Empressite is a

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

form of

silver telluride Silver telluride (Ag2Te) is a chemical compound, a telluride (chemistry), telluride of silver, also known as disilver telluride or silver(I) telluride. It forms a monoclinic crystal. In a wider sense, ''silver telluride'' can be used to denote AgTe ...

, AgTe. It is a rare, grey, orthorhombic mineral with which can form compact masses, rarely as bipyramidal crystals. Recent crystallographic analysis has confirmed that empressite is a distinct mineral with orthorhombic crystal structure, different from the hexagonal Ag_5−xTe₃ with which empressite has been commonly confused in mineralogy literature. At the same time, empressite does not appear on the equilibrium Ag-Te phase diagram,Karakaya, I., Thompson, W.T.: J. Phase Equilibria 12, 56 (1991). and therefore it is only metastable at ambient conditions. Given infinite time, it would phase separate into pure Ag₅Te₃ and pure Te. The name empressite comes from the location of its discovery – the Empress Josephine mine,

Saguache County, Colorado Saguache County (suh-WATCH ) is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 6,368. The county seat is Saguache. History Saguache County was formed December 29, 1866 in the Territory of Colorado ...

, US. It was first described in 1914.

References

Silver minerals Telluride minerals Orthorhombic minerals Minerals in space group 62 Minerals described in 1914 {{Mineral-stub