Tredouxite, NiSb2O6: another new Ni-bearing mineral from the Bon Accord oxide body, South Africa

Tredouxite, NiSb2O6, is a new Ni-rich mineral discovered in the Bon Accord ultramafic body located in South Africa. The mineral forms grains variable in size from 10 to 500mm and occurs closely associated with trevorite in a Ni-rich hydroxysilicate (likely willemseite) matrix. It commonly forms subhedral to anhedral grains intergrown with bottinoite. In polished section using plane-polarized light, tredouxite is light gray in color, shows weak bireflectance and is not pleochroic. The mineral has weak redbrown internal reflections. Between crossed polars, tredouxite is very weakly anisotropic with light brownish gray to gray colors. Tredouxite has a submetallic luster and a gray streak. It is brittle with no cleavage. Values of VHN10 range between 218 and 240 kg·mm-2, with a mean value of 226 kg·mm-2 corresponding to a Mohs hardness of approximately 3–3½. Reflectance values (R1 and R2) are 15.6–15.8 (471.1 nm), 14.6–14.4 (548.3 nm), 14.5–14.3 (586.6 nm) and 14.6–14.4 (652.3 nm). The empirical formula of tredouxite (based on 3 cations pfu) is ðNi2+0:92Fe3+0:08ÞðSb5þ1:97As5+0:02Fe3+0:01ÞO6:03, ideally NiSb2O6. Single-crystal X-ray studies (R1 = 5.59%) indicate that tredouxite is tetragonal, space group P42/mnm, with cell parameters: a = 4.6342(5)A, c = 9.2154(8)A V = 197.91(3)z 3, and Z = 2. Tredouxite exhibits a structure that can be described as a cation-ordered derivative of the rutile structure. It consists of an hexagonal closest-packing of oxygen atoms with cations occupying one half of the octahedral interstices, producing straight chains of edge-sharing octahedra, linked to adjacent chains by corner sharing. The four strongest X-ray powder-diffraction lines [d in A (I/I0) (hkl)] are: 3.28 (100) (1 1 0), 2.561 (65) (1 0 3), 1.716 (60) (2 1 3), and 1.379 (20) (3 0 3). The mineral likely formed at relatively high temperature. Successive H2O-rich fluids impregnated it and formed secondary hydrated minerals like bottinoite. The name of the mineral honours Professor Marian Tredoux for her contribution to the knowledge of the origin of the enigmatic hypernickeliferous Bon Accord body.