DRIEKOPITE, IDEALLY PtBi, A NEW MINERAL SPECIES FROM THE DRIEKOP PLATINUM PIPE, REPUBLIC OF SOUTH AFRICA

The new mineral driekopite, ideally PtBi, was found in a concentrate from the Driekop mine, one of three zoned Pt pipes (mined from 1925 to 1930) that crosscut the layered mafic and ultramafic sequences of the eastern Bushveld Complex, Republic of South Africa. The holotype grain of driekopite (similar to 22x13 mu m) occurs in a complex, rounded aggregate similar to 120 mu m in diameter, in association with isoferroplatinum (Pt3Fe), hollingworthite (RhAsS), geversite (PtSb2), insizwaite (PtBi2), andrieslombaardite (RhSbS), stibiopalladinite (Pd5Sb2), sobolevskite (PdBi), possible tatyanaite (Pt9Cu3Sn4), osmium-bearing tulameenite (Pt2FeCu), and Pt-Fe alloy (similar to Pt2Fe). Driekopite appears slightly orange under reflected light compared to Pt-Fe alloys. It shows moderate to strong bireflectance, varying from light yellow to brownish yellow, no pleochroism or internal reflections, and moderate to strong anisotropism. The empirical formula, calculated from the average of six wavelengthdispersive spectrometry analyses made on five grains, on the basis of two atoms, is (Pt0.68Pd0.31Fe0.01)(Sigma 1.00)(Bi0.53Sb0.43As0.02 Sn0.02S0.01)(Sigma 1.01). The mineral is hexagonal, space group P63/mmc (#194) with the refined unit-cell dimensions a = 4.1993(5), c = 5.6194(6) degrees A, V = 85.82 degrees A 3, Z = 2, and D-calc - 12.91 g/cm(3). Driekopite is isostructural with NiAs, with mixed compositions of Pt and Pd at the 2a site (0.55:0.45, respectively) and Bi and Sb at the 2c site (0.63:0.37, respectively). Its crystal structure was refined to wR = 6.3% using 13 unique Laue reflections obtained using synchrotron radiation. The six strongest lines for the powder X-ray diffraction pattern calculated from the crystal structure refined from synchrotron data is [d in degrees A (I) (hkl)]: 3.0531 (92) (101), 2.2234 (100) (102), 2.0997 (77) (2 (1) over bar0, 1.5266 (28) (202), 1.2347 (24) similar to 12THORN, 1.1676 (18) o2 14+/-. The holotype grain of driekopite is observed to be paragenetically later than isoferroplatinum and hollingworthite and is considered to be synformational with Bi-bearing geversite, insizwaite, andrieslombaardite, and sobolevskite. The entire aggregate containing these platinum-group minerals is overgrown by a rim of tulameenite and Pt-Fe alloy (similar to Pt2Fe), indicating they are paragenetically the last minerals to form. Experiments designed to synthesize PtBi over the range of 200 to 500 8C were all successful. Synthetic PtBi melts congruently at 765 degrees C, suggesting that driekopite likely crystallized at sub-magmatic temperatures.