Feinglosite, a new mineral related to brackebuschite, from Tsumeb, Namibia

Feinglosite, the zinc analog of arsenbrackebuschite, was found lining a cavity in a sample of massive chalcocite from Tsumeb, Namibia. In this cavity it is assocd. with wulfenite, anglesite and goethite. The mean of seven electron-microprobe analyses (wt.%) is: PbO 61.4, ZnO 7.3, FeO 1.8, As2O5 22.1, SO3, 5.3, H2O (by difference) [2.1], total = [100.00]%, leading to the ideal formula: Pb2(Zn,Fe)[(As,S)O4]2·H2O. Feinglosite is monoclinic, space group P21 or P21/m, with unit-cell parameters a=8.973(6), b=5.955(3), c=7.766(6) Å, β=112.20(6)°, with Z = 2. The strongest five reflections of the X-ray powder diffraction pattern are [d in Å (I) (hkl)]: 4.85 (50) (110), 3.246 (100) (112), 2.988 (60) (301), 2.769 (60) (300/211), 2.107 (50) (321). The mineral is pale olive-green, transparent, sectile, and has a white streak and adamantine luster. It overgrows clusters of goethite crystals and forms globular microcryst. aggregates up to 0.5-0.75 mm in size. The hardness on Mohs scale is 4-5; the mean micro-indentation hardness is 263 at VHN100. Its calcd. d. is 6.52 g cm-3. The mineral is pale brownish gray in reflected light (when compared with goethite). Visible spectrum reflectance data are presented. Feinglosite is named for Mark N. Feinglos who first recognized the mineral on a specimen of chalcocite in his collection.