Zaccariniite, RhNiAs, a new platinum-group mineral from Loma Peguera, Dominican Republic

Zaccariniite, RhNiAs, is a new platinum-group mineral discovered in the Loma Peguera ophiolitic chromitite, Dominican Republic. The mineral forms anhedral grains (1-20 mu m in size) and occurs intergrown with garutiite, in association with hexaferrum, Ru-Os-Ir-Fe alloys, Ru-Os-Ir-Fe oxygenated compounds, and chromite. Zaccariniite is opaque and has a metallic luster and a grey streak. The mineral is brittle, with no cleavage. Values of VHN (sub 5) (10 indentations on different grains of synthetic RhNiAs) fall between 166 and 286 kg/mm (super 2) , with a mean value of 218 kg/mm (super 2) , corresponding to a Mohs hardness of approximately 3.5-4. Under plane-polarized reflected light, the mineral is white with brownish to pinkish tints, has moderate to strong bireflectance, a strong white to pinkish brownish white pleochroism, and strong anisotropy, with rotation tints from orange to blue-green; it exhibits no internal reflections. Reflectance values of zaccariniite in air (R1, R2 in %) are: 49.4, 49.4 at 470 nm; 52.4, 53.2 at 546 nm; 54.2, 53.2 at 589 nm; and 56.6, 53.3 at 650 nm. Three electron microprobe analyses of natural zaccariniite gave an average composition of: Rh 41.77, Os 0.51, Ir 0.64, Ru 0.46, Pd 0.34, Ni 23.75, Fe 0.53, As 27.84, S 0.10, for a total of 96.09 wt.%, corresponding to the empirical formula (Rh (sub 1.01) Os (sub 0.01) Ir (sub 0.01) Ru (sub 0.01) Pd (sub 0.01) ) (sub Sigma 1.05) (Ni (sub 1.00) Fe (sub 0.02) ) (sub Sigma 1.02) (As (sub 0.92) S (sub 0.01) ) (sub Sigma 0.93) based on three atoms. The average result of twenty-eight analyses of synthetic zaccariniite is: Rh 44.57, Ni 24.50, and As 31.82 total 100.88 wt.%, corresponding to Rh (sub 1.02) Ni (sub 0.98) As (sub 1.00) . The mineral is tetragonal, belongs to space group P4/nmm with lattice parameters a = 3.5498(1), c = 6.1573(2) Aa, V = 77.59(1) Aa (super 3) , Z = 2, and crystallizes with the Cu (sub 2) Sb structure type. The strongest lines in the X-ray powder-diffraction pattern of synthetic zaccariniite [d in Aa(I)(hkl)] are: 2.5092(40)(110), 2.3252(100)(111,102), 1.9453(51)(112), 1.7758(80)(103,200), 1.2555(40)(213,220), 1.1044(22)(302,311), 1.0547(23)(312), 0.9730(42)(215). The new Rh-Ni arsenide is named after Federica Zaccarini in recognition of her important contributions to the mineralogy of platinum-group elements.