinc and bauxite deposits) can contribute to the development of quantitative grade-tonnage models and new approaches to evaluating the geologic availability of gallium as a byproduct. New models to estimate the occurrence of gallium in poly metallic resources of the Kipushi type could also contribute to more accurate quantitative assessments of the geologic avail ability of gallium. Also, models can assist in the evaluation of resources of gallium currently locked up in unprocessed mine waste, such as the extensive waste piles left over from the mining of Mississippi Valley-type deposits in the midcontinent region of the United States. Experimental studies.—Studies of gallium at conditions applicable to biochemical processes occurring in surface environments would provide for a better understanding of the natural gallium lifecycle and the potential issues related to gallium waste disposal. New technologies.—Devising new and more efficient extrac tion technologies for both raw ore and recycled products would help prevent or alleviate future bottlenecks in the supply chain that may result from increased demand. Recycling.—Because so little gallium in used products gets recycled, greater gallium recycling efficiency in the future can reduce reliance on mining of primary gallium resources. Future research could be focused on improving gallium recycling from current products, as well as future product design that allows greater gallium recycling efficiency. References Cited Note: All Web links listed were active as of the access date but may no longer be available. Agency for Toxic Substances and Disease Registry, 2008, Toxicological profile for aluminum: Atlanta, Ga., U.S. Department of Health and Human Services, Public Health Service, September, 310 p. plus 4 appendixes. [Also available at ://www.atsdr.cdc.gov/ToxProfiles/tp22.pdf.] Alaska Department of Environmental Conservation, 2005, table 2-1, Composition of Red Dog lead and zinc concen trates, accessed March 6, 2013, at ://dec.alaska.gov/ spar/csp/docs/reddog/03_ra_tables-april2005.pdf. Andrén, C.M., and Rydin, E., 2012, Toxicity of inorganic aluminum at spring snowmelt—In-stream bioassays with brown trout (Salmo trutta L.): Science of the Total Environment, v. 437, p. 422-432. 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For more information concerning this report, please contact: Mineral Resources Program Coordinator U.S. Geological Survey 913 National Center Reston, VA 20192 Telephone: 703- 648- 6100 Fax: 703- 648- 6057 Email: minerals@usgs.gov Home page: ://minerals.usgs.gov Prepared by the USGS Science Publishing Network Reston Publishing Service Center Edited by J.C. Ishee and Stokely J. Klasovsky Illustrations by Caryl J. Wipperfurth Layout by Caryl J. Wipperfurth and Cathy Y. Knutson Posting by Angela E. Hall
Foley and others—Critical Mineral Resources of the United States—Gallium —Professional Paper 1802-H ISSN 2330-7102 (online) ://doi.org/10.3133/pp1802H