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Synchrotron radiation X-ray fluorescence (SR-XRF) was used to characterize As speciation within natural fluid inclusions from three deposits with different hydrogeochemical and geological settings. The studied samples represent different compositions of Au-bearing fluids: typical orogenic Au deposit (low-salinity, ~6 mol% C[O.sub.2] [+ or -] C[H.sub.4]; Brusson, Western Italian Alps); brines from a Proterozoic (Fe)- Cu-Au deposit (Starra, Queensland, Australia); and an As-rich magmatic fluid with a bulk composition similar to that typical of orogenic gold (Muiane pegmatite, Mozambique). Arsenic K-edge X-ray absorption spectra (XAS) were obtained from fluid inclusions at temperatures ranging from 25 to 200[degrees]C, and compared with spectra of aqueous As(III) and As(V) solutions and minerals. X-ray absorption near edge structure (XANES) data show that initially the fluid inclusions from all three regions contain some As in reduced form [As(III) at Brusson and Muiane; As-sulfide or possibly As(0) at Starra]. However, this reduced As is readily oxidized under the beam to As(V). Therefore, extended X-ray absorption fine structure (EXAFS) spectra for the As(III) aqueous complex could be collected only on the sample from the Muiane pegmatite containing large fluid inclusions with high As concentrations (>>1000 ppm). Analysis of these EXAFS data shows that As[(OH).sub.3(aq)] (coordination number of 3.0 [+ or -] 0.2 atoms, bond length of 1.76 [+ or -] 0.01 [Angstrom]) is the dominant arsenic aqueous species in the Muiane fluid inclusions at 100[degrees]C, in accordance with predictions based on studies conducted using autoclaves. The As(V) complex resulting from photooxidation in the Muiane inclusions was characterized at 200[degrees]C; the As-O bond distance (1.711 [+ or -] 0.025 [Angstrom]) corresponds to that found in the arsenate group in minerals, and to that measured for the [(HAs[O.sub.4]).sup.2-] complex at room temperature (1.700 [+ or -] 0.023 [Angstrom]). The extent of the XAS information that could be obtained for As in this study was limited by the rapid photooxidation that occurred in all inclusions, despite the relatively low photon flux density used (~4.4 x [10.sup.6] photons/s/[micro][m.sup.2]). Photosensitivity was not observed in autoclave experiments and is the result of a complex interaction between redox-sensitive complexes in solution and the products of water radiolysis generated by the beam. Even under such challenging experimental conditions, the information gathered provides some precious information about As chemistry in ore-forming fluids. Keywords: Fluid inclusion, X-ray absorption spectroscopy, arsenic speciation, hydrothermal gold deposits DOI: 10.2138/am.2010.3411

Xocolatlite, [Ca.sub.2][Mn.sup.4+.sub.2][Te.sup.6+.sub.2][O.sub.12] x [H.sub.2]O, is a rare new mineral from the Moctezuma deposit in Sonora, Mexico. It occurs as chocolate-brown crystalline crusts on a quartz matrix. Xocolatlite has a copper-brown streak, vitreous luster, and is transparent. Individual crystals show a micaceous habit. Refractive indices were found to be higher than 2.0. Density calculated from the empirical formula is 4.97 g/[cm.sup.3], and immersion in Clerici solution indicated a density higher than 4.1 g/[cm.sup.3]. The mineral is named after the word used by the Aztecs for chocolate, in reference to its brown color and provenance. The crystallographic characteristics of this monoclinic mineral are space group P2, P2/m, or Pro, with the following unit-cell parameters refined from synchrotron X-ray powder diffraction data: a = 10.757(3) [Angstrom], b = 4.928(3) [Angstrom], c = 8.942(2) [Angstrom], [beta] = 102.39(3)[omicron], V = 463.0(3) [[Angstrom].sup.3], and Z = 2. The unavailability of a suitable crystal prevented single-crystal X-ray studies. The strongest 10 lines of the X-ray powder diffraction pattern are [d in [Angstrom] (I) (hkl)]: 3.267(100)(012), 2.52(71)(30[bar.3]), 4.361(51) (002), 1.762(39)(32[bar.3]), 4.924 (34)(010), 2.244(32)(31[bar.3]), 1.455(24)(006), 1.996(21)(014), 1.565(20) (611), and 2.353(18)(41[bar.1]). XANES Te [L.sub.III]-edge spectra of a selection of Te minerals (including xocolatlite) and inorganic compounds showed that the position of the absorption edge can be reliably related to the oxidation state of Te. XANES demonstrated that xocolatlite contains [Te.sup.6+] as a tellurate group. Water has been tentatively included in the formula based on IR spectroscopy that indicated the presence of a small amount of water. Raman, IR, XANES, and X-ray diffraction data together with the chemical composition show a similarity of xocolatlite to kuranakhite. A possible series may exist between these two species, xocolatlite being the Ca-rich end-member and kuranakhite the Pb-rich one. Keywords: Xocolatlite, Kuranakhite, new mineral, XANES spectroscopy, tellurium oxidation state, Moctezuma, Sonora, Mexico