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

The new mineral species whitecapsite, H (sub 16) Fe (super 2+) (sub 5) Fe (super 3+) (sub 14) Sb (super 3+) (sub 6) (AsO (sub 4) ) (sub 18) O (sub 16) .120H (sub 2) O, 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 X 0.3 X 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 2-2 1/2. The mineral is brittle, cleavage was not observed, fracture is uneven. D (sub meas) = 2.30(3), D (sub calc) = 2.297 g cm (super -3) . Whitecapsite is optically uniaxial (+),omega = 1.590(2) and epsilon = 1.603(3). Chemical composition (wt%, electron probe) is: MnO 0.09, FeO 5.32, Fe (sub 2) O (sub 3) 16.82, Sb (sub 2) O (sub 3) 13.30, As (sub 2) O (sub 5) 30.78, H (sub 2) O (sub calc) 34.53, total 100.84. The empirical formula calculated on the basis of 208 O apfu with 256 H apfu is: H (sub 16) Mn (super 2+) (sub 0.08) Fe (super 2+) (sub 4.95) Fe (super 3+) 14.07Sb (super 3+) (sub 6.10) As (super 5+) (sub 17. 89) O (sub 88) .120H (sub 2) O. Both the IR and Raman spectra show high content of H (sub 2) O, the presence of hydrated protons (the complex cations H (super +) .nH (sub 2) O) and an absence of OH groups. Sb L (sub 3) and Fe K XANES spectra show the presence of only Sb (super 3+) and of both Fe (super 3+) and Fe (super 2+) (Fe (super 3+) > Fe (super 2+) ). Whitecapsite is hexagonal, P6 (sub 3) /m, a = 16.0916(8), c = 21.7127(9) Aa, V = 4869.0(4) Aa (super 3) and Z = 1. The strongest reflections of the X-ray powder-diffraction pattern [d,Aa (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 [([],Fe (super 2+) ) (sub 6) Fe (super 3+) (sub 7) Sb (sub 3) O (sub 8) (AsO (sub 4) ) (sub 9) (H (sub 2) O ) (sub 30) ] with the Fe (super 3+) O (sub 6) trigonal prism as a central polyhedron. These clusters are isolated from each other and linked only via hydrogen bonds to the H (sub 2) O molecules located in the interstitial space.