A new natural Au-Sb oxide

A gold-antimony X-ray amorphous oxide, resulting from a hydrothermal alteration of aurostibite, AuSb2, occurs in the Krasna Horn gold deposit Czech Republic. Its reflectivity is close to that of goethite. The average composition obtained by electron microprobe analyses (wt. %) is: Au - 68.32; Cu - 0.10; Sb - 21.26; As - 0.30; Si - 0.21; O - 8.44; total 98.63. This yields the empirical formula (Au0.677Cu0.003Sb0.341As0.008)(1.029)O. The [(Au + Cu)/(Sb + AS)](at) ratio varies from 1.86 to 1.95. Among possible formulae satisfying the equilibrium of charges, that implying unique valence states for Au and/or Sb was retained. It can be written Au21+Sb3+O2(OH) with the theoretical composition (wt. %): Au - 69.76; Sb - 21.54; O - 8.50; H - 0.20. This Au- and Sb-bearing oxide is associated with native gold electrum; aurostibite, arsenopyrite and pyrite in a quartz gangue. The powder pattern of AuSbO3 was indexed on an orthorhombic unit-cell with a = 5.00(2); b = 12.46(4); c = 5.43(2) Angstrom, Z = 4, rho(calc) = 7.20 g.cm(-3), assuming a replacement of Sb3+ by Au3+ in the valentinite-type crystal structure. (C) Academie des sciences/Elsevier, Paris.