Calaverite

Calaverite
General
Category	Telluride mineral
Chemical formula	AuTe2
Identification
Molar mass	452.17 gm
Color	Brass yellow to silver white
Crystal habit	Bladed and slender striated prisms, also massive granular
Crystal system	Monoclinic - Prismatic
Twinning	Common on [110]
Cleavage	None
Fracture	Uneven to subconchoidal
Tenacity	Brittle
Mohs scale hardness	2.5-3
Luster	Metallic
Streak	Green to yellow grey
Diaphaneity	opaque
Specific gravity	9.1-9.3
Optical properties	Anisotropic
Pleochroism	Weak
Ultraviolet fluorescence	None
References	[1] [2] [3]

Calverite crystalline structure

Calaverite, or gold telluride, is an uncommon telluride of gold; it is a metallic mineral. It was first discovered in Calaveras County, California in 1861. Its chemical formula is AuTe₂. Its color may range from a silvery white to a brassy yellow. It is closely related to the gold - silver telluride sylvanite. Another mineral containing AuTe₂ is krennerite. Calaverite represents a minor ore of gold and tellurium.

Physical properties

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.

Chemical properties

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

Calaverite’s structure has been both an object of fascination and frustration in the scientific community for many years. (Goldschmidt et al.(1931)) succeeded in indexing 105 crystals resulting in 92 forms but needed five different lattices to index all of the faces (Balzuwelt et al.(1991)). This lead to consideration that Calaverite violated Hauy’s Law of Rational Indices (Goldschmidt et al.( 1931)). The introduction of diffraction did not completely solve this problem. It was first Tunnel and Ksanda in 1936 and then Tunnel and Pauling in 1952 who were able to solve the C2/m general structure of (Calaverite Bindi et al.(2009)). however, additional diffraction spots which they could not interpret were present in the survey (Bindi et al.(2009)). Later, based on a transmission electron microscopy study (VanLoosdrecht et al.(1993)) discovered that the satellite reflections in Calaverite were due to Au in commensurately displacive modulation superimposed on the average C2/m structure (Bindi et al.(2009)). In 1988, Schutte and DeBoer solved the structure by using the 3H super space group C2/m (α O γ)Os (Schutte et al.(1988)). Schutte and DeBoer also showed that these modulations consist mainly of the displacements of Te atoms and the observed modulations are 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 (Schutte et al.(1988)). In 2009 Luca Bindi, Alla Araecheeou and Gervais Chapvis concluded that the different coordination numbers associated with Calaverite were indeed associated with a significant differentiation in the valence sum of Au (Bindi et al.(2009)). Furthermore, they concluded the random distribution of Ag suppresses the fluctuation of Au+ and Au3, whereas the ordered distribution reinforces it.

Occurrence

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

In the Kalgoorlie gold rush of the 1890s, large amounts of calaverite were 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 minor gold rush to excavate the streets.

See also

• List of minerals

References

• D. M. Chizhikov, V. P. Shchastlivyi, 1966, Tellurium and Tellurides, Nauka Publishing, Moscow
• Klein, Cornelis and Hurlbut, Cornelius S., 1985, Manual of Mineralogy 20th ed., p. 289, Wiley, ISBN 0-471-80580-7
• Fortey, Richard, 2004, The Earth, Harper Collins
• Schutte W.J and de Boer, J.L (1988) Valance fluctuations in the incommensuratly modulated structure of Calaverite (AuTe2). Acta Crystallographia. B 44. 486-494
• Goldschmidt. V. Palache, C. and Peacock, M. (1931) Uber Calaverit Neves Jahrbuch fur Mineralogic.63. 1-58
• Bindi.L,Arakcheeva.A,Chapuis.G,(2009) The role of silver in the stabilization of the incommensurately modulated structure in calaverite (AuTe2). American Mineralogist. 94. 728-736
• Balzuwelt.K,Meeks.H,Bennems.P (1991) Morphology and crystal growth of pure Calaverite, Applied Physics. 24. 203-208
• VanLoosdrecht.P, Gerrits.A,Balzuweit.K,Koing.W,Wittlin.A,VanBantum.P(1993) Optical Properties f incommensurately modulated Calaverite, Phys. Condas. 5. 3977-3986

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