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

摘要

Fischesserite, Ag_3AuSe_2, from the De Lamar mine, Owyhee County, Idaho, USA, was chemically and structurally analyzed. Electron-microprobe analyses give the chemical formula Ag_(2.98)Cu_(0.01)Au_(0.97)Se_(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 percent (652.3 nm). Fischesserite is cubic, space group 14_132, with a 9.965(1) A, V 989.5(2) A~3 and Z = 8. The crystal structure has been refined to R = 2.69 percent. Fischesserite exhibits a garnet-like structure consisting of edge-sharing Ag[Se_4Ag_2Au_2] and Au[Ag_6Se_2] polyhedra. Fischesserite is confirmed to be isotypic with petzite, Ag_3AuTe_2. The different crystal-chemical environments of noble metals in fischesserite and petzite are compared with those found in other silver-gold chalcogenides.