FETTELITE, A NEW HG - SULFOSALT MINERAL FROM ODENWALD

Fettelite, Ag24HgAs5S20, is a new Hg-bearing species found in hydrothe rmal veins cutting basic intrusives of Variscan age at the Nieder-Beer bach mine near Darmstadt, Odenwald. This low-temperature mineral occur s within calcite-quartz-prehnite veins in close association with nativ e silver, proustite, pearceite, safflorite, xanthoconite, polybasite, cinnabar, sphalerite, chalcopyrite, pyrite, and other minerals. Fettel ite has a mica-like appearance. It occurs as individual hexagonal flak es or as clusters of subparallel growth or rosette-like groups, with f lake dimensions up to 0.2 mm across and mostly 5-10 mu m in thickness. It has a reddish color, is brittle with perfect [0001] cleavage. Frac ture is irregular and luster is submetallic to adamantine. Measured Vi cker's hardness is 1.58 which corresponds to Mohs hardness of 3 3/4. T he calculated density for the idealized formula is 6.29 g/cm(3). Under reflected light, fettelite is grey in color, with weak but still perc eptible anisotropism. The internal reflection is red. Reflectance valu es measured at 420-700 nm range between 9.2-14.4 (oil) and 22.5-31.3 ( air), respectively. The empirical formula from microprobe analysis (ba sed on 24 Ag atoms) is: Ag24HgCu0.04Fe0.03Tl0.02Pb0.01Sb0.07As5.01S20. 07, ideally Ag24HgAs5S20. Fettelite is trigonal, with a(h) = 15.00 Ang strom and c(h) = 15.46 Angstrom, and crystallizes in one of the six po ssible space groups P312, P321, P31m, P3m1, P (3) over bar 1m, P (3) o ver bar m1. The five strongest powder lines are 3.091 (10)(005), 1.878 (8)(440), 3.175 (6)(401), 2.998 (4)(402), 2.755 (3)(043). The name ho nours Mr. M. FETTEL, an experienced field worker in the area, who foun d the mineral.