Lenaite from the Gies gold-silver telluride deposit, Judith Mountains, Montana, USA: Occurrence, composition, and crystal structure

Lenaite. AgFeS2, a very rare silver-iron sulfide, was found in a sample in the mineralogical collection of the Natural History Museum of Florence. The sample is from the Gies gold-silver telluride deposit, Judith Mountains, Montana. Lenaite in this sample occurs as anhedral grains up to 0.2 mm across. It is steel-grey in color and has a black streak. No cleavage is observed, and the Vickers hardness (VHN10) is 285 kg/mm(2). Lenaite is greyish white. non-pleochroic, moderately anisotropic in yellowish tints, and it shows no bireflectance. Reflectance values in air for R-min and R-max are 28.2, 30.6 (471.1 nm), 31.8, 35.8 (548.3 nm), 32.8, 36.5 (586.6 nm). and 30.9, 35.0% (652.3 nm), respectively. Lenaite is tetragonal and belongs to the space group I (4) over bar 2d, with the following unit-cell parameters: a 5.4371(2), c 10.8479(9) angstrom, V 320.69(3) angstrom(3) and Z = 4. Electron-microprobe analyses gave the chemical formula Ag0.96Fe1.01S2.02. The crystal structure has been solved and refined to R = 3.63%. It consists of corner-sharing M-[4] and X-[4] tetrahedra forming cubic or pseudocubic frameworks, as in sphalerite, but with cation order. The crystal-chemical relationships with other members of the chalcopyrite group are outlined.