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Calaverite, or gold telluride, is an uncommon telluride of gold, a mineral with the chemical formula AuTe2, with approximately 3% of the gold Silver telluride. 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.
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.
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. 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.
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.
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