Oscarkempffite, Ag10Pb4(Sb17Bi9)∑26S48, a new Sb-Bi member of the lillianite homologous series. (2nd January 2018) - Topa, Dan
Abstract: Oscarkempffite, ideally Ag10 Pb4 (Sb17 Bi9 )∑=26 S48, is a new mineral species found in old material (1929–30) from the Colorada vein, Animas mine, Chocaya Province, Department of Potosi, Bolivia. It is associated with aramayoite, stannite, miargyrite, pyrargyrite and tetrahedrite. Oscarkempffite forms anhedral grains and grain aggregates up to 10 mm across. The mineral is opaque, greyish black with a metallic lustre; it is brittle without any discernible cleavage. In reflected light oscarkempffite is greyish white, pleochroism is distinct, white to dark grey. Internal reflections are absent. In crossed polars, anisotropism is distinct with rotation tints in shades of grey. The reflectance data (%, air) are: 39.9, 42.6 at 470 nm, 38.6, 41.7 at 546 nm, 38.1, 41.2 at 589 nm and 37.3, 40.6 at 650 nm. Mohs hardness is 3–3½, microhardness VHN50 exhibits a range 189–208, with a mean value 200 kg mm –2 . The average results of four electron-microprobe analyses in a grain are: Cu 0.24(7), Ag 14.50(8), Pb 11.16(14), Sb 28.72(16), Bi 24.56(17), S 20.87(5), total 100.05(6) wt.%, corresponding to Cu0.24 Ag9.92 Pb4.00 Sb17.36 Bi8.64 S47.84 (on the basis of Me + S = 88 apfu). The simplified formula, Ag10 Pb4 Sb17 Bi9 S 48, is in accordance with the results of a crystal-structure determination. The density, 5.8 g cm– 3, was calculated using the ideal formula. Oscarkempffite has an orthorhombic cell with a = 13.199(2), b = 19.332(3), c = 8.249(1) Å, V = 2116.3(5) Å 3, space group Pnca and Z = 1. The strongest eight lines in the (calculated) powder-diffraction pattern are [ d in Å(I) hkl ]: 3.66(35)(122), 3.37(70)(132), 3.34(100)(250), 2.982(55)(312), 2.881(86)(322), 2.733(29)(332), 2.073(27)(004) and 2.062(31)(182). Comparison with gustavite, andorite and roshchinite confirms its independence as a mineral species.