New Reference Materials and Assessment of Matrix Effects for SIMS Measurements of Oxygen Isotopes in Garnet

Accurate ion microprobe analysis of oxygen isotope ratios in garnet requires appropriate reference materials to correct for instrumental mass fractionation that partly depends on the garnet chemistry (matrix effect). The matrix effect correlated with grossular, spessartine and andradite components was characterised for the Cameca IMS 1280HR at the SwissSIMS laboratory based on sixteen reference garnet samples. The correlations fit a second‐degree polynomial with maximum bias of ca. 4‰, 2‰ and 8‰, respectively. While the grossular composition range 0–25% is adequately covered by available reference materials, there is a paucity of them for intermediate compositions. We characterise three new garnet reference materials GRS2, GRS‐JH2 and CAP02 with a grossular content of 88.3 ± 1.2% (2s), 83.3 ± 0.8% and 32.5 ± 3.0%, respectively. Their micro scale homogeneity in oxygen isotope composition was evaluated by multiple SIMS sessions. The reference δ¹⁸O value was determined by CO₂ laser fluorination (δ¹⁸O_LF). GRS2 has δ¹⁸O_LF = 8.01 ± 0.10‰ (2s) and repeatability within each SIMS session of 0.30–0.60‰ (2s), GRS‐JH2 has δ¹⁸O_LF = 18.70 ± 0.08‰ and repeatability of 0.24–0.42‰ and CAP02 has δ¹⁸O_LF = 4.64 ± 0.16‰ and repeatability of 0.40–0.46‰.     

A study of synthetic and natural uranium oxides by X-ray photoelectron spectroscopy

Synthetic and natural uranium oxides UO _x (2≦×≦3) have been studied with X-ray photoelectron spectroscopy (XPS) to determine the phase composition and content of uranium ions in uraninites with a varying degree of oxidation. A strong hybridization of U_6p and O_2s orbitals has been found which permits a quantitative assessment of the U-O bond lengths. The values of such bonds in some substances have been found to be smaller than those in synthetic U(VI) oxide. The oxides U₂O₅ and U₃O₈ contain two types of uranium ions with a varying degree of oxidation.     

Uranium minerals from the San Marcos District, Chihuahua, Mexico

The mineralogy of the two uranium deposits (Victorino and San Marcos I) of Sierra San Marcos, located 30 km northwest of Chihuahua City, Mexico, was studied by optical microscopy, powder X-ray diffraction with Rietveld analysis, scanning electron microscopy with energy dispersive X-ray analysis, inductively coupled plasma spectrometry, and gamma spectrometry. At the San Marcos I deposit, uranophane Ca(UO₂)₂Si₂O₇·6(H₂O) (the dominant mineral at both deposits) and metatyuyamunite Ca(UO₂)(V₂O₈)·3(H₂O) were observed. Uranophane, uraninite (UO_2+x), masuyite Pb(UO₂)₃O₃(OH)·3(H₂O), and becquerelite Ca(UO₂)₆O₄(OH)₆ ·(8H₂O) are present at the Victorino deposit. Field observations, coupled with analytical data, suggest the following sequence of mineralization: (1) deposition of uraninite, (2) alteration of uraninite to masuyite, (3) deposition of uranophane, (4) micro-fracturing, (5) calcite deposition in the micro-fractures, and (6) formation of becquerelite. The investigated deposits were formed by high-to low-temperature hydrothermal activity during post-orogenic evolution of Sierra San Marcos. The secondary mineralization occurred through a combination of hydrothermal and supergene alteration events. Becquerelite was formed in situ by reaction of uraninite with geothermal carbonated solutions, which led to almost complete dissolution of the precursor uraninite. The Victorino deposit represents the second known occurrence of becquerelite in Mexico, the other being the uranium deposits at Peña Blanca in Chihuahua State.     

Determination of the uranium valence state in the brannerite structure using EELS, XPS, and EDX

In this study, the valence states of uranium in synthetic and natural brannerite samples were studied using a combination of transmission electron microscopy-electron energy loss spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), and X-ray photoelectron spectroscopy (XPS) techniques. We used a set of five (UO₂, CaUO₄, SrCa₂UO₆, UTi₂O₆, and Y_0.5U_0.5Ti₂O₆) U standard samples, including two synthetic brannerites, to calibrate the EELS branching ratio, M₅/(M₄ +M₅), against the number of f electrons. The EELS data were collected at liquid nitrogen temperature in order to minimise the effects of electron beam reduction of U⁶⁺ and U⁵⁺. Test samples consisted of three additional synthetic brannerites (Th_0.7U_0.3Ti₂O₆, Ca_0.2U_0.8Ti₂O₆, and Th_0.55U_0.3Ca_0.15Ti₂O₆) and three natural brannerites from different localities. The natural brannerite samples are all completely amorphous, due to cumulative alpha decay events over geological time periods (24–508 Ma). Our U valence calibration results are in reasonable agreement with previous work, suggesting possibly a non-linear relationship between the branching ratio and the number of f electrons (and hence the average valence state) of U in solids. We found excellent agreement between the nominal valence states of U and the average valence states determined directly by EELS and estimated by EDX analysis (with assumptions regarding stoichiometry) in two of the three synthetic brannerite test samples. The average U oxidation states of the five synthetic brannerite samples, as derived from XPS analyses, are also in good agreement with those determined by other techniques. The average valence states of U in three amorphous (metamict) natural brannerite samples with alpha decay doses ranging from 3.6×10¹⁶ to 6.9×10¹⁷ /mg were found to be 4.4, 4.7, and 4.8, consistent with the presence of U⁵⁺ and/or U⁶⁺ as well as U⁴⁺ in these samples. These results are in general agreement with previous wet chemical analyses of natural brannerite. However, the average valence states inferred by SEM-EDX for two of the natural brannerite samples do not show satisfactory agreement with the EELS determined valence. This may be due to the occurrence of OH^– groups, cation vacancies, anion vacancies, or excess oxygen in the radiation-damaged structure of natural brannerite.     

Transformation of synthetic U3O8 into different uranium oxide hydrates

Summary Referring to the natural formation of secondary uranium minerals, the primary transformation of U₃O₈ into schoepite has been investigated. The transformation is realized in a continuous system with O₂, CO₂ and H₂O. At 100°C schoepite III, UO₃ · zH₂O (z 1), is formed (a = 14.12; b = 16.83; c = 15.22 Å) with a density of 4.460 g/cm³. At 25°C a mixture of schoepite II (UO₃ · yH₂O, 1 < y < 2; a = 13.99; b = 16.72; c = 14.73 Å) and schoepite I (UO₃ · xH₂O, x 2; a = 14.33; b = 16.79; c = 14.73 Å) is obatined. From thermogravimetric analysis the activation energy of dehydration for schoepite III is determined as 49(3) · 10³ J/mole.

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Umwandlung von synthetischem U₃O₈ in verschiedene Uranoxidhydrate

Zusammenfassung In Hinblick auf die natürliche Bildung sekundärer Uranminerale wurde die primäre Umwandlung von U₃O₈ in Schoepit untersucht. Die Umwandlung wurde in einem kontinuierlichen System mit O₂, CO₂ und H₂O bewerkstelligt. Bei 100°C bildet sich Schoepit III (UO₃ · zH₂O, z 1; a = 14.12, b = 16.83, c = 15.22 Å; Dichte: 4.460 g/cm³). Bei 25°C wird eine Mischung von Schoepit II (UO₃ · yH₂O, 1 < y < 2; a = 13.99, b = 16.72, c = 14.73 Å) und Schoepit I (UO₃ · xH₂O, x 2; a = 14.33, b = 16.79, c = 14.73 Å) erhalten. Aus der thermogravimetrischen Analyse wurde die Aktivierungsenergie der Dehydratation von Schoepit III mit 49(3) · 10³ J/mole berechnet.

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Who wishes to dedicate the paper to the memory of his father, Hendrik Vochten.

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With 3 Figures     

Quartz Reference Materials for Oxygen Isotope Analysis by SIMS

Two quartz samples of igneous origin, UNIL‐Q1 (Torres del Paine Intrusion, Chile) and BGI‐Q1 (Shandong province, China), were calibrated for their oxygen isotope composition for SIMS measurements. UNIL‐Q1 and BGI‐Q1 were evaluated for homogeneity using SIMS. Their reference δ¹⁸O values were determined by CO₂ laser fluorination. The average δ¹⁸O value found for UNIL‐Q1 is 9.8 ± 0.06‰ and that for BGI‐Q1 is 7.7 ± 0.11‰ (1s). The intermediate measurement precision of SIMS oxygen isotope measurements was 0.32–0.41‰ (2s; UNIL‐Q1) and 0.40–0.48‰ (2s; BGI‐Q1), respectively. While less homogeneous in its oxygen isotope composition, BGI‐Q1 is also suitable for SIMS trace element measurements.     

Uranotungstit,ein neues sekundäres Urnmineral aus dem Schwarzwald

Zusammenfassung Uranotungstit kommt als Sekundärbildung in der Uranlagerstätte von Menzenschwand im südlichen Schwarzwald und in der Grube Clara bei Oberwolfach im mittleren Schwarzwald vor. Er bildet Krusten auf Quarz, Meta-Uranocircit und Meta-Heinrichit. Zu den weiteren Begleitmineralien gehören Bergenit, Meta-Torbernit, Meta-Zeunerit und Schoepit.Das Mineral bildet sphärolithische Aggregate, deren Durchmesser 0,3 mm erreicht. Die rhombischen Kristalle sind lattenförmig ausgebildet mit (010) als Tafelfläche. Farbe gelb, orange oder bräunlich. Strich gelb. Etwas durchscheinend, Glanz stumpf, zum Teil perlmutterartig, Spaltbarkeit nach (010) vollkommen, Bruch unregelmäßig, Härte ungefähr 2, Dichte (gem.)>4,03 (ber.) 4,27 g/cm³. Optische Eigenschaften:n =1,682,n =1,845,n =1,855 (jeweils±0,005), negativ, 2V=42^o,r>v, Pleochroismus:X ± farblos,Y, Z gelb,X=b.Gitterkonstanten:a ₀=9,22,b ₀=13,81,c ₀=7,17 Å,Z=2. Stärkste Linien des Pulver-diagramms: 6,96 (10) 020; 4,60 (6) 200, 030; 3,46 (5) 012, 040; 3,21 (7) 140, 022. Die Analyse des Minerals von Menzeschwand ergab. FeO 2,3, BaO 4,9, PbO 6,9, UO₃ 45,1, WO₃ 19,8, H₂O 22,5, Summe 101,5, was auf der Basis von 24 Sauerstoffatomen zur Formel (Fe_0,38Ba_0,37Pb_0,36) _1,11U_1,82W_0,99H_28,94O₂₄ führt oder, idealisiert, (Fe,Ba,Pb)(UO₂)₂(WO₄) (OH)₄·12H₂O.Das Mineral ist das erste in der Natur nachgewiesene Uranylwolframat.

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Uranotungstite, a new secondary uranium mineral from the Black Forest

Summary Uranotungstite has been found as secondary mineral at the uranium deposit of Menzenschwand in the Southern Black Forest and at the Clara Mine near Oberwolfach in the Central Black. Forest (Germany). It forms crusts on quartz, meta-uranocircite and meta-heinrichite. Other associated secondary minerals are bergenite, meta-torbernite, meta-zeunerite and schoepite.The mineral forms spherulitic aggregates reaching 0.3 mm in diameter. The orthorhombic crystals are lath-like with (010) as plane of flattening. Colour yellow, orange or brownish. Streak yellow. Somewhat translucent, luster dull, in part pearly, perfect cleavage (010), fracture irregular, hardness about 2, density (meas.)>4.03, (calc.) 4.27 g/cm³. Optical properties:n =1.682,n =1.845,n =1.855 (±0.005 each), negative, 2V=42^o,r>v, pleochroism:X ± colourless,Y, Z yellow,X=b.Unit-cell dimensions:a ₀=9.22,b ₀=13.81,c ₀=7.17 Å,Z=2. Strongest lines of the powder pattern: 6.96 (10) 020; 4.60 (6) 200, 030; 3.46 (5) 012, 040; 3.21 (7) 140, 022. The analysis of the mineral from Menzenschwand, FeO 2.3, BaO 4.9, PbO 6.9, UO₃ 45.1, WO₃ 19.8, H₂O 22.5, total 101.5, yields the following formula on the basis of 24 oxygen atoms: (Fe_0.38Ba_0.37Pb_0.36) _1.11U_1.82W_0.99H_28.94O₂₄ or, idealized, (Fe,Ba,Pb) (UO₂)₂ (WO₄) (OH)₄·12H₂O.The mineral is the first uranyl tungstate discovered in nature.

     

Fluid Inclusions and Hydrogen,Oxygen, and Lead Isotopes of the Antuoling Molybdenum Deposit,Northern Taihang Mountains,China

The Antuoling Mo deposit is a major porphyry‐type deposit in the polymetallic metallogenic belt of the northern Taihang Mountains, China. The processes of mineralization in this deposit can be divided into three stages: an early quartz–pyrite stage, a middle quartz–polymetallic sulfide stage, and a late quartz–carbonate stage. Four types of primary fluid inclusions are found in the deposit: two‐phase aqueous inclusions, daughter‐mineral‐bearing multiphase inclusions, CO₂–H₂O inclusions, and pure CO₂ inclusions. From the early to the late ore‐forming stages, the homogenization temperatures of the fluid inclusions are 300 to >500°C, 270–425°C, and 195–330°C, respectively, with salinities of up to 50.2 wt%, 5.3–47.3 wt%, and 2.2–10.4 wt% NaCl equivalent, revealing that the ore‐forming fluids changed from high temperature and high salinity to lower temperature and lower salinity. Moreover, based on the laser Raman spectra, the compositions of the fluid inclusions evolved from the NaCl–CO₂–H₂O to the NaCl–H₂O system. The δ¹⁸O_H2O and δD values of quartz in the deposit range from +3.9‰ to +7.0‰ and ?117.5‰ to ?134.2‰, respectively, reflecting the δD of local meteoric water after oxygen isotopic exchange with host rocks. The Pb isotope values of the sulfides (²⁰⁸Pb/²⁰⁴Pb, 36.320–37.428; ²⁰⁷Pb/²⁰⁴Pb, 15.210–15.495; ²⁰⁶Pb/²⁰⁴Pb, 16.366–17.822) indicate that the ore‐forming materials originated from a mixed upper mantle–lower crust source.     

Interaction of model F-bearing silicic melt with chloride fluid,uraninite, and columbite at 750°C and 1000–2000 bar and its implications for estimation of the ore-forming capability of the upper crustal magma chamber beneath the Strel’tsovka caldera,eastern Transbaikalia

The experimental study of an F-bearing silicic melt—U, Nb, Ta minerals—chloride-fluoride fluid system is focused on ascertaining the origin of uranium deposits spatially related to intraplate silicic volcanism. The first series of experiments on uranium solubility in silicic melts close in composition to ore-bearing rhyolite of the unique Strel’tsovka Mo-U ore field has been performed in order to determine more precisely the ore genesis. As starting solid phases, model homogeneous glass of the chemical composition (wt %) 72.18 SiO₂, 12.19 Al₂O₃, 1.02 FeO, 0.20 MgO, 0.33 CaO, 4.78 Na₂O, 3.82 K₂O, 1.44 Li₂O, and 2.4 F (LiF, NaF, KF, CaF₂, MgF₂); synthetic UO₂ and UO₃·0.33H₂O; and natural columbite were used. The starting solutions contained 1.0 m Cl and 10⁻² m F. The runs were conducted in a gas vessel at a pressure of 1000 bar and in a high-pressure hydrothermal vessel at 2000 bar. The O₂ (H₂) fugacity was set by Ni-NiO, Co-CoO, Fe₃O₄-Fe₂O₃, and Cu-Cu₂O buffers. The equilibrium between melt and solution for major elements is reached during the first day, whereas 5–7 days are required for ore elements (U, Nb, Ta) to come into equilibrium. The solubility of Nb and especially Ta in Cl-F solutions equilibrated with F-bearing melt is extremely low. The solubility of U is much higher (10⁻⁴−10⁻⁵ mol/kg H₂O). The energy dispersive spectroscopy of run products allowed us to establish that columbite dissolved incongruently with formation of U- and F-bearing pyrochlores. The performed experiments have shown that a silicic melt close to the rhyolitic magma of the Strel’tsovka caldera in composition is not able to generate postmagmatic ore-forming solutions containing more than 10⁻⁶−10⁻⁵ mol U/kg H₂O under the relatively low pressure necessary for the existence of the first type of fluid. The amount of uranium that could have precipitated from this fluid in the zone of ore deposition is estimated at 216–9000 t. This estimate is two orders of magnitude lower than the total uranium resources of the deposits localized in the Strel’tsovka caldera. Thus, the upper crustal silicic magma chamber hardly was a source of uranium for Mo-U deposits of the Strel’tsovka ore field.     

The crystal structure of curite, [Pb6.56(H2O,OH)4] [(UO2)8O8(OH)6]2

Summary The crystal structure of curite, of which until now only the arrangement of the U and Pb atoms was known, has been redetermined with a synthetic crystal using three-dimensional X-ray techniques.R=0.043 for 1270 observed reflections. Curite is orthorhombic, space groupPnam-D _2h ¹⁶ ,a=12.513,b=13.002,c=8.373 ,Z=6.56 PbO·16UO₃·9.44H₂O. The structure consists of a novel type of washboard like puckered layers ² [(UO₂)₈O₈ (OH)₆]^6– formed by tetragonal bipyramidal [(UO₂)O₃(OH)] and pentagonal bipyramidal [(UO₂)O₃ (OH)₂] polyhedra. The layers are parallel to {100} and are directly connected by hydrogen bonds. Lead atoms and oxygen atoms (H₂O+OH) are located in folds between the layers, helping to connect them. The interlayer atomic positions are slightly disordered and one of them is partially occupied. The variable concentrations of the interlayer atoms are responsible for the changes in chemical composition.The structural formula [Pb_8–x (OH)_4–2x (H₂O)_2x ] [(UO₂)₈(OH)₆]₂ is suggested for curite;x=1.44 for the investigated synthetic curite. Within the three different U–O polyhedra the axial U–O distances are 1.81–1.88 , the equatorial 2.14–2.57 . The two different Pb atoms have ionic coordinations, each by ten oxygens with Pb–O distances of 2.46–3.32 , on the average 2.82 .

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Die Kristallstruktur von Curit, [Pb _6,56 (H ₂ O, OH) ₄ ] [(UO ₂)₈ O ₈(OH)₆]₂

Zusammenfassung Die Kristallstruktur von Curit, von der bisher nur die Lagen der Uran- und Bleiatome bekannt waren, wurde anhand eines künstlichen Kristalls mit dreidimensionalen Röntgendaten neu bearbeitet und für 1270 Reflexe aufR=0,043 verfeinert. Curit kristallisiert rhombisch, RaumgruppePnam-D _2h ¹⁶ ,a=12,513,b=13,002,c=8,373 ,Z=6,56 PbO·16 UO₃·9,44 H₂O. Die Struktur enthält gewellte Schichten eines neuen Typs, ² [(UO₂)₈O₈(OH)₆]^6–, die sich aus tetragonal bipyramidalen [(UO₂)O₃(OH)]- und pentagonal-bipyramidalen [(UO₂)O₃(OH)₂]-Polyedern zusammensetzen. Die Schichten verlaufen parallel {100} und sind über Wasserstoffbrücken miteinander unmittelbar verknüpft. Zwischen den Schichten befinden sich Bleiatome und zusätzliche Sauerstoffatome (H₂O+OH). Diese Atome weisen zum Teil Fehlordnung auf; eine der beiden Pb-Lagen ist nur partiell besetzt. Für Schwankungen in der chemischen Zusammensetzung von Curit ist der unterschiedliche Gehalt an Zwischenschichtatomen verantwortlich. Aufgrund dieser Untersuchung wird die Strukturformel [Pb_8–x (OH)_4–2x (H₂O)_2x ] [(UO₂)₈O₈(OH)₆]₂ vorgeschlagen; für den untersuchten Curit istx=1,44. Die drei verschiedenen U–O-Polyeder der Struktur besitzen axiale bzw. äquatoriale U–O-Abstände von 1,81–1,88 bzw. 2,14–2,57 . Die zwei Arten von Bleiatomen besitzen eine ionische Koordination; beide sind von 10 Sauerstoffatomen in Abständen von 2,46–3,32 (Mittelwert 2,82 ) umgeben.

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With 3 Figures     

Geochemical Characteristics of Deep—Seated Potassium—Rich Brine in the Middle Triassic Leikoupo Formation,Sichuan,China

Brine extremely rich in potassium, boron and bromine has been discovered from the Middle Triassic Leikoupo Formation at a depth of 4300 m in Sichuan Province. It contains ～50 g/L of K⁺, >92 g/L of Na⁺, >12 g/L of B₂O₃, >2.36 g/L of Br^? and ～0.030 g/L of I⁺. The solid precipitates during evaporation at 25°C include KB₅O₈·4H₂O, K₂B₄O₇·3H₂O, MgCl₂·6H₂O and KMgCl₃·6H₂O. The brine ranges from 2.2‰ to 2.8‰ (SMOW) in δ¹⁸O, ? 38‰ – ? 53‰ (SMOW) in δD, 15.6‰ in δ³⁴S, and 13.5‰–15.1‰ in δ¹¹B. These data, particularly the isotopic composition of boron, indicate that the brine has a composite derivation from marine and nonmarine brines and dissolved marine evaporites in the Triassic system.     

The effect of species on lacustrine δ18Odiatom and its implications for palaeoenvironmental reconstructions

The oxygen isotope composition of diatom silica (δ¹⁸O_diatom) is increasingly being used to reconstruct climate from marine and lacustrine sedimentary archives. Although diatoms are assumed to precipitate their frustule in isotopic equilibrium with their surrounding water, it is unclear whether internal processes of a given species affect the fractionation of oxygen between the water and the diatom. We present δ¹⁸O_diatom data from two diatom size fractions (3–38 and >38 µm) characterized by different species in a sediment core from Heart Lake, Alaska. Differences in δ¹⁸O_diatom between the two size fractions varies from 0 to 1.2‰, with a mean offset of 0.01‰ (n = 20). Fourier transform infrared spectroscopy confirms our samples consist of pure biogenic silica (SiO₂) and δ¹⁸O_diatom trends are not driven by contamination. The maximum offset is outside the range of error, but the mean is within analytical error of the technique (± 1.06‰), demonstrating no discernible species‐dependent fractionation in δ¹⁸O_diatom. We conclude that lacustrine δ¹⁸O_diatom measurements offer a reliable and valuable method for reconstructing δ¹⁸O_water. Considering the presence of small offsets in our two records, we advise interpreting shifts in δ¹⁸O_diatom only where the magnitude of change is greater than the combined analytical error.     

Crystal chemistry of selenates with mineral-like structures. VI. Hydrogen bonds in the crystal structure of [(H5O2)(H3O)(H2O)][(UO2)(SeO4)2]

The crystal structure of a new compound, [(H₅O₂)(H₃O)(H₂O)][(UO₂)(SeO₄)₂] (monoclinic, P2₁/n a = 8.3105(15), b = 11.0799(14), c = 13.227(2) Å, β = 103.880(13)°, V = 1182.4(3) ų), has been solved by direct methods and refined to R ₁ = 0.036. The structure is based on [(UO₂)(SeO₄)₂]^2? sheet complexes formed by corner-shared UO₇ pentagonal bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the ( $ \bar 1 The crystal structure of a new compound, [(H₅O₂)(H₃O)(H₂O)][(UO₂)(SeO₄)₂] (monoclinic, P2₁/n a = 8.3105(15), b = 11.0799(14), c = 13.227(2) ?, β = 103.880(13)°, V = 1182.4(3) ?³), has been solved by direct methods and refined to R ₁ = 0.036. The structure is based on [(UO₂)(SeO₄)₂]²⁻ sheet complexes formed by corner-shared UO₇ pentagonal bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the (01) plane. Oxonium ions and water molecules forming [(H₃O)·(H₂O)·(H₅O₂)]²⁺ complexes are interlayer. Among minerals, the existence of (H₅O₂)⁺ has been unambiguously confirmed only in rhomboclase, (H₅O₂)⁺[Fe₂(SO₄)₂(H₂O)₂]. Original Russian Text ? S.V. Krivovichev, 2008, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2008, No. 2, pp. 123–130.     

Alteration of uraninite from the Nopal I deposit,Pen˜a Blanca District,Chihuahua, Mexico,compared to degradation of spent nuclear fuel in the proposed U.S. high-level nuclear waste repository at Yucca Mountain,Nevada

At the Nopal I uranium deposit, primary uraninite (nominally UO_2+x) has altered almost completely to a suite of secondary uranyl minerals. The deposit is located in a Basin and Range horst composed of welded silicic tuff; uranium mineralization presently occurs in a chemically oxidizing and hydrologically unsaturated zone of the structural block. These characteristics are similar to those of the proposed U.S. high-level nuclear waste (HLW) repository at Yucca Mountain, Nevada. Petrographic analyses indicate that residual Nopal I uraninite is fine grained (5–10 μm) and has a low trace element content (average about 3 wt%). These characteristics compare well with spent nuclear fuel. The oxidation and formation of secondary minerals from the uraninite have occurred in an environment dominated by components common in host rocks of the Nopal I system (e.g. Si, Ca, K, Na and H₂O) and also common to Yucca Mountain. In contrast, secondary phases in most other uranium deposits form from elements largely absent from spent fuel and from the Yucca Mountain environment (e.g. Pb, P and V). The oxidation of Nopal I uraninite and the sequence of alteration products, their intergrowths and morphologies are remarkably similar to those observed in reported corrosion experiments using spent fuel and unirradiated UO₂ under conditions intended to approximate those anticipated for the proposed Yucca Mountain repository. The end products of these reported laboratory experiments and the natural alteration of Nopal I uraninite are dominated by uranophane [nominally Ca(UO₂)₂Si₂O₇·6H₂O] with lesser amounts of soddyite [nominally (UO₂)₂SiO₄·2H₂O] and other uranyl minerals. These similarities in reaction product occurrence developed despite the differences in time and physical—chemical environment between Yucca Mountain-approximate laboratory experiments and Yucca Mountain-approximate uraninite alteration at Nopal I, suggesting that the results may reasonably represent phases likely to form during long-term alteration of spent fuel in a Yucca Mountain repository. From this analogy, it may be concluded that the likely compositional ranges of dominant spent fuel alteration phases in the Yucca Mountain environment may be relatively limited and may be insensitive to small variations in system conditions.     

Mechanism of mineralization in the Changjiang uranium ore field,South China: Evidence from fluid inclusions,hydrothermal alteration,and H–O isotopes

The Changjiang uranium ore field, which contains >10,000 tonnes of recoverable U with a grade of 0.1–0.5%, is hosted by Triassic two-mica and Jurassic biotite granites, and is one of the most important uranium ore fields in South China. The minerals associated with alteration and mineralization can be divided into two stages, namely syn-ore and post-ore. The syn-ore minerals are primarily quartz, pitchblende, hematite, hydromica, chlorite, fluorite, and pyrite; the post-ore minerals include quartz, calcite, fluorite, pyrite, and hematite. The fluid inclusions of the early syn-ore stage characteristically contain O₂, and those of the late syn-ore and post-ore stage contain H₂ and CH₄. The fluid inclusions in quartz of the syn-ore stage include H₂O, H₂O–CO₂, and CO₂ types, and they occur in clusters or along trails. Homogenization temperatures (T_h) for the H₂O–CO₂ and two-phase H₂O inclusions range from 106 °C to >350 °C and cluster in two distinct groups for each type; salinities are lower than 10 wt% NaCl equiv. The ore-forming fluids underwent CO₂ effervescence or phase separation at ∼250 °C under a pressure of 1000–1100 bar. The U/Th values of the altered granites are lowest close to the ore, increase outwards, but subsequently decrease close to unaltered granites. From the unaltered granites to the ore, the lowest Fe₂O₃/FeO values become lower and the highest values higher. The REE patterns of the altered granites and the ores are similar to each other. The U contents of the ores show a positive correlation with total REE contents but a negative correlation with LREE/HREE ratios, indicating the pitchblende is REE-bearing and selectively HREE-rich. The δEu values of the ore show a positive correlation with U contents, indicating the early syn-ore fluids were oxidizing. The δCe values show a negative correlation, indicating the later mineralization environment became reducing. The water–rock interactions of the early syn-ore stage resulted in oxidization of altered granites and reduction of the ore-forming fluids, and it was this reduction that led to the uranium mineralization. During alteration in the early syn-ore stage, the oxidizing fluids leached uranium from granites close to faults, and Fe₂O₃/FeO ratios increased in the alteration zones. The late syn-ore and post-ore alteration decreased the Fe₂O₃/FeO ratios in the alteration zones. The δ¹⁸O_W–SMOW values of the ore-forming fluids range from −1.8‰ to 5.4‰, and the δD_W–SMOW values range from −104.4‰ to −51.6‰, suggesting meteoric water. The meteoric water underwent at least two stages of water–rock interaction: the first caused the fluids to become uranium-bearing, and the second stage, which was primarily associated with ore-bearing faults, led to uranium deposition as pitchblende, accompanied by CO₂ effervescence.     

Thermodynamic characterization of boltwoodite and uranophane: Enthalpy of formation and aqueous solubility

Boltwoodite and uranophane are uranyl silicates common in oxidized zones of uranium ore deposits. An understanding of processes that impact uranium transport in the environment, especially pertaining to the distribution of uranium between solid phases and aqueous solutions, ultimately requires determination of thermodynamic parameters for such crystalline materials. We measured formation enthalpies of synthetic boltwoodites, K(UO₂)(HSiO₄)·H₂O and Na(UO₂)(HSiO₄)·H₂O, and uranophane, Ca(UO₂)₂(HSiO₄)₂·5H₂O, by high temperature oxide melt solution calorimetry. We also studied the aqueous solubility of these phases from both saturated and undersaturated conditions at a variety of pH. The combined data permit the determination of standard enthalpies, entropies and Gibbs free energies of formation for each phase and analysis of its potential geological impact from a thermodynamic point of view.     

Synthesis,crystallographic data,solubility and electrokinetic properties of meta-zeunerite,meta-kirchheimerite and nickel-uranylarsenate

{M[UO₂¦AsO₄]₂ · nH₂O} with M=Cu²⁺, Co²⁺, Ni²⁺ has been synthesized from reagent grade chemicals and by ion exchange of trögerite {HUO₂AsO₄ · 4 H₂O}. Synthetic meta-zeunerite (M=Cu²⁺), meta-kirchheimerite (M=Co²⁺) and nickel-uranylarsenate are all tetragonal. The cell parameters determined from Guinier-Hägg diffraction data for {Cu[UO₂¦AsO₄]₂ · 8 H₂O} are a=b=7.10 Å and c=17.42 Å, with Z=2 and the measured density 3.70 g cm^?3. The cell parameters for {Co[UO₂¦AsO₄]₂ · 7 H₂O} and {Ni[UO²¦AsO⁴]² · 7 H₂O} are a=b=20.25 Å and c=17.20 Å, with Z=16 and the measured density 3.82 and 3.74 g cm^?3, respectively. The solubility products for synthetic Cu-, Co- and Ni-uranylarsenate at 25° C are 10^?49.20, 10^?45.34 and 10^?45.10, respectively. The zeta-potential remains negative between pH=2 and pH=9 and is strongly affected by the presence of different cations.     

Uranium-rich opal from the Nopal I uranium deposit, Peña Blanca, Mexico: Evidence for the uptake and retardation of radionuclides

The Nopal I uranium deposit of the Sierra Peña Blanca, Mexico, has been the focus of numerous studies because of its economic importance and its use as a natural analog for nuclear-waste disposal in volcanic tuff. Secondary uranyl minerals such as uranophane, Ca[(UO₂)(SiO₃OH)]₂(H₂O)₅, and weeksite, (K,Na)₂[(UO₂)₂(Si₅O₁₃)](H₂O)₃, occur in the vadose zone of the deposit and are overgrown by silica glaze. These glazes consist mainly of opal A, which contains small particles of uraninite, UO₂, and weeksite. Close to a fault between brecciated volcanic rocks and welded tuff, a greenish silica glaze coats the altered breccia. Yellow silica glazes from the center of the breccia pipe and from the high-grade pile coat uranyl-silicates, predominantly uranophane and weeksite. All silica glazes are strongly zoned with respect to U and Ca, and the distribution of these elements indicates curved features and spherical particles inside the coatings. The concentrations of U and Ca correlate in the different zones and both elements inversely correlate with the concentration of Si. Zones within the silica glazes contain U and Ca in a 1:1 ratio with maximum concentrations of 0.08 and 0.15 at.% for the greenish and yellow glazes, respectively, suggesting trapping of either Ca₁U₁-aqueous species or -particles in the colloidal silica. X-ray photoelectron spectroscopy (XPS), Fourier-transform infra-red spectroscopy (FTIR), and oxygen-isotope ratios measured by secondary-ion mass spectrometry (SIMS) indicate higher U⁶⁺/U⁴⁺ ratios, higher proportions of Si-OH groups and lower δ¹⁸O values for the greenish silica glaze than for the yellow silica glaze. These differences in composition reflect increasing brecciation, porosity, and permeability from the center of the breccia pipe (yellow silica glaze) toward the fault (green silica glaze), where the seepage of meteoric water and Eh are higher.     

Genesis of metapelitic migmatites in the Adirondack Mountains, New York

Oxygen isotope ratios and rare earth element (REE) concentrations provide independent tests of competing models of injection v. anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the Adirondack Mountains, New York. Values of δ¹⁸O and REE profiles were measured by ion microprobe in garnet–zircon pairs from 10 sample localities. Prior U–Pb SIMS dating of zircon grains indicates that inherited cores (1.7–1.2 Ga) are surrounded by overgrowths crystallized during the Grenville orogenic cycle (～1.2–1.0 Ga). Cathodoluminescence imaging records three populations of zircon: (i) featureless rounded ‘whole grains’ (interpreted as metamorphic or anatectic), and rhythmically zoned (igneous) cores truncated by rims that are either (ii) discordant rhythmically zoned (igneous) or (iii) unzoned (metamorphic or anatectic). These textural interpretations are supported by geochronology and oxygen isotope analysis. In both the amphibolite facies NW Adirondacks and the granulite facies SE Adirondacks, δ¹⁸O_(Zrc) values in overgrowths and whole zircon are highly variable for metamorphic zircon (6.1–13.4‰; n = 95, 10 μm spot). In contrast, garnet is typically unzoned and δ¹⁸O_(Grt) values are constant at each locality, differing only between leucosomes and corresponding melanosomes. None of the analysed metamorphic zircon–garnet pairs attained oxygen isotope equilibrium, indicating that zircon rims and garnet are not coeval. Furthermore, REE profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth. Thus, petrological estimates from garnet equilibria (e.g. P–T) cannot be associated uncritically with ages determined from zircon. The unusually high δ¹⁸O values (>10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ¹⁸O_(Zrc) < 10‰) indicating that these leucosomes are not injected magmas derived from known igneous rocks. Surrounding melanosomes have similarly high δ¹⁸O_(Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ¹⁸O metasedimentary rocks.     

Synthesis of In‐House Produced Calibrated Silver Phosphate with a Large Range of Oxygen Isotope Compositions

The large range of stable oxygen isotope values of phosphate‐bearing minerals and dissolved phosphate of inorganic or organic origin requires the availability of in‐house produced calibrated silver phosphate of which isotopic ratios must closely bracket those of studied samples. We propose a simple protocol to synthesise Ag₃PO₄ in a wide range of oxygen isotope compositions based on the equilibrium isotopic fractionation factor and the kinetics and temperature of isotopic exchange in the phosphate–water system. Ag₃PO₄ crystals were obtained from KH₂PO₄ that was dissolved in water of known oxygen isotope composition. Isotopic exchange between dissolved phosphate and water took place at a desired and constant temperature into PYREX? tubes that were placed in a high precision oven for defined run‐times. Samples were withdrawn at desired times, quenched in cold water and precipitated as Ag₃PO₄. We provide a calculation sheet that computes the δ¹⁸O of precipitated Ag₃PO₄ as a function of time, temperature and δ¹⁸O of both reactants KH₂PO₄ and H₂O at t = 0. Predicted oxygen isotope compositions of synthesised silver phosphate range from ?7 to +31‰ VSMOW for a temperature range comprised between 110 and 130 °C and a range of water δ¹⁸O from ?20 to +15‰ VSMOW.