J. Brugger

Contribution to the mineralogy of acid drainage of Uranium minerals: Marecottite and the zippeite-group

Abstract Sulfate-rich acid waters produced by oxidation of sulfide minerals enhance U mobility around U ores and U-bearing radioactive waste. Upon evaporation, several secondary uranyl minerals, including many uranyl sulfates, precipitate from these waters. The zippeite-group of minerals is one of the most common and diverse in such settings. To decipher the nature and crystal chemistry of the zippeite-group, the crystal structure of a new natural hydrated Mg uranyl sulfate related to Mgzippeite was determined. The mineral is named marecottite after the type locality, the La Creusaz U prospect near Les Marécottes, Western Swiss Alps. Marecottite is triclinic, P1, with a = 10.815(4), b = 11.249(4), c = 13.851(6) Å, a = 66.224(7), b = 72.412(7), and g = 69.95(2)°. The ideal structural formula is Mg3(H2O)18[(UO2)4O3(OH)(SO4)2]2(H2O)10. The crystal structure of marecottite contains uranyl sulfate sheets composed of chains of edge-sharing uranyl pentagonal bipyramids that are linked by vertex-sharing with sulfate tetrahedra. The uranyl sulfate sheets are topologically identical to those in zippeite, K(UO2)2(SO4)O2·2H2O. The zippeite-type sheets alternate with layers containing isolated Mg(H2O)6 octahedra and uncoordinated H2O groups. The uranyl sulfate and Mg layers are linked by hydrogen bonding only. Magnesium-zippeite is redefined as Mg(H2O)3.5(UO2)2(SO4)O2, based on comparison of the powder X-ray diffraction pattern of micro-crystalline co-type material with the pattern of a synthetic phase. Magnesium-zippeite contains zippeite-type uranyl sulfate sheets with Mg located between the layers, where it is in octahedral coordination. In Mg-zippeite, distorted Mg octahedra are linked by sharing vertices, resulting in dimers. The apices of the Mg octahedra correspond to two O atoms of uranyl ions, and four H2O groups. Magnesium-zippeite and marecottite co-exist, sometimes in the same sample, at Lucky Strike no. 2 mine, Emery County, Utah (type locality of Mg-zippeite), at Jáchymov, Czech Republic, and at La Creusaz. This study provides insight into the complexity of the zippeite-group minerals containing divalent cations, where different arrangements in the interlayers result in different unit cells and space groups.

Mechanism and kinetics of a mineral transformation under hydrothermal conditions: Calaverite to metallic gold

Abstract The transformation of calaverite to gold under hydrothermal conditions was studied experimentally by probing the effects of temperature (140 to 220 °C), pH (2-12), oxidant concentration, geometric specific surface area, and solid-weight to fluid-volume ratio on the sample textures and the reaction kinetics. Under all of the experimental conditions explored, calaverite transformed to various extents to metallic gold. The replacement is pseudomorphic, as gold preserves the external dimensions of calaverite. The resulting elemental gold is porous; consisting of filament-shaped aggregates with diameters ranging from 200 to 500 nm and lengths up to 25 μm. Gold crystals appear to be randomly oriented with respect to the twinned calaverite grains. The transformation proceeds via a coupled calaverite dissolution-gold precipitation mechanism, with calaverite dissolution being rate-limiting relative to gold precipitation. Tellurium is lost to the bulk solution as Te(IV) complexes, and may further precipitate away from the dissolution site (e.g., autoclave walls) as TeO2(s). In contrast, gold precipitates locally near the calaverite dissolution site. Such local gold precipitation is facilitated by fast heterogeneous nucleation onto the calaverite surface. The dissolution of calaverite and the overall reaction are oxidation reactions, and oxygen diffusion through the porous metallic gold layer probably plays an important role in sustaining the reaction. A similar dissolution-reprecipitation process may be responsible for the formation of mustard gold during the weathering of gold-telluride ores. At 220°C, solid-state replacement of calaverite by gold is slow (months), but calaverite grains ~100 μm in size are fully replaced in <24 h under hydrothermal conditions, providing a possible alternative to roasting as a pre-treatment of telluride-rich gold ores

Transformation of pentlandite to violarite under mild hydrothermal conditions

Abstract The transformation of pentlandite, (Ni,Fe)9S8, to violarite, (Ni,Fe)3S4, has been investigated under mild hydrothermal conditions, at constant values of pH (range 3 to 5) controlled by the acetic acid/sodium acetate buffer. At 80 °C, 20(4) wt% of the pentlandite transforms to violarite in 33 days; with the addition of small amounts of Fe3+(CH3COO)2(OH) and H2S the reaction reaches 40(4) wt% completion in this time. At 120 °C and a pressure of 3.5 bars the reaction is complete in 3 days at pH 3.9. Electron backscatter diffraction and backscattered electron imaging reveal that the reaction textures are typical of a coupled dissolution-reprecipitation reaction, rather than a solid state electrolytic process as has been previously reported. The gap between the dissolution front and the precipitation front of violarite is less than 400 nm. The violarite produced by these hydrothermal transformations is texturally similar to supergene violarite, being fine grained, porous and finely cracked.

Tripuhyite, FeSbO4, revisited

Abstract The exact nature of tripuhyite remains controversial more than 100 years after the first description of the mineral. Different stoichiometries and crystal structures (rutile or tri-rutile types) have been suggested for this Fe-Sb-oxide. To address these uncertainties, we studied tripuhyite from Tripuhy, Minas Gerais, Brazil (type material) and Falotta, Grisons, Switzerland using single-crystal and powder X-ray diffraction (XRD), optical microscopy and electron microprobe analysis. Electron microprobe analyses showed the Fe/Sb ratios to be close to one in tripuhyite from both localities. Single crystal XRD studies revealed that tripuhyite fromthe type locality and fromFalotta have the rutile structure (P42/mnm, a = 4.625(4) c = 3.059(5) and a = 4.6433(10) c = 3.0815(9) Å , respectively). Despite careful examination, no evidence for a tripled c parameter, characteristic of the tri-rutile structure, was found and hence the structure was refined with the rutile model and complete Fe-Sb disorder over the cationic sites in both cases (type material: R1 = 3.61%; Falotta material: R1 = 3.96%). The specular reflectance values of type material tripuhyite and lewisite were measured and the following refractive indices calculated (after Koenigsberger): tripuhyite nmin = 2.14, nmax = 2.27; lewisite (cubic) n = 2.04. These results, together with those of 57Fe and 121Sb Mössbauer spectroscopy on natural and synthetic tripuhyites reported in the literature, indicate that the chemical formula of tripuhyite is Fe3+Sb5+O4 (FeSbO4). Thus, tripuhyite can no longer be attributed to the tapiolite group of minerals of general type AB2O6. A comparison of the results presented with the mineralogical data of squawcreekite suggests that tripuhyite and squawcreekite are identical. In consequence, tripuhyite was redefined as Fe3+Sb5+O4 with a rutile-type structure. Both the proposed new formula and unit cell (rutile-type) of tripuhyite as well as the discreditation of squawcreekite have been approved by the Commission on New Mineral and Mineral Names (CNMMN) of the International Mineralogical Association (IMA).

Deriving formation constants for aqueous metal complexes from XANES spectra: Zn2+ and Fe2+ chloride complexes in hypersaline solutions

Abstract The development of numerical modeling of reactive transport relies on the availability of thermodynamic properties for the solid, surface, aqueous, and vapor species stable at the conditions of interest. The lack of experimental studies and comprehensive activity-composition models severely limits the predictive capabilities of these models in systems involving highly saline fluids or low-density volatile-rich fluids. X-ray absorption near-edge structure spectroscopy (XANES) is a powerful technique to study the speciation of transition metals in aqueous fluids: it is element specific; sensitive to the oxidation state of the metal, the ligand field and the coordination geometry of the complex; and it is suitable for measuring trace amounts of metals (<1 wt%) in solutions over a wide pressure and temperature range. Formation constants for the aqueous complexes of transition metals can be determined from a series of XANES spectra obtained on solutions containing a constant amount of the metal of interest and variable concentrations of a ligand. The method relies on a non-linear, least-squares-fitting approach, with full distribution of species calculations based on a complete thermodynamic model for the experimental system under consideration. The technique is particularly suitable for following octahedral to tetrahedral transitions among weak chloride complexes of transition metals. The log K for the reaction Zn2+(octahedral) + 4Cl- = ZnCl42-(tetrahedral) at 25 °C is retrieved to be 0.1(6), within error of the accepted literature value. The same method applied to Fe2+-chloride complexes shows that the log K for the reaction Fe2+(octahedral) + 4Cl- = FeCl42-(tetrahedral) increases from -6.2(6) at 25 °C to -2.9(3) at 150 °C. This study confirms that tetrahedral chloride complexes play an important role in Fe transport in hypersaline brines especially at elevated temperatures, and shows that XANES is well suited to study systems that may be difficult to study with other techniques.

Arsenic speciation in fluid inclusions using micro-beam X-ray absorption spectroscopy

Abstract 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% CO2 ± CH4; 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°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)3(aq) (coordination number of 3.0 ± 0.2 atoms, bond length of 1.76 ± 0.01 Å) is the dominant arsenic aqueous species in the Muiane fluid inclusions at 100 °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 °C; the As-O bond distance (1.711 ± 0.025 Å) corresponds to that found in the arsenate group in minerals, and to that measured for the (HAsO4)2- complex at room temperature (1.700 ± 0.023 Å). 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 × 106 photons/s/μm2). 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

Spriggite, Pb3 [(UO2)6O8(OH)2] (H2O)3, a new mineral with β-U3O8 –type sheets: Description and crystal structure

Abstract Spriggite, Pb3[(UO2)6O8(OH)2](H2O)3, is a new hydrated Pb uranyl oxyhydroxide found near Arkaroola, Northern Flinders Ranges, South Australia. The new mineralʼs name honors geologist and conservationist Reginald Claude Sprigg (1919-1994), founder of the Arkaroola Tourist Station. Together with beta-uranophane, soddyite, kasolite, Ce-rich francoisite-(Nd), metatorbernite, billietite, Ba-bearing boltwoodite, schoepite, metaschoepite, and weeksite, spriggite results from the supergene alteration of U-Nb-REE-bearing hydrothermal hematite breccia. Spriggite forms prismatic crystals up to about 150 μm in length and up to 40 μm across. It is transparent, bright orange in color with vitreous luster, biaxial, nmin= 1.807; nmax = 1.891 (NaD, 22.5 °C), non-fluorescent, brittle with an uneven fracture. It has a pale orange streak, Mohsʼ hardness ~4, good cleavage along (100) and Dcalc = 7.64(6) g/ cm3. The empirical formula is (Pb2.77Ca0.06Ba0.04)Σ2.87U6O19.9(OH)2⋅3H2O, and the simplified formula is Pb3[(UO2)6O8(OH)2](H2O)3. Spriggite is monoclinic, C2/c, a = 28.355(9), b = 11.990(4), c = 13.998(4) Å, β = 104.248(5); V = 4613(3) Å3, Z = 8. The strongest eight lines in the powder X-ray diffraction pattern are [d in Å (I)(hkl)]: 6.92(60)(400), 6.02(30)(112̅;020), 3.46(80)(800), 3.10(100)(204;6̅04;33̅2;5̅32), 2̅.74(30)(4̅40), 2.01(30)(336̅), 1.918(60)(10.04̅;14.04̅;11.3̅2;1̅3̅-.31), 1.738(30)(53̅6;1̅1̅.36). The structure has been solved from a crystal twinned on (001) and refined to R1 = 9.7%. The structure is based upon the [(UO2)6O8(OH)2]6+ sheets of uranyl polyhedra of the β-U3O8 anion topology with Pb2+ cations and H2O groups in the interlayer. Billietite and spriggite contain only hexavalent U in the uranyl sheets, whereas the similar sheets in β-U3O8 contain U5+ and U6+, and those in ianthinite U4+ and U6+. Spriggite has the highest Pb:U ratio among the known hydrated Pb uranyl oxyhydroxide minerals.

Mapping REE distribution in scheelite using luminescence

Abstract In situ laser ablation high resolution ICP-MS analyses of scheelite from hydrothermal veins at the Archaean Mt. Charlotte gold deposit (Western Australia) show inhomogeneous REE distribution at small scale (<100 μm). In a limited number of samples, variations of the cathodoluminescence (CL) colours from blue to yellow are linked to the REE content of scheelite, and reveal oscillatory zoning of the REE with zone widths between 1 μm and 100 μm. However, CL failed to reveal the zoning in most inhomogeneous scheelite samples. A nuclear microprobe has been used to characterize the distribution of REE in these samples. No reasonable map for the distribution of REE could be obtained by particle induced X-ray emission, because of interferences with W-L lines. However, monochromatic ionoluminescence (IL) maps collected at the wavelength of the main REE3+ luminescence peaks revealed oscillatory zoning. Therefore, IL is a powerful tool for mapping the distribution of REE in natural scheelite. Monochromatic IL maps allow us to determine the nature of the inhomogeneous distribution of REE in scheelite, fundamental information for using the REE in this mineral as a marker for the chemistry of ore-forming fluids, and for interpreting Sm-Nd isotopic data.

Xocolatlite, Ca2Mn24+Te2O12·H2O, a new tellurate related to kuranakhite: Description and measurement of Te oxidation state by XANES spectroscopy

Abstract Xocolatlite, Ca2Mn4+2 Te6+2O12·H2O, 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 copperbrown 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/cm3, and immersion in Clerici solution indicated a density higher than 4.1 g/cm3. 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 Pm, with the following unit-cell parameters refined from synchrotron X-ray powder diffraction data: a = 10.757(3) Å, b = 4.928(3) Å, c = 8.942(2) Å, β = 102.39(3)°, V = 463.0(3) Å3, 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 Å (I) (hkl)]: 3.267(100)(012), 2.52(71)(303̄), 4.361(51) (002), 1.762(39)(323̄), 4.924 (34)(010), 2.244(32)(313̄), 1.455(24)(006), 1.996(21)(014), 1.565(20) (611), and 2.353(18)(411̄). XANES Te LIII-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 Te6+ 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.

Pseudojohannite from Jáchymov, Musonoï, and La Creusaz : A new member of the zippeite-group

Abstract Pseudojohannite is a hydrated copper(II) uranyl sulfate described from Jáchymov, Northern Bohemia, Czech Republic (type locality). Pseudojohannite also occurs at the Musonoï quarry near Kolwezi, Shaba, Congo, and the La Creusaz prospect, Western Swiss Alps. At all three localities, pseudojohannite formed through the interaction of acid sulfate mine drainage waters with uraninite (Jáchymov and La Creusaz) or uranyl silicates (Musonoï). Pseudojohannite forms moss green, non UV-ß uorescent aggregates consisting of irregularly shaped crystals measuring up to 25 μm in length and displaying an excellent cleavage parallel to (.101). dmeas is 4.31 g/cm3, dcalc 4.38 g/cm3, and the refractive indices are nmin = 1.725 and nmax = 1.740. A high-resolution synchrotron powder diffraction pattern on the material from Musonoï shows that pseudojohannite is triclinic (P1 or P1̅), with a = 10.027(1) Å, b = 10.822(1) Å, c = 13.396(1) Å, α = 87.97(1)°, β = 109.20(1)°, γ = 90.89(1)°, V = 1371.9(5) Å3. The location of the uranium and sulfur atoms in the cell was obtained by direct methods using 1807 reflections extracted from the powder diffractogram. Pseudojohannite contains zippeite-type layers oriented parallel to (.101). The empirical chemical formula calculated for a total of 70 O atoms is Cu6.52U7.85S4.02O70H55.74, leading to the simplified chemical formula Cu6.5[(UO2)4O4(SO4)2]2(OH)5·25H2O. The distance of 9.16 Å between the uranylsulfate sheets in pseudojohannite shows that neighboring layers do not share O atoms with the same CuΦ6 [Φ = (O,OH)] distorted octahedrons, such as in magnesium-zippeite. Rather, it is expected that CuΦ6 forms a layer bound to the zippeite-type layers by hydrogen bonding, as in marécottite, or one apex of the CuΦ6 polyhedron only is shared with a zippeite-type layer, as in synthetic SZIPPMg. The higher number of cations in the interlayer of pseudojohannite (Cu:S = 1.6:1) compared to marécottite (3:4) and SZIPPMg (1:1) indicates that pseudojohannite has a unique interlayer topology. Ab-initio powder structure solution techniques can be used to obtain important structural information on complex micro-crystalline minerals such as those found in the weathering environment. Pseudojohannite represents a new member of the zippeite group of minerals, and further illustrates the structural complexity of zippeite-group minerals containing divalent cations, which have diverse arrangements in the interlayer. Peudojohannite and other divalent zippeites are common, easily overlooked minerals in acid drainage environments around uranium deposits and wastes.