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Calaverite
Calaverite
Calaverite
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Calaverite
Platy calaverite crystals on matrix from the Cripple Creek District (size: 6 x 5 x 3.5 cm)
General
CategoryTelluride mineral
FormulaAuTe2
IMA symbolClv[1]
Strunz classification2.EA.10
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Unit cella = 7.19 Å, b = 4.4 Å,
c = 5.08 Å; β = 90.3°; Z = 2
Identification
Formula mass452.17 g/mol
ColorBrass yellow to silver white
Crystal habitBladed and slender striated prisms, also massive granular
TwinningCommon on [110]
CleavageNone
FractureUneven to subconchoidal
TenacityBrittle
Mohs scale hardness2.5–3
LusterMetallic
StreakGreen to yellow grey
DiaphaneityOpaque
Specific gravity9.1–9.3
Optical propertiesAnisotropic
PleochroismWeak
Ultraviolet fluorescenceNone
References[2][3][4][5]

Calaverite, or gold telluride, is an uncommon telluride of gold, a metallic mineral with the chemical formula AuTe2, with approximately 3% of the gold replaced by silver. It was first discovered in Calaveras County, California in 1861, and was named for the county in 1868.

The mineral often has a metallic luster, and its color may range from a silvery white to a brassy yellow. It is closely related to the gold-silver telluride mineral sylvanite, which, however, contains significantly more silver. Another AuTe2 mineral (but with a quite different crystal structure) is krennerite. Calaverite and sylvanite represent the major telluride ores of gold, although such ores are minor sources of gold in general. As a major gold mineral found in Western Australia, calaverite played a major role in the 1890s gold rushes in that area.

Physical and chemical properties

[edit]

Calaverite occurs as monoclinic crystals, which do not possess cleavage planes. It has a specific gravity of 9.35 and a hardness of 2.5.

Calaverite can be dissolved in concentrated sulfuric acid. In hot sulfuric acid the mineral dissolves, leaving a spongy mass of gold in a red solution of tellurium.

Structure

[edit]
Calaverite ball-and-stick crystalline structure. The yellow-colored atoms represent gold.

Calaverite's structure has been both an object of fascination and frustration in the scientific community for many years. Goldschmidt et al. indexed calaverite 105 crystals resulting in 92 forms[6] but needed five different lattices to index all of the faces.[7] This led to consideration that calaverite violated Haüy's Law of Rational Indices.[6]

The introduction of X-ray diffraction did not completely solve this problem. Tunell and Ksanda in 1936 and then Tunell and Pauling in 1952 solved the C2/m general structure of calaverite. However, additional diffraction spots which they could not interpret were present in the survey. Later, transmission electron microscopy study suggested that the satellite reflections in calaverite were due to Au in incommensurately displacive modulation superimposed on the average C2/m structure.[8] In 1988, Schutte and DeBoer solved the structure by using the 3H super space group C2/m (α O γ)Os. They also showed that these modulations consist mainly of the displacements of tellurium atoms and the observed modulations were interpreted in terms of valence fluctuations between the Au+ and Au3+. According to Schutte and DeBoer, those displacements also affect the coordination number of calaverite.[9]

In 2009, Bindi et al. concluded that the different coordination numbers associated with calaverite were indeed associated with a significant differentiation in the valence sum of Au, and that the random distribution of Ag suppresses the fluctuation of Au+ and Au3+, whereas the ordered distribution reinforces it.[10]

Occurrence

[edit]
Calaverite from the Cresson Mine, Cripple Creek, Colorado. Largest crystal is 9 mm

Calaverite occurrences include Cripple Creek, Colorado, Calaveras County, California, US (from where it gets its name), Nagyag, Romania, Kirkland Lake Gold District, Ontario, Rouyn District, Quebec, and Kalgoorlie, Australia.

History

[edit]

Calaverite was first recognized and obtained in 1861 from the Stanislaus Mine, Carson Hill, Angels Camp, in Calaveras Co., California.[3] It was named for the County of origin by chemist and mineralogist Frederick Augustus Genth who differentiated it from the known gold telluride mineral sylvanite, and formally reported it as a new gold mineral in 1868.[11][12] Genth found that the telluride formula for calaverite generally corresponded with the gold-silver telluride mineral sylvanite, but had a far lower percentage of ionic silver in place of ionic gold (3 to 3.5% in Genth's analysis, vs. 11 to 13% silver typical for sylvanite). Since silver is isomorphous with gold in telluride minerals (i.e. gold atoms replace silver without automatically changing the crystal character), Genth more importantly reported the calaverite differed from sylvanite in having no distinct crystalline cleavage line, whereas sylvanite was known to have a distinct line of cleavage. (As discussed above, both sylvanite and calaverite have since been found to be basically monoclinic, whereas the third known gold-silver telluride mineral krennerite is orthorhombic, with yet a different characteristic line of cleavage parallel to the crystal base). Genth was later also able to characterize a sample of calaverite from Boulder, Colorado, finding that his two specimens from that location were 2.04 and 3.03% silver.[13]

In the initial phase of the Kalgoorlie gold rush in Western Australia in 1893, large amounts of calaverite were initially mistaken for fool's gold, and were discarded. The mineral deposits were used as a building material, and for the filling of potholes and ruts. Several years later, the nature of the mineral was identified, leading to a second gold rush of 1896 that included excavating the town's streets.[14]

References

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See also

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Calaverite

View on Grokipedia
from Grokipedia
Calaverite is a rare with the AuTe₂, composed primarily of and , and recognized as one of the few natural compounds from which is extracted on an industrial scale. It occurs as metallic, bladed or prismatic crystals with a distinctive brass-yellow to silver-white color, a metallic luster, and a greenish to yellowish-gray streak. Named in 1868 after its type locality in Calaveras County, California, calaverite is monoclinic in crystal system, with a Mohs hardness of 2.5–3 and a specific gravity of 9.10–9.40, making it relatively soft and dense compared to many other minerals. Calaverite forms in low- to high-temperature hydrothermal veins, often associated with minerals such as altaite, coloradoite, krennerite, , and , and is dimorphous with the related krennerite. Its features an incommensurately modulated lattice based on a distorted CdI₂-type arrangement, which has long puzzled crystallographers due to periodic displacive modulations along the b-axis linked to charge redistribution on sites. Historically, calaverite played a significant role in , particularly during the Australian gold rush, where it was initially mistaken for worthless before its high content—up to 42% by weight—was discovered, leading to roasting processes to recover the metal. Notable deposits occur in Cripple Creek, Colorado; , ; and various sites in and .

Discovery and Naming

Etymology

Calaverite was named in 1868 by the chemist and mineralogist Frederick Augustus Genth for its type locality at the Stanislaus Mine in , USA. The county's name derives from the Spanish word calaveras, meaning "skulls," which was applied to the Calaveras River by Spanish explorer Gabriel Moraga in 1806 after his expedition encountered numerous human skulls—likely those of Native Americans—along its banks. This etymological root reflects the region's early exploration history rather than any direct association with the mineral's appearance. Unlike calaverite, which honors a North American locality, the similar gold telluride mineral sylvanite, which was named in 1835 after its type locality in Transylvania (modern-day Romania), combined with the suffix -ite commonly used in mineral nomenclature. Similarly, krennerite, another orthorhombic gold telluride, was named in 1877 after József Krenner, a Hungarian mineralogist and professor at the University of Budapest, highlighting the role of individual scientists in mineral naming conventions. These distinctions underscore the diverse origins in geographic, linguistic, and personal inspirations for naming telluride minerals during the 19th century.

Initial Identification

Calaverite was first discovered by miners during in , in 1861, at the Stanislaus Mine near Carson Hill. The specimen examined was obtained from this locality, where it occurred in association with petzite in veins. The mineral's scientific identification occurred in 1868, when chemist Frederick Augustus Genth conducted detailed chemical assays on the sample, revealing a composition of approximately 40-41% , 3% silver, and 56% , consistent with the formula AuTe₂. Genth's analysis involved methods, including dissolution and tests, to distinguish it from other -bearing minerals. Early recognition was complicated by its metallic, bronze-yellow appearance, which led miners to confuse it with and discard it as worthless "fool's gold" in some deposits. Genth's findings were published in the American Journal of Science and Arts in 1868, formally establishing calaverite as a new species and contributing to the understanding of tellurium compounds in gold ores. This publication marked the initial scientific documentation of calaverite, highlighting its distinct properties and setting the stage for further studies on .

Properties

Physical Properties

Calaverite exhibits a distinctive brass-yellow to silver-white color with a metallic luster, often appearing as bladed or prismatic crystals up to 1 cm in length, or in massive granular forms. Its streak is greenish to yellowish-gray, aiding in its identification during mineral assays. Mechanically, calaverite has a Mohs hardness of 2.5–3, making it relatively soft compared to other gold-bearing minerals, and a specific gravity ranging from 9.10 to 9.40, reflecting its high density due to gold and tellurium content. It displays no cleavage, instead fracturing in an uneven to subconchoidal manner, and possesses brittle tenacity, which contributes to its granular habit in ore deposits. Optically, calaverite is opaque and shows weak and anisotropism in polished sections under reflected polarized light. Thermally, calaverite has a of approximately 464 °C, a relatively low temperature that historically facilitated its decomposition and recovery during fire assays in mining operations. This property underscores its behavior under heat.

Chemical Properties

Calaverite has the ideal AuTe₂, with a theoretical composition of 43.56% and 56.44% by weight. Natural specimens may exhibit minor substitutions, including up to approximately 3 wt.% silver and trace amounts of . The mineral is insoluble in , reflecting its stability under mildly acidic reducing conditions. However, it dissolves in hot concentrated , yielding a spongy deposit of metallic and a red solution containing (TeO₂). Nitric acid reacts similarly by darkening the mineral and precipitating metallic , while achieves dissolution with minor formation if substitutions are present. Calaverite oxidizes upon heating in air at elevated temperatures, decomposing to metallic and . During , a common pretreatment for ores, it breaks down further, releasing vapors and producing a porous calcine enriched in . The mineral demonstrates resistance to in low-temperature subsurface environments but undergoes oxidative alteration to secondary tellurides and native in near-surface oxidizing conditions.

Crystal Structure

Unit Cell and Symmetry

Calaverite crystallizes in the with C2/m, describing the average structure of this . The unit cell dimensions are a = 7.1947(4) Å, b = 4.4146(2) Å, c = 5.0703(3) Å, and β = 90.038(4)°, as determined from single-crystal X-ray diffraction studies. These parameters reflect the distorted layered arrangement fundamental to calaverite's framework, with the monoclinic distortion evident in the slight deviation of β from 90°. The unit cell contains Z = 2 formula units (AuTe₂), yielding a cell volume of approximately 160.8 ų. From these dimensions, the calculated density is 9.31 g/cm³, which closely aligns with measured specific gravity values ranging from 9.10 to 9.40 g/cm³, confirming the consistency of the structural model with empirical observations. Calaverite typically forms bladed or short to slender prismatic crystals elongated parallel to , often exhibiting striations along this direction, with maximum dimensions up to 1 cm; massive and granular aggregates are also common. Twinning is prevalent, most commonly on {110}, with less frequent occurrences on {031} and {111}, contributing to the mineral's characteristic bent or reticulated forms in natural specimens.

Atomic Arrangement and Bonding

Calaverite features a layered atomic structure derived from a distorted CdI₂-type , consisting of triangular layers of (Au) atoms interleaved with sheets of (Te) atoms. The Au atoms occupy positions within these layers, forming zig-zag chains along the a-axis, while Te atoms bridge the Au layers, resulting in a pseudo-hexagonal packing distorted by the incommensurate modulation. This modulation is characterized by a q ≈ −0.40 a* + 0.45 c*, leading to periodic displacements primarily of the Te atoms along the b-axis, which create a characterized by alternating short and long bonds. In terms of coordination, the Te atoms adopt a distorted octahedral around each Au atom in the average , with Au-Te bond lengths varying significantly due to the modulation: typically two short bonds (≈2.65 ) and four longer ones (≈2.8–3.0 ), reflecting the local distortions. The incommensurate nature arises from these Te displacements, which adjust the local environments to accommodate structural instabilities. Additionally, short Au-Au contacts of approximately 2.7 occur within the triangular Au layers, contributing to the metallic character observed in the mineral's properties. Te-Te interactions are also present, with distances around 3.3–3.7 , forming weak dimer-like pairs that influence the overall layering. The bonding in calaverite involves Au in the +1 valence state, with charge fluctuations that drive the incommensurate modulation as a mechanism primarily on Te sites. This is stabilized by negative charge-transfer energy, with excess holes localized on Te 5p orbitals. The local coordination around Au shows distortions to linear (two Te) and square-planar (four Te), but without Au valence change beyond +1. The Au-Te bonds exhibit partial covalent character, as evidenced by concentrations along these directions, while the short Au-Au bonds suggest contributions, and Te-Te pairs indicate weaker homopolar interactions. These electronic features, resolved by ab initio calculations in 2018, underpin the structural complexity and relate to the mineral's bronze-like luster through the metallic bonding component. The atomic arrangement and bonding have been elucidated through single-crystal diffraction studies using methods to model the incommensurate modulation. Early work by Schutte and de Boer (1988) refined the (3+1)-dimensional structure, revealing the Te displacements and coordination details. Subsequent analyses, such as Bindi et al. (2009), confirmed the modulation's dependence on trace silver content, which suppresses valence fluctuations when randomly distributed. Electron microscopy, including (TEM), has further visualized superstructure variations and satellite reflections, supporting the diffraction-based models.

Geological Occurrence

Formation Processes

Calaverite primarily forms in low-temperature hydrothermal environments, particularly within epithermal deposits, where it precipitates from gold-tellurium-rich fluids at temperatures typically ranging from 200 to 300 °C. These fluids originate from magmatic sources and circulate through shallow crustal levels, often at depths less than 1,500 meters, depositing calaverite in or veins as part of low-sulfidation or high-sulfidation systems. The process involves , cooling, fluid mixing, or vapor , which trigger the rapid precipitation of tellurides from the hydrothermal solutions. High-temperature magmatic fluids, around 300 °C, mix with cooler meteoric near 250 °C in adularia-sericite epithermal systems, facilitating the incorporation of into gold-bearing phases. In terms of paragenesis, calaverite commonly occurs alongside , , , and other tellurides such as altaite (PbTe), petzite, and krennerite, reflecting its deposition in polymetallic stages of mineralization. It often forms in late-stage sequences following initial quartz- or base-metal precipitation, and it may replace or be replaced by native due to the volatility of under changing fluid conditions. This textural relationship highlights calaverite's role in the evolution of Au-Te systems, where 's affinity for stabilizes the mineral until destabilization occurs through fluid evolution or oxidation. Calaverite's stability is tied to late-stage mineralization events linked to volcanic or intrusive activity in tectonically active settings, such as convergent-margin or extensional basins, where prolonged hydrothermal activity sustains the necessary Au-Te fluid chemistry. It is rare in metamorphic settings, as its formation favors the dynamic, open-system conditions of epithermal hydrothermal systems over high-pressure, closed-system . These deposits typically develop over timescales of about 1,000 years, with calaverite precipitating at greater depths than associated Au-Ag phases due to its higher temperature stability field.

Major Localities and Associated Minerals

Calaverite is primarily found in epithermal deposits formed through low-temperature hydrothermal processes. One of the most significant localities is Cripple Creek in , USA, where it occurs in rich veins within alkaline volcanic rocks, contributing substantially to the district's production. The type locality is in , USA, specifically at the Stanislaus Mine in the Carson Hill district, where calaverite was first identified in -bearing veins. Other major deposits include the goldfield in , particularly the Lake View and North Kalgoorlie mines, which have yielded large quantities of calaverite as a key gold telluride in veins. In , notable occurrences are at in , including the Lake Shore and Wright-Hargreaves mines, where calaverite is disseminated in and associated with other sulfides. In , calaverite occurs in significant deposits such as the Dongping Au-Te deposit in Province. Calaverite commonly occurs with other gold and silver tellurides such as , krennerite, petzite, and hessite, as well as native and in these deposits. In some settings, it is associated with coloradoite or melonite, along with sulfides like and . These occurrences are characteristically vein fillings in volcanic or intrusive rocks, often in low-sulfidation epithermal systems. Rare placer deposits, derived from the erosion of primary veins, have been reported in areas like , where detrital calaverite grains contribute to alluvial concentrations.

Economic and Historical Importance

Role in Gold Mining

Calaverite (AuTe₂), containing approximately 40–43% gold by weight, represents a significant economic resource in telluride-rich gold districts due to its high precious metal content. As a primary gold-bearing mineral, it has been a major ore in low-temperature vein deposits, contributing substantially to production in specific locales. In the Kalgoorlie Golden Mile, Western Australia, tellurides like calaverite accounted for roughly 20% of the district's total gold, yielding approximately 291 tonnes from these minerals amid over 1,457 tonnes of gold extracted historically. This concentration made calaverite essential for high-grade ore processing in telluride districts, where it often occurs alongside other gold tellurides, enhancing the viability of otherwise marginal deposits. Historically, calaverite played a pivotal role in fueling major gold rushes during the late 19th and early 20th centuries. In , its presence in the 1893 discovery by prospectors , Tom Flanagan, and Dan Shea ignited the state's most prolific gold boom, transforming the region into a global production hub and driving annual outputs that peaked at 38 tonnes of gold in 1903 alone. Similarly, in the United States, calaverite was central to the Cripple Creek mining district's explosive growth following its 1891 discovery, where it formed the bulk of the telluride ores that propelled the area to become Colorado's richest gold producer, with total district output of approximately 650 tonnes of gold through underground vein mining. These booms not only spurred —such as expansion and population surges—but also established calaverite as a symbol of untapped wealth in alkaline-hosted epithermal systems. Early mining efforts faced notable challenges due to calaverite's superficial resemblance to , leading to widespread misconceptions that it was worthless "fool's gold." In , prior to its identification in 1896, telluride-bearing rocks were routinely discarded as barren waste, resulting in significant initial losses and delaying recognition of their potential. This misidentification, common in telluride districts like Cripple Creek where calaverite's brassy luster mimicked non-precious sulfides, underscored the need for advanced mineralogical assays and contributed to erratic early production rates despite the ore's inherent value.

Extraction Methods and Challenges

Calaverite, a refractory gold telluride mineral (AuTe₂), requires specialized pre-treatment to liberate gold for extraction, as the metal is encapsulated within the telluride lattice, rendering direct cyanidation inefficient. The primary industrial method involves roasting the ore at 500–600°C in the presence of oxygen, which decomposes the telluride structure and volatilizes tellurium primarily as tellurium dioxide (TeO₂) and sulfur dioxide (SO₂) if sulfides are present, producing a porous calcine amenable to subsequent leaching. This is followed by cyanidation, where the oxidized gold is dissolved using a sodium cyanide solution under alkaline conditions (pH 10–11), achieving gold recovery rates of up to 95% after optimization. Modern alternatives to roasting include pressure oxidation at elevated temperatures (e.g., 190°C and 1060 kPa oxygen pressure), which enhances sulfide and telluride breakdown while minimizing atmospheric emissions, and bioleaching using acidophilic bacteria like Thiobacillus ferrooxidans to selectively oxidize the matrix, reducing reliance on high-energy processes and toxic gas releases. The nature of calaverite poses significant challenges, as the telluride bonding hinders gold dissolution in , leading to low recovery (often below 50%) without pre-treatment and increased consumption due to interference from residual species. High content exacerbates environmental concerns, including the generation of SO₂ emissions during roasting, which contribute to , and potential from oxidation of associated sulfide minerals in telluride deposits, releasing acidic, metal-laden effluents that harm aquatic ecosystems. Historically, early 20th-century cyanidation efforts were inefficient, with losses attributed to tellurium's interference and incomplete oxidation, until refinements like the bromocyanide (Diehl) process around improved yields by enhancing telluride decomposition. As of 2016, practices at operations in Australia's region—where calaverite has been a key —included to concentrate tellurides (recovering ~88% of at pH 8 with collectors), followed by or bio-oxidation and carbon-in-pulp (CIP) cyanidation, yielding overall recoveries of 95–98%. These methods emphasize emission controls and water recycling to mitigate environmental impacts, though challenges persist in managing byproducts, which require specialized handling to prevent soil and water contamination. As of 2024, the Super Pit produced approximately 14 tonnes of annually, with a mill rebuild underway to increase output starting in 2026. At Cripple Creek, heap leach operations produced about 10 tonnes in 2019.

References

  1. https://www.pnas.org/doi/10.1073/pnas.1802836115
  2. https://www.handbookofmineralogy.org/pdfs/calaverite.pdf
  3. https://www.mindat.org/min-852.html
  4. https://www.calaverashistory.org/calaveras-county
  5. https://www.mindat.org/min-3849.html
  6. https://www.mindat.org/min-2274.html
  7. https://archive.org/details/minesandmineral02clar
  8. https://rruff.info/uploads/The_American_Journal_of_Science_and_Arts95_1868_314.pdf
  9. https://www.rockngem.com/exploring-legendary-hard-to-find-gold-and-silver/
  10. https://rruff.geo.arizona.edu/doclib/hom/calaverite.pdf
  11. https://link.springer.com/chapter/10.1007/978-3-031-12654-3_25
  12. https://www.sciencedirect.com/science/article/abs/pii/S016745280515039X
  13. http://webmineral.com/data/Calaverite.shtml
  14. https://www.researchgate.net/publication/247860972_Phase_relations_in_the_Au-Ag-Te_systems_and_their_mineralogical_significance
  15. https://arxiv.org/html/2404.16239v1
  16. https://trekgeo.net/m/b/61insolubleHCle.htm
  17. http://www.primaryinfo.com/industry/calaverite.htm
  18. https://www.911metallurgist.com/wp-content/uploads/2016/09/Recovering-Gold-from-Telluride-Ore.pdf
  19. https://www.sciencedirect.com/science/article/abs/pii/S0892687509002933
  20. https://www.gktoday.in/calaverite/
  21. https://pubs.geoscienceworld.org/msa/ammin/article/94/5-6/728/44918/The-role-of-silver-on-the-stabilization-of-the
  22. https://pubmed.ncbi.nlm.nih.gov/11574718/
  23. https://core.ac.uk/download/pdf/148033156.pdf
  24. https://www.researchgate.net/publication/233732546_High-pressure_single-crystal_structure_study_on_calaverite_AuTe_2
  25. https://pubs.usgs.gov/sir/2010/5070/q/sir20105070q.pdf
  26. https://pubs.geoscienceworld.org/msa/ammin/article/110/5/709/649529/Formation-of-bonanza-Au-Ag-telluride-ores-in
  27. https://home.wgnhs.wisc.edu/minerals/calaverite/
  28. https://www.mindat.org/locentry-269536.html
  29. https://pubs.geoscienceworld.org/segweb/economicgeology/article/116/4/963/594205/Age-and-Origin-of-the-Dongping-Au-Te-Deposit-in
  30. https://www.sciencedirect.com/science/article/abs/pii/B9780444636584000517
  31. https://pubs.usgs.gov/of/1995/ofr-95-26/ofr95-26.pdf
  32. https://www.superpit.com.au/about/
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