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STRUCTURAL AND PHYSICAL PROPERTIES OF FISCHESSERITE, A RARE GOLD–SILVER SELENIDE FROM THE DE LAMAR MINE, OWYHEE COUNTY, IDAHO, USA

Luca Bindi and Curzio Cipriani

Museo di Storia Naturale - sez. di Mineralogia e Litologia, Università degli Studi di Firenze, via La Pira 4, I-50121 Firenze, Italy

E-mail address: lbindi{at}steno.geo.unifi.it

Fischesserite, Ag₃AuSe₂, from the De Lamar mine, Owyhee County, Idaho, USA, was chemically and structurally analyzed. Electron-microprobe analyses give the chemical formula Ag_2.98Cu_0.01Au_0.97Se_2.03. Fischesserite is optically isotropic, silver white in color with a metallic luster. In reflected light, the mineral is pink. Where observed near naumannite, it appears darker with slightly bluish tints. Reflectivity values are 32.5 (471.1 nm), 30.0 (548.3 nm), 31.1 (586.6 nm), and 33.7% (652.3 nm). Fischesserite is cubic, space group I4₁32, with a 9.965(1) Å, V 989.5(2) ų and Z = 8. The crystal structure has been refined to R= 2.69%. Fischesserite exhibits a garnet-like structure consisting of edge-sharing Ag[Se₄Ag₂Au₂] and Au[Ag₆Se₂] polyhedra. Fischesserite is confirmed to be isotypic with petzite, Ag₃AuTe₂. The different crystal-chemical environments of noble metals in fischesserite and petzite are compared with those found in other silver–gold chalcogenides.

Keywords: fischesserite, Ag–Au chalcogenides, chemical composition, physical properties, crystal- structure refinement, De Lamar mine, Owyhee County, Idaho.

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				L. Bindi, M. Evain, and S. Menchetti SELENOPOLYBASITE, [(Ag,Cu)6(Sb,As)2(S,Se)7][Ag9Cu(S,Se)2Se2], A NEW MEMBER OF THE PEARCEITE-POLYBASITE GROUP FROM THE DE LAMAR MINE, OWYHEE COUNTY, IDAHO, USA Can Mineral, December 1, 2007; 45(6): 1525 - 1528. [Abstract] [Full Text] [PDF]