Abstract. Lopatkaite, ideally Pb10As2Sb6S22 (Z=4), is a new arsenic-bearing sulfosalt found in the Madoc deposit, Taylor Pit, Ontario, Canada. Associated minerals in the holotype specimen are boulangerite, veenite, and sterryite, all embedded in a calcite matrix. Lopatkaite is greyish black and opaque, with metallic lustre and dark-grey streak. It is brittle without any discernible cleavage and parting and has a Mohs hardness of 3–3.5. In reflected light lopatkaite is greyish white, with distinct bireflectance and pleochroism from white to grey, especially in oil. Under crossed polarisers, anisotropism is distinct, with rotation tints in shades of grey. Reflectance measurements in air yield the following Rmin/Rmax values based on the standard wavelengths (Commission on Ore Mineralogy, COM): 37.0 % / 39.3 % (470 nm), 34.1 % / 36.9 % (546 nm), 33.1 % / 36.2 % (589 nm), and 31.3 % / 34.1 % at (650 nm). The average result of four electron probe microanalyses for the structurally investigated grain is as follows (in wt %): Pb 57.81(4), As 3.53(8), Sb 20.03(6), S 19.08(6), and total 100.46(22), corresponding to Pb10.28(3)As1.74(4)Sb6.06(3)S21.92(3) (based on 18Me + 22S = 40 atoms per asymmetric unit). The density calculated using the empirical formula is 6.168 Mg m−3. Single-crystal X-ray diffraction data show lopatkaite to be monoclinic, space group P21/c (no. 14), with a=8.0806(6), b=23.3597(18), c=21.4880(16) Å, β=100.7090(10)°, V=3985.4(5) Å3, and Z=4. The seven strongest lines in the (calculated) powder diffraction pattern are as follows (d in Å (intensity) (hkl)): 3.728(39) 211, 3.712(100) 035, 3.653(35) 062, 2.804(41) −261, 2.780(43) 260, 2.779(38) −262, and 2.020(47) −402. The ideal formula is in accordance with the results of the crystal structure analysis, Pb10.336As1.567Sb6.088S22 , and may be derived from the ideal boulangerite formula, Pb10Sb8S22 (Z=4), by means of substitution of two Sb atoms with two As atoms. Lopatkaite is an isotype of boulangerite, differing by dominant As occupancy at two crystallographically independent mixed (Sb, As) sites. This dominant-site substitution defines lopatkaite as the arsenic-dominant isotype of boulangerite and justifies its recognition as a distinct mineral species. Lopatkaite is also a new member of the rod-based family of sulfosalts.
Topa et al. (Thu,) studied this question.