Calaverite
Encyclopedia
Calaverite, or gold telluride, is an uncommon telluride
of gold
, a metal
lic 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.
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.
's Law of Rational Indices.
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 commensurately displacive modulation superimposed on the average C2/m structure. 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.
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.
, Colorado
, Calaveras County, California, USA (from where it gets its name), Nagyag, Romania
, Kirkland Lake Gold District, Ontario
, Rouyn
District, Quebec
, and Kalgoorlie, Australia.
who differentiated it from the known gold telluride mineral sylvanite
, and formally reported it as a new gold mineral in 1868. 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 isomorphic 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.
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.
Telluride (chemistry)
The telluride ion is Te2−. It is the final stable member of the series of dianions O2−, S2−, and Se2− ....
of gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
, a metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
lic mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
with the chemical formula AuTe2, with approximately 3% of the gold replaced by silver. It was first discovered in Calaveras County, California
Calaveras County, California
Calaveras County is a county located in the Gold Country of the U.S. state of California. Calaveras is the Spanish word for skulls; the county was reportedly named for the remains of Native Americans discovered by the Spanish explorer Captain Gabriel Moraga. As of the 2010 census, the county had a...
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
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
telluride mineral sylvanite
Sylvanite
Sylvanite or silver gold telluride, Te2, is the most common telluride of gold.-Properties:The gold:silver ratio varies from 3:1 to 1:1. It is a metallic mineral with a color that ranges from a steely gray to almost white. It is closely related to calaverite, which is more purely gold telluride with...
, which, however, contains significantly more silver. Another AuTe2 mineral (but with a quite different crystal structure) is krennerite
Krennerite
Krennerite is an orthorhombic gold telluride mineral which can contain a relatively small amount of silver in the structure. The formula is AuTe2 varying to Te2. Both of the chemically similar gold-silver tellurides, calaverite and sylvanite are in the monoclinic crystal system, whereas 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
Calaverite occurs as monoclinic crystals, which do not possess cleavage planes. It has a specific gravitySpecific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
of 9.35 and a hardness of 2.5.
Calaverite can be dissolved in concentrated sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
. In hot sulfuric acid the mineral dissolves, leaving a spongy mass of gold in a red solution of tellurium.
Structure
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 but needed five different lattices to index all of the faces. This led to consideration that calaverite violated HaüyRené Just Haüy
René Just Haüy – 3 June 1822 in Paris) was a French mineralogist, commonly styled the Abbé Haüy after he was made an honorary canon of Notre Dame. He is often referred to as the "Father of Modern Crystallography." -Biography:...
's Law of Rational Indices.
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 commensurately displacive modulation superimposed on the average C2/m structure. 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.
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.
Occurrence
Calaverite occurrences include Cripple CreekCripple Creek, Colorado
The City of Cripple Creek is a Statutory City that is the county seat of Teller County, Colorado, United States. Cripple Creek is a former gold mining camp located southwest of Colorado Springs near the base of Pikes Peak. The Cripple Creek Historic District, which received National Historic...
, Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
, Calaveras County, California, USA (from where it gets its name), Nagyag, Romania
Romania
Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...
, Kirkland Lake Gold District, Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
, Rouyn
Rouyn-Noranda, Quebec
Rouyn-Noranda is a city on Osisko Lake in northwestern Quebec, Canada.The city of Rouyn-Noranda is coextensive with a territory equivalent to a regional county municipality and census division of Quebec of the same name...
District, Quebec
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....
, and Kalgoorlie, Australia.
History
Calaverite was first recognized and obtained in 1861 from the Stanislaus Mine, Carson Hill, Angels Camp, in Calaveras Co., California. It was named for the County of origin by chemist and mineralogist Frederick Augustus GenthFrederick Augustus Genth
Frederick Augustus Ludwig Karl Wilhelm Genth was a United States chemist.-Biography:...
who differentiated it from the known gold telluride mineral sylvanite
Sylvanite
Sylvanite or silver gold telluride, Te2, is the most common telluride of gold.-Properties:The gold:silver ratio varies from 3:1 to 1:1. It is a metallic mineral with a color that ranges from a steely gray to almost white. It is closely related to calaverite, which is more purely gold telluride with...
, and formally reported it as a new gold mineral in 1868. 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 isomorphic 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
Krennerite
Krennerite is an orthorhombic gold telluride mineral which can contain a relatively small amount of silver in the structure. The formula is AuTe2 varying to Te2. Both of the chemically similar gold-silver tellurides, calaverite and sylvanite are in the monoclinic crystal system, whereas 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.
In the initial phase of the Kalgoorlie gold rush
Gold rush
A gold rush is a period of feverish migration of workers to an area that has had a dramatic discovery of gold. Major gold rushes took place in the 19th century in Australia, Brazil, Canada, South Africa, and the United States, while smaller gold rushes took place elsewhere.In the 19th and early...
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.