Bowlesite, PtSnS, a new platinum group mineral (PGM) from the Merensky Reef of the Bushveld Complex, South Africa

Abstract

AbstractBowlesite is a new mineral discovered in the Merensky Reef of the Rustenburg Platinum Mine, Bushveld complex, South Africa. Bowlesite forms tiny grains (maximum dimension 20 μm). It is associated with sulfides including chalcopyrite, pyrrhotite and pentlandite, in contact with silicates including plagioclase, pyroxene- and minor serpentine-subgroup and amphibole-supergroup minerals. Bowlesite is brittle and has a metallic lustre. In plane-polarised light, bowlesite has a light bluish grey colour. It shows weak bireflectance, no pleochroism and has weak anisotropism. Internal reflections were not observed. Reflectance values of bowlesite in air (R1, R2 in %) are: 50.3–51.4 at 470 nm, 48.5–48.9 at 546 nm, 47.9–48.6 at 589 nm and 47.8–48.7 at 650 nm. Ten spot analyses of bowlesite give the average composition: Pt 56.85, Pd 0.02, Sn 34.03 and S 9.15, total 100.05 wt.%, corresponding to the empirical formula (Pt1.001Pd0.001)Σ1.002Sn0.997S1.001, based on 3 atoms per formula unit. The simplified formula is PtSnS. Due to the small size of bowlesite, the crystal structure was solved and refined from the powder X-ray-diffraction data of synthetic PtSnS. The calculated density is 10.06 g⋅cm–3. The mineral is orthorhombic, space group: Pca21 (#29) with a = 6.11511(10), b = 6.12383(10), c = 6.09667(11) Å, V = 228.31(1) Å3 and Z = 4. Bowlesite is isotypic with cobaltite, CoAsS. The origin of bowlesite is probably related to low-T exsolution of Pt–Sn phases from high-T sulfides crystallised from the sulfide melt. The mineral honours Dr. John Bowles (Manchester University, UK) for his contributions to ore mineralogy and mineral deposits related to mafic–ultramafic rocks.