Mazzettiite, A93HgPbSbTe5, a new mineral species from Findley Gulch, Saguache County, Colorado, USA

Mazzettiite, ideally Ag3HgPbSbTe5, is a new mineral species from Findley Gulch, Saguache County, Colorado. It occurs as anhedral to subhedral grains up to 200 mum across, closely associated with altaite together with galena in a gangue of quartz. Mazzettiite is opaque with a metallic luster and a black streak. It is brittle; the Vickers hardness (VHN25) is 101 kg/mm(2) (range: 96-106) (Mobs hardness of similar to3-3 1/2). The density could not be measured because of the small grain-size [density (calc.) = 9.04 g/cm(3)]. In reflected light, mazzettiite is light grey, weakly to moderately bireflectant, and weakly pleochroic from slightly greenish grey to a slightly darker bluish grey. Under crossed polars, it is moderately anisotropic, without characteristic rotation-tints. Internal reflections are absent. Reflectance percentages in the range 400-700 nm are tabulated, with values for the four COM wavelengths interpolated. The valued for R-min and R-max are 45.6, 46.1% (470 nm), 46.1, 46.6% (546 nm), 46.3, 47.0% (589 nm), and 46.4, 47.1% (650 nm), respectively. The unit-cell parameters of mazzettiite were derived by comparison with petrovicite, Cu3HgPbBiSe5, to which it shows strong similarities, both from a chemical and a structural point of view. Mazzettiite is orthorhombic, space group Pna2(1) or Pnam, with a 16.495(6), b 14.762(7), c 4.506(2) Angstrom, V = 1097.2(8) Angstrom(3), Z = 4. The strongest seven powder-diffraction lines [a in Angstrom (I/I-0)(hkl)] are: 3.65(60)(330), 3.26(50)(131), 3.17(60)(430), 3.01(100)(520), 2.754(60)(440), 2.137(50)(122), and 1.806(55)(561). A mean of 20 electron-microprobe analyses gave Ag 21.6(3), Hg 13.6(2), Pb 14.1(3), Sb 8.1(1), Te 42.7(3), total 100.1 wt.%, corresponding, on the basis of a total of 11 atoms, to Ag2.99Hg1.01Pb1.02 Sb0.99Te4.99. Requirements of the ideal formula are: Ag 21.70, Hg 13.45, Pb 13.90, Sb 8.16, Te 42.79. The new mineral and mineral name have been approved by the IMA Commission on New Minerals and Mineral Names (2004-03). It is named after Giuseppe Mazzetti, curator-in-chief of the Mineralogy Section of the Museo di Storia Naturale, Universita di Firenze.