Showing papers on "Silicate minerals published in 2000"

The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3

Abstract: Mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (KFMASHTO) using thermocalc and its internally consistent thermodynamic dataset constrain the effect of TiO2 and Fe2O3 on greenschist and amphibolite facies mineral equilibria in metapelites. The end-member data and activity–composition relationships for biotite and chloritoid, calibrated with natural rock data, and activity–composition data for garnet, calibrated using experimental data, provide new constraints on the effects of TiO2 and Fe2O3 on the stability of these minerals. Thermodynamic models for ilmenite–hematite and magnetite–ulvospinel solid solutions accounting for order–disorder in these phases allow the distribution of TiO2 and Fe2O3 between oxide minerals and silicate minerals to be calculated. The calculations indicate that small to moderate amounts of TiO2 and Fe2O3 in typical metapelitic bulk compositions have little effect on silicate mineral equilibria in metapelites at greenschist to amphibolite facies, compared with those calculated in KFMASH. The addition of large amounts of TiO2 to typical pelitic bulk compositions has little effect on the stability of silicate assemblages; in contrast, rocks rich in Fe2O3 develop a markedly different metamorphic succession from that of common Barrovian sequences. In particular, Fe2O3-rich metapelites show a marked reduction in the stability fields of staurolite and garnet to higher pressures, in comparison to those predicted by KFMASH grids.

Barite–Celestine Geochemistry and Environments of Formation

Abstract: Minerals in the barite (BaSO4)–celestine (SrSO4) solid solution series, (Ba,Sr)SO4, occur in a remarkably diverse range of sedimentary, metamorphic, and igneous geological environments which span geological time from the Early Archean (~3.5 Ga) to the present. The purpose of this chapter is to review: (1) the controls on the chemical and isotopic composition of barite and celestine and (2) the geological environments in which these minerals form. Some health risks are associated with barite, and these are discussed near the end of this chapter. Although complete solid solution exists between BaSO4 and SrSO4 most representatives of the series are either distinctly Ba-rich or Sr-rich. Hence, it is convenient to use the term barite to refer to not only the stochiometric BaSO4 endmember but also to those (Ba,Sr)SO4 solid solutions dominated by Ba. Similarly, the term celestine will refer here not only to the stoichiometric SrSO4 endmember but to solid solutions dominated by Sr. Such usage is in accord with standard mineral nomenclature. The Committee on Mineral Names and Nomenclature of the International Mineralogical Association recognizes “celestine” as the official name for SrSO4. However, the name “celestite” is still commonly used in the literature. ### Geological significance of barite and celestine Most of the barite which exists in the Earth’s crust has formed through the mixing of fluids, one containing Ba leached from silicate minerals, and the other an oxidized shallow fluid, such as seawater, which contains sulfate. Large deposits of barite represent areas of focused fluid flow and mineral precipitation and thus aid in the reconstruction of the hydrogeologic history of the Earth’s crust. The stability of barite is redox sensitive, and the presence or absence of this mineral helps to constrain interpretation of paleoredox conditions. Pelagic barite, the precipitation of which is biologically mediated, may …

Surface area and porosity of primary silicate minerals

Abstract: Surface area is important in quantifying mineral-water reaction rates. Specific surface area (SSA) was measured to investigate controls on this parameter for several primary silicate minerals (PSM) used to estimate rates of weathering. The SSA measured by gas adsorption for a given particle size of relatively impurity-free, laboratory-ground samples generally increases in the order: quartz ≈ olivine ≈ albite 4000 cm2/g) and values measured with N2 were observed to be up to 50% larger than values measured with Kr. For laboratory-ground Amelia albite and San Carlos olivine, SSA can be calculated using log (SSA , cm2/g) = b + m log ( d ), where d = grain diameter (μm), b = 5.2 ± 0.2 and m = −1.0 ± 0.1. A similar equation was previously published for laboratory-ground quartz. Some other samples showed SSA higher than predicted by these equations. In some cases, high SSA is attributed to significant second phase particulate content, but for other laboratory-ground samples, high SSA increased with observed hysteresis in the adsorption-desorption isotherms. Such hysteresis is consistent with the presence of pores with diameters in the range 2 to 50 nm (mesopores). In particular, porosity that contributes to BET-measured SSA is inferred for examples of laboratory-ground diopside, hornblende, and all compositions of plagioclase except albite, plus naturally weathered quartz, plagioclase, and potassium feldspar. Previous workers documented similar porosity in laboratory-ground potassium feldspar. Surface area measured by gas adsorption may not be appropriate for extrapolation of interface-limited rates of dissolution of many silicates if internal surface is present and if it does not dissolve equivalently to external surface. In addition, the large errors associated in measuring SSA of coarse and/or impurity-containing silicates suggest that surface area-normalized kinetics in both field and laboratory systems will be difficult to estimate precisely. Quantification of the porosity in laboratory-ground and naturally weathered samples may help to alleviate some of the discrepancy between laboratory- and field-based estimates of weathering rate.

Factors influencing the release of plant nutrient elements from silicate rock powders: a geochemical overview.

Abstract: Rock-forming minerals of igneous and metamorphic rocks contain most of the nutrients required by higher plants for growth and development. Ground rock fertilisers may provide a source of nutrients to depleted topsoils where bulk soil solutions are not in equilibrium with fresh primary minerals. Slow dissolution rates of silicate minerals may inhibit the use of rock powders in agriculture unless suitable soils are identified and optimum rock powder properties developed. This review identifies previous research where the agronomic effectiveness of ground rock fertilisers has been evaluated. There are many contradictory findings that need to be evaluated by reference to basic geochemical knowledge. Geochemical studies of mineral dissolution indicate the general reaction pathways by which nutrients are released, assuming that equilibrium between the soil solution and primary mineral is achieved. In soils, mineral dissolution is enhanced by disequilibrium between soil solution and mineral surfaces through the removal of ions by processes such as leaching and nutrient uptake. Rhizosphere processes and other biological activity may further enhance mineral dissolution through the release of H-ions and complexing organic compounds which react with mineral surfaces. Geochemical principles can be used to predict some of the reactions that occur when ground silicate minerals are added to soils as mineral fertilisers. A range of weathering rates for minerals have been identified in the laboratory and the field and are dependent on physical, mineralogical and biogeochemical factors. The rate limiting steps may be those that involve reactions between the soil solution and mineral surface. Dissolution primarily occurs at defects at the mineral surfaces and an understanding of these surface reactions may lead to preparative procedures to enhance nutrient release from the mineral surface. Normalising the release rates of nutrients to a unit surface area basis can aid in predicting nutrient release during dissolution from various ground rock materials. Identifying the relationships between release rates of minerals and plant uptake is vital to developing an understanding the effectiveness of rock dust applied to vegetated soils.

Ca/Sr and 87Sr/86Sr geochemistry of disseminated calcite in Himalayan silicate rocks from Nanga Parbat: Influence on river-water chemistry

Abstract: Trace amounts of disseminated calcite were identified in gneiss, schist, and granite bedrock sampled from the Raikhot watershed and other locations within the Nanga Parbat massif of northern Pakistan. The calcite grains occur interstitially within individual silicate minerals, at grain boundaries, and as fracture fillings that transect the mineralogic fabric of the rock. Disseminated calcite composes is ≤ 0.29 wt% of silicate rocks sampled in the Raikhot watershed and has Ca/Sr (µmol/nmol) and 87Sr/86Sr ratios that range from 0.878 to 5.33 and from 0.794 039 to 0.930 619, respectively. Elsewhere in the Nanga Parbat region, disseminated calcite composes ≤ 3.6 wt% of the silicate rock samples and has Ca/Sr (µmol/nmol) and 87Sr/86Sr ratios that range from 0.535 to 3.33 and from 0.715 757 to 0.771 244, respectively. For all samples, the 87Sr/86Sr ratios of disseminated calcite are similar to the 87Sr/86Sr ratios measured in the silicate host rock. Within the partially glaciated Raikhot watershed, the rapid weathering of disseminated calcite with high Ca/Sr and 87Sr/86Sr ratios has a strong influence on the chemical composition of stream water and exceeds contributions from silicate mineral dissolution. Comparisons of disseminated calcite compositions with source-water chemistry throughout the Himalaya suggest that disseminated calcite may be a more important component of Himalayan silicate rocks than previously recognized. Therefore, calculations relating the Sr isotope geochemistry of Himalayan rivers to atmospheric CO2 consumption should consider this widespread and compositionally variable carbonate end member.

Comment on "Abiotic controls on H2 production from basalt-water reactions and implications for aquifer biogeochemistry".

Abstract: Abiotic production of H{sub 2} from basalt reactions in aqueous solutions is hypothesized to support microbial ecosystems in deep subsurface aquifers, such as those found in the Columbia River Basalt group (CRB). The authors investigated factors controlling this phenomenon, including rock composition, pH, temperature, sterilization method, reducing agents, and product removal. Ferrous silicate minerals were found to be the basalt components responsible for H{sub 2} production from anaerobic water-rock interactions. H{sub 2} evolution was faster at pH < 7 but occurred from pH 5 to pH 11, which covers the pH range (7--10) measured in CRB groundwaters. The onset of H{sub 2} evolution coincided with the appearance of dissolved Fe{sup 2+}, and an apparent alkaline inhibition could be alleviated by the addition of excess FeCl{sub 2}. This may reflect, in part, the low redox conditions needed for H{sub 2} evolution and suggests that H{sub 2} may be controlled by reaction rates of ferrous silicate minerals. Rates were higher at 60 C than at 30 C, suggesting that the geothermal gradient may lead to increased H{sub 2} production at depth. The results were consistent with the hypothesis at depth. The results were consistent with the hypothesis that basalt-redox reactions support primarymore » production by microorganisms in some terrestrial subsurface environments.« less

Carbon dioxide sequestration by direct mineral carbonation with carbonic acid

Abstract: The Albany Research Center (ARC) of the U.S. Dept. of Energy (DOE) has been conducting a series of mineral carbonation tests at its Albany, Oregon, facility over the past 2 years as part of a Mineral Carbonation Study Program within the DOE. Other participants in this Program include the Los Alamos National Laboratory, Arizona State University, Science Applications International Corporation, and the DOE National Energy Technology Laboratory. The ARC tests have focused on ex-situ mineral carbonation in an aqueous system. The process developed at ARC utilizes a slurry of water mixed with a magnesium silicate mineral, olivine [forsterite end member (Mg2SiO4)], or serpentine [Mg3Si2O5(OH)4]. This slurry is reacted with supercritical carbon dioxide (CO2) to produce magnesite (MgCO3). The CO2 is dissolved in water to form carbonic acid (H2CO3), which dissociates to H+ and HCO3 -. The H+ reacts with the mineral, liberating Mg2+ cations which react with the bicarbonate to form the solid carbonate. The process is designed to simulate the natural serpentinization reaction of ultramafic minerals, and for this reason, these results may also be applicable to in-situ geological sequestration regimes. Results of the baseline tests, conducted on ground products of the natural minerals, have been encouraging. Tests conductedmore » at ambient temperature (22 C) and subcritical CO2 pressures (below 73 atm) resulted in very slow conversion to the carbonate. However, when elevated temperatures and pressures are utilized, coupled with continuous stirring of the slurry and gas dispersion within the water column, significant reaction occurs within much shorter reaction times. Extent of reaction, as measured by the stoichiometric conversion of the silicate mineral (olivine) to the carbonate, is roughly 90% within 24 hours, using distilled water, and a reaction temperature of 185?C and a partial pressure of CO2 (PCO2) of 115 atm. Recent tests using a bicarbonate solution, under identical reaction conditions, have achieved roughly 83% conversion of heat treated serpentine and 84% conversion of olivine to the carbonate in 6 hours. The results from the current studies suggest that reaction kinetics can be improved by pretreatment of the mineral, catalysis of the reaction, or some combination of the two. Future tests are intended to examine a broader pressure/temperature regime, various pretreatment options, as well as other mineral groups.« less

Complexities on the acapulcoite-lodranite parent body: Evidence from trace element distributions in silicate minerals

Abstract: — The trace element distributions of individual minerals from seven acapulcoites and lodranites have been studied. Systematic differences are evident between some members of the two groups. Specifically, pyroxenes from the lodranites MacAlpine Hills (MAC) 88177 and Lewis Cliff (LEW) 88280 exhibit depletions of the rare earth elements (REE) and other incompatible trace elements (Ti, Zr, Y), relative to acapulcoite (Acapulco, Allan Hills (ALH) A81261) pyroxenes, that are consistent with the formation and removal of 15% or more silicate partial melts from these meteorites. Phosphate REE patterns in these lodranites also support this scenario. However, other members of the acapulcoite-lodranite clan exhibit more complex trace element variations. Elephant Moraine (EET) 84302, which has been classified as transitional between the acapulcoites and lodranites, generally has trace element characteristics similar to the acapulcoites. However, its plagioclase REE compositions suggest a somewhat greater degree of metamorphism than that experienced by acapulcoites such as Acapulco and ALHA81261. Similar and elevated REE abundances in the silicate phases from acapulcoite ALHA81187 and lodranite Graves Nunataks (GRA) 95209 suggest that these two meteorites, in fact, experienced similar thermal histories. This probably included some silicate partial melting, although little melt appears to have been lost from the samples. The observed variations in the trace element abundances of these samples from the acapulcoite-lodranite clan emphasize the complex and varied processes that have acted on their parent body. The simple bimodal classification of these meteorites based primarily on petrographic criteria, which has been used to date, appears to be inadequate to describe this diverse group of samples, as they represent a range of degrees of partial melting, both with and without accompanying melt migration. In some instances, secondary processes on the parent body, such as cryptic metasomatism, have further modified sample compositions.

Remobilization in the cratonic lithosphere recorded in polycrystalline diamond

Abstract: Polycrystalline diamonds (framesites) from the Venetia kimberlite in South Africa contain silicate minerals whose isotopic and trace element characteristics document remobilization of older carbon and silicate components to form the framesites shortly before kimberlite eruption. Chemical variations within the garnets correlate with carbon isotopes in the diamonds, indicating contemporaneous formation. Trace element, radiogenic, and stable isotope variations can be explained by the interaction of eclogites with a carbonatitic melt, derived by remobilization of material that had been stored for a considerable time in the lithosphere. These results indicate more recent formation of diamonds from older materials within the cratonic lithosphere.

Leaching behaviour of rock material and slag used in road construction ‐ amineralogical interpretation

Abstract: Rock materials used in road construction contain heavy metal elements bound in minerals that are more or less soluble. There are no requirements for investigations of leaching behaviour before use of rock materials in Sweden, which is the case regarding other materials as, e.g., slags. This implies that there is a lack of data to be used when other materials are evaluated. Seven rock materials and two gravels representing non-weathered material for use in base or sub-base course from three counties in Sweden have been investigated regarding mineral composition in order to explain the leaching behaviour. Microscopic studies of the mineral composition, acid-base-accounting and pH-measurements have been used to explain the leaching results achieved with the availability test. The identified transparent minerals were the expected silicate minerals for the sampled rock-forming materials. Overall, the content of identified opaque minerals was low. How an element is bound in the mineral is decisive for the dissolution of the heavy metal elements. Sulphide bound elements have a notably high fraction that is soluble, especially under oxidising conditions. Chromium and vanadium present as substituted ions in the crystal lattice of oxides are not dissolved. The dissolution of the buffering rock forming silicates is much slower than the dissolution of the acid-producing sulphides. The results have been compared to similar leaching tests of metallurgical slag used in road construction. The dissolution of the major phase, the solubility of the heavy metal mineral and secondary reactions are factors influencing the dissolved amounts of heavy metal elements. Compared to the crystalline rock materials, the amorphous fuming slag from a copper smelter has very low solubility, while blast furnace slag is easily dissolved due to hydrolysis. The soluble amounts of sulphide bound elements in rock material is higher compared to blast furnace slag. The kinetics of the acid-producing and acid-consuming reactions of the rock materials needs to be further investigated. The blast furnace slag and the fuming slag can be used in road construction without any risk of harmful environmental impact due to heavy metal leaching.

Fluid regime of platinum group elements (PGE) and gold-bearing reef formation in the Dovyren mafic–ultramafic layered complex, eastern Siberia, Russia

Abstract: The Dovyren layered dunite–troctolite–gabbro massif (northern Transbaikalia region, Russia) contains precious metal mineralization related to sparsely disseminated sulfides (Stillwater type). The distribution of gases trapped in micro-inclusions and intergranular pores of the Dovyren massif has been investigated. This type of study had previously only been undertaken on the traps or peridotite–pyroxenite–norite intrusions hosting copper–nickel sulfide deposits. A novel method of analyzing trapped gases, involving the grinding of samples under high vacuum at room temperature, was employed. A modified gas-chromatography and mass-spectrometry approach was used to analyze the composition of the extracted gases. The concentrations of reduced gases (CH4 and H2) are higher in inclusions trapped by silicate minerals, whereas oxidized gases (H2O, CO2) are less common. The content of reduced gases (H2, CH4, CO), N2, He, radiogenic Ar, and C2H6 increases upward through the layered series of the massif. The distribution of all gases, especially methane and hydrogen, show peak concentrations coincident with the PGE and gold reef type horizons. A correlation of the gas peaks and noble metal contents appears to be related to their geochemical affinities. This conclusion is supported by the experimental modeling.

The crystal structure of TlAlSiO4: The role of inert pairs in exclusion of Tl from silicate minerals

Abstract: Thallium aluminosilicate, TlAlSiO 4 , synthesized hydrothermally is monoclinic with space group P 2 1 / n [ a = 5.4095(3), b = 9.4232(7), c = 8.2629(6) A, γ = 90.01(2) o , V = 421.20(6) A 3 , Z = 4]. The crystal structure was refined to an R index of 3.8% based on 1852 observed unique reflections. The compound is a unique framework silicate with a topology similar to that of the tridymite structure. The TlO 8 polyhedron resembles a truncated rectangular pyramid, and shares its edges with three adjacent AlO 4 tetrahedra, three SiO 4 tetrahedra, and six TlO 8 polyhedra. Local understaturation at the Tl position suggested by bond-valence analysis implies that lone-pair electrons are present. The geometrical data indicate that the inert pair causes distortion of the Tl-polyhedron. Polyhedral distortion analysis using the software IVTON places the lone-pair parallel to [010], pointing to the largest base of Tl polyhedron. The rule in the valence shell electron pair repulsion model that a nonbonding pair occupies more space on the “surface” of the central atom than a bonding pair supports the orientation of inert-pair electrons in thallium provided by IVTON. The remarkable structure distortion caused by the inert-pair effect explains the rarity of Tl as a major element in silicate minerals because these cannot accommodate extremely distorted polyhedra. In contrast, about forty species of Tl-sulfide minerals exist because these structures are more flexible. Furthermore this effect probably explains why atoms such as Ge 2+ , Pb 2+ , Sn 2+ , Sb 3+ , and Bi 3+ , crystallize not as silicate phases but mainly as sulfide ones in nature.

Fe Oxidizing Bacteria and the Weathering of Fe Silicate Minerals

Abstract: This study was designed to determine whether chemolithotrophic bacteria that derive “ metabolic energy from iron oxidation can be sustained by the flux of ferrous iron released by dissolution of fayalite (Fe2Si04) and to evaluate the impact of metabolic products on the dissolution kinetics. When fayalite was dissolvqd abiotically, almost all dissolved iron remained as Fe2+over the duration of the experiment. However, in the presence of l%iobacihsferrooxidans aqueous iron was oxidized to Fe3+.Dead cells did not induce si-gificant ferrous iron oxidation and had minimal effect on the reaction rate. These results confirm that T.ferrooxidans was active in our experiments and can be sustained by fayalite dissolution. In the presence of Z ferrooxidam the total Fe accumulated in solution was only 5 to 50% of that in abiotic experiments. Silicon was not strongly incorporated into secondary minerals. Thus, Si release rates can be used to evaluate fayalite dissolution rates. The data indicate inhibition of fayalite dissolution in the presence of microorganisms. TEM and SEM analyses confirm that fayalite in the biotic experiments was much less extensively reacted than in the organism-free experiments. However, some precipitation of secondary iron oxyhydroxide minerals, especially in . proximity to cells, leads to solutions with different stoichiometry than that of fayalite. The iron oxyhydroxides occur as aggregates of few nanometer diameter particles. These are unlikely to suppress the reaction rate via diffusion inhibition. Consequently, abiotic experiments were conducted to test the hypothesis that the ferric iron byproduct of $ microbial metabolism was responsible for the decreased dissolution rates in biological experiments. .4ddition of aqueous Fe3+inhibited both silica and iron release by

Iron oxidation in sediment cores (Site 1062) during six months of storage in the Ocean Drilling Program archive

Abstract: Changes in bulk sediment Fe(II)/Fe(III) ratio and in the distribution of iron among different minerals as a result of Ocean Drilling Program archive storage in the Bremen Core Repository were investigated using Mossbauer spectroscopy. Massive Fe(II) to Fe(III) oxidation, which involved between 24% and 45% of the initial Fe(II), occurred within only 6 months of refrigerated storage. Prior to archive storage, >95% of the Fe(II) in the sediment samples under investigation was structural iron in silicate minerals. Hence, virtually the entire oxidation process took place within silicate mineral lattices, and the sediment mineral assemblage was not changed in this case. Nevertheless, the observed oxidation of the comparatively shielded silicate lattice Fe(II) suggests that Fe(II) bound in authigenic carbonates, phosphates, or sulfides—such as that found in many marine sediments—would likely be oxidized at least as fast. Those minerals, however, would be replaced by Fe(III)-bearing oxides and oxyhydroxides, which implies a change of sediment composition, and thus, of various sediment properties, including the magnetic signal, within a few months of storage. Furthermore, changes in the silicate lattice Fe(II)/Fe(III) ratio during storage, such as those reported here, also signify loss of information. This is because oxidation of the structural Fe(II) upon contact with atmospheric oxygen may occur only inasmuch as the inverse Fe(III)–Fe(II) redox transition has taken place in the seabed. Therefore, the reversible shift, if it were measured under controlled reoxidation in the laboratory, may suggest the chemical stress that was suffered by the iron oxide minerals at the ocean bottom. Concerning Site 1062, this process might help to judge both the authenticity of magnetic field excursion records and the lithostratigraphic value of red lutites at given sediment depths. Although the nature and extent of information loss or alteration during storage depend on sediment type, the reported observations emphasize the need for special sample protection with respect to properties that might be affected.

Solution Chemistry of Flotation Separation of Diaspore-type Bauxite (I) Crystal Structure and Floatability

Abstract: The crystal structure and surface property of kaolinite, illite and pyrophyllite have been discussed The influence of residual bond, lattice impurities and grinding behavior on wettability, electrokinetics and floatability have been studied based on crystal chemistry and solution chemistry of flotation The number of breaking bonds per unit area of carystal plane lies in the following order: N Si-O(110) N Si-O(010) N Si-O(100) ,and N Al-O(110) N Al-O(010) N Al-O(100) in the three minerals This accounts for the anisotropy of wettability and electrokinetics of different crystal planes of the three minerals The difference in crystal structure between diaspore and aluminury silicate minerals results in the difference in residual bond and surface active site affecting their wettability and floatability

Partitioning of Actinides, Rare Earth Elements, and Other Trace Elements In Titanium-Rich Veins From Adamello, Italy

Abstract: Extensive mineralogical and chemical studies have been carried out on the Ti-rich hydrothermal veins emplaced within the contact aureole of the Adamello batholith. In addition to other actinide and rare earth element host phases, the veins contain both zirconolite and betafite and provide information relevant to ceramic wasteforms designed for the disposal of actinide-rich nuclear wastes. In this paper, we describe the results of element partitioning studies based on dissolution experiments using 9M HCl. Generally, the acid-resistant minerals include allanite, baddeleyite, betafite, chalcopyrite, geikielite, titanite, spinel, and zirconolite. We also found that the major silicate minerals forsterite, phlogopite, and titanian clinohumite and the sulfide mineral pyrrhotite are partially dissolved by the acid treatment, whereas calcite and apatite are highly soluble (as expected). In particular, the distributions of Th and U between the acid-resistant and acid-soluble fractions indicate that they partition mainly between zirconolite, titanite, betafite, and apatite. However, there is a considerable increase in the amounts of Zr, Nb, Th, and U released in certain actinide-rich samples that may result from enhanced dissolution following radiation damage.

Oxidative alteration of spent fuel in a silica-rich environment: SEM/AEM investigation and geochemical modeling

Abstract: Correctly identifying the possible alteration products and accurately predicting their occurrence in a repository-relevant environment are the key for the source-term calculation in a repository performance assessment. Uraninite in uranium deposits has long been used as a natural analog to spent fuel in a repository because of their chemical and structural similarity. In this paper, a SEM/AEM investigation has been conducted on a partially alternated uraninite sample from a uranium ore deposit of Shinkolobwe of Congo. The mineral formation sequences were identified: uraninite {yields} uranyl hydrates {yields} uranyl silicates {yields} Ca-uranyl silicates or uraninite {yields} uranyl silicates {yields} Ca-uranyl silicates. Reaction-path calculations were conducted for the oxidative dissolution of spent fuel in a representative Yucca Mountain groundwater. The predicted sequence is in general consistent with the SEM observations. The calculations also show that uranium carbonate minerals are unlikely to become major solubility-controlling mineral phases in a Yucca Mountain environment. Some discrepancies between model predictions and field observations are observed. Those discrepancies may result from poorly constrained thermodynamic data for uranyl silicate minerals.