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. Dmeas = 2.30(3), Dcalc = 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, H2Ocalc 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) Å3 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.

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