Whitecapsite, a new hydrous iron and trivalent antimony arsenate mineral from the White Caps mine, Nevada, USA

The new mineral species whitecapsite, H₁₆Fe²⁺₅Fe³⁺₁₄Sb³⁺₆(AsO₄)₁₈O₁₆·120H₂O, was found in the oxidation zone of the White Caps gold deposit (White Caps mine), Manhattan district, Nye Co., Nevada, USA, and named after the holotype locality. The mineral was found in cavities in a breccia consisting of fragments of jasperoid, silicified marble and mica schist, in association with picropharmacolite, guerinite, pitticite, gypsum, jarosite, goethite, sulfur, metastibnite and earlier quartz, calcite, kaolinite, realgar, stibnite, arsenopyrite, pyrite, sphalerite, orpiment, cinnabar and metacinnabar. Whitecapsite occurs as well-shaped hexagonal prismatic crystals (up to 0.3 × 0.3 × 1.5 mm) terminated with bipyramidal faces (forms: {110} and {111}) and as clusters of crystal up to 1 mm in maximal dimension. It is bright orange to golden-brownish orange, transparent, with a vitreous lustre. Mohs’ hardness is 2–2½. The mineral is brittle, cleavage was not observed, fracture is uneven. D_meas = 2.30(3), D_calc = 2.297 g cm⁻³. Whitecapsite is optically uniaxial (+),ω = 1.590(2) and ε = 1.603(3). Chemical composition (wt%, electron probe) is: MnO 0.09, FeO 5.32, Fe₂O₃ 16.82, Sb₂O₃ 13.30, As₂O₅ 30.78, H₂O_calc 34.53, total 100.84. The empirical formula calculated on the basis of 208 O apfu with 256 H apfu is: H₁₆Mn²⁺_0.08Fe²⁺_4.95Fe³⁺14.07Sb³⁺_6.10As⁵⁺_17.89O₈₈·120H₂O. Both the IR and Raman spectra show high content of H₂O, the presence of hydrated protons (the complex cations H⁺·nH₂O) and an absence of OH groups. Sb L₃ and Fe K XANES spectra show the presence of only Sb³⁺ and of both Fe³⁺ and Fe²⁺ (Fe³⁺ > Fe²⁺). Whitecapsite is hexagonal, P6₃/m, a = 16.0916(8), c = 21.7127(9) Å, V = 4869.0(4) ų and Z = 1. The strongest reflections of the X-ray powder-diffraction pattern [d,Å (I,%) (hkl)] are: 13.99 (49) (100); 11.73 (100) (101); 5.267 (6) (120); 3.644 (4) (205, 132, 006); 2.999 (8) (411, 126, 225); 2.757 (4) (501, 234); 2.648 (5) (331, 414, 316). The crystal structure was solved by direct methods and refined to R = 0.058. Its major building unit is a heteropolyhedral cluster [(□,Fe²⁺)₆Fe³⁺₇Sb₃O₈(AsO₄)₉(H₂O)₃₀] with the Fe³⁺O₆ trigonal prism as a central polyhedron. These clusters are isolated from each other and linked only via hydrogen bonds to the H₂O molecules located in the interstitial space.