Laforetite, AgInS ₂ , a new mineral of the chalcopyrite group from the Montogros Mine, Haute-Loire, France

Musee Geologique & Institut de Mineralogie et de Geochimie, Lausanne, Switzerland

Laforetite, AgInS ₂ , is a new mineral from the Montgros Pb-Zn mine, Haute-Loire, France. It occurs as very small, less than 30 mu m, anhedral inclusions in galena, which is associated with sphalerite, barite, carbonates and quartz. The new species is brownish grey with abundant red internal reflections in plane polarized light; under crossed nicols in oil immersion, it presents a strong anisotropy with red brick orange to bluish-grey purplish tints and polysynthetic twins. Electron-microprobe analysis yields (mean of 8 analyses): Ag 37.68, Cu 0.04, In 40.22, Fe 0.02, S 22.42, Se 0.01, total 100.39 wt.%. The simplified formula is AgInS ₂ . Because of the material paucity, X-ray powder patterns were obtained from a very small inclusion (20X30 mu m) with a microdiffractometer using the reflection geometry. Parameters refined from a tetragonal cell (by comparison with the synthetic low-temperature from of AgInS ₂ ) are: a = 5.880 (2), c = 11.21 (1) Aa, V = 387.57 (1) Aa ³ ; space group I 42d (no. 122) and Z = 4. Laforetite is the silver analogue of roquesite and belongs to the chalcopyrite group. The name honours Claude Laforet (BRGM, Orleans), a metallographer who first identified this mineral in the Montgros mine.

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