Laforetite, AgInS2, a new mineral of the chalcopyrite group from the Montgros mine, Haute-Loire, France

Laforetite, AgInS2, is a new mineral from the Montgros Pb-Zn mine, Haute-Lo ire, France. It occurs as very small, less than 30 mu m, anhedral inclusion s in galena, which is associated with sphalerite, barite, carbonates and qu artz. The new species is brownish grey with abundant red internal reflectio ns in plane polarized light; under crossed nicols in oil immersion, it pres ents a strong anisotropy with red brick orange to bluish-grey purplish tint s and polysynthetic twins. Electron-microprobe analysis yields (mean of 8 a nalyses): 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 AgInS2. Because of the material paucity , X-ray powder patterns were obtained from a very small inclusion (20 x 30 mu m) with a microdiffractometer using the reflection geometry. Parameters refined from a tetragonal cell (by comparison with the synthetic low-temper ature form of AgInS2) are: a = 5.880 (2), c = 11.21 (1) Angstrom, V = 387.5 7 (1) Angstrom(3); space group 1 4 (2) over bar d (no. 122) and Z = 4. Lafo retite is the silver analogue of roquesite and belongs to the chalcopyrite group. The name honours Claude Laforet (BRGM, Orleans), a metallographer wh o first identified this mineral in the Montgros mine.