Incorporation Of Minor Elements Research Articles

Incorporation mechanism of tungsten in W-Fe-Cr-V-bearing rutile

Abstract Rutile is a common mineral in many types of ore deposits and can carry chemical or isotopic information about the ore formation. For closer understanding of this information, the mechanisms of incorporation of minor elements should be known. In this work, we have investigated natural rutile crystals with elevated concentrations of WO3 (up to 17.7 wt%), Cr2O3,tot (7.5), V2O3,tot (4.1), FeOtot (7.3), and other metals. X-ray absorption spectroscopy (XAS) of rutile at the Fe K, Cr K, V K, and W L1 and L3 edges shows that all cations are coordinated octahedrally. The average oxidation state of V is +3.8, and that of Cr is near +4. Shell-by-shell fitting of the W L3 EXAFS data shows that W resides in the rutile structure. Raman spectroscopy excludes the possibility of hydrogen as a charge-compensating species. High-resolution TEM and electron diffraction confirm this conclusion as the entire inspected area consists of rutile single crystal with variable amounts of metals other than Ti. Our results show that rutile or its precursors can be efficient vehicles for tungsten in sedimentary rocks, leading to their enrichment in W and possibly later fertility with respect to igneous ore deposits. Leucoxene, a nanocrystalline mixture of Ti and Fe oxides, is an especially suitable candidate for such a vehicle.

Helvine-group minerals from Norwegian granitic pegmatites and some other granitic rocks: Cases of significant Sc and Sn contents

ABSTRACTHelvine-group minerals from two granitic pegmatites have disparate compositions, from nearly pure helvine (Ågskardet, northern Norway; Devonian) to helvine close to ternary compositions (Heftetjern, southern Norway; Precambrian). Metagranite from Høgtuva (northern Norway; Precambrian with Caledonian metamorphic overprint) contains Zn-rich danalite. The Ågskardet helvine contains up to 0.46 wt.% SnO2, and the Heftetjern ternary helvine shows a maximum of 1.74 wt.% Sc2O3. Helvine minerals were also analyzed from three occurrences connected to peralkaline granite (ekerite) of the Permian Oslo Rift. Nearly pure genthelvite (99.19 mol.%) occurs in miarolitic cavities at Gjerdingselva. Two mineralogically different granitic pegmatites derived from the same ekerite pluton in the southern part of the Oslo Rift show quite distinct helvine compositions, from nearly continuous solid solution between helvine and genthelvite in crystals with oscillatory zonation (Rundemyr) to solid solutions midway between danalite and genthelvite (Bakstevalåsen). The Rundemyr crystals have a maximum SnO2 content of 1.28 wt.%. The incorporation of minor elements (Ca, Mg, Al, Sn, Sc) in helvine-group minerals is discussed with emphasis on their chalcophilicity characteristics. For stereochemical reasons, Sn in helvine minerals must be tetravalent, even if Sn2+ is more chalcophile than Sn4+.

The Mineral-Water Interface: Where Minerals React with the Environment

The reactions that occur at the mineral–water interface are central to all geochemical processes. They affect a wide range of important Earth processes, all of which involve geochemical element cycling. Examples include weathering and soil formation, nutrient availability, biomineralization, acid mine drainage, the fate of contaminants, nuclear waste disposal, and minor element incorporation and partitioning during mineral growth. Each of these processes, and its reaction rates, is ultimately controlled by reactions that occur at mineral surfaces. Through the development of advanced analytical methods, direct observations of mineral reactions at the nanoscale have enabled exciting new possibilities for clarifying the mechanisms governing mineral–fluid reactions.

Focussed ion beam–transmission electron microscopy applications in ore mineralogy: Bridging micro- and nanoscale observations

Focussed ion beam–scanning electron microscopy (FIB–SEM) is a relatively new analytical tool that has been little applied to problems of ore genesis. The technique enables high-resolution (cross-section) imaging and can be used to prepare thinned foils for study by transmission electron microscopy (TEM). FIB–SEM methods applied to sulphides and related compounds represent an in-situ approach for sample characterisation and thus provides for crystal–chemical data that can be placed into the geological context of a given ore deposit. We present four study cases: these deal with minor element incorporation and release in ZnS; intergrowths and replacement among Cu–(Fe)-sulphides; fabrics in Au-bearing, As-free pyrite; and symplectites of Bi-sulphosalts within galena. The data is discussed in the context of polytypism and planar defects for minor element incorporation and release, superstructure ordering and formation of fine particles (100–2500 nm) or nanoparticles (< 100 nm) during replacement processes. Several analytical difficulties encountered when preparing FIB–TEM samples from sulphides are discussed, in particular mechanical and chemical damage to the surface. The FIB–TEM foils are difficult to thin for direct high-resolution TEM imaging but are usable for Scanning Transmission Electron Microscopy (STEM) and High-Angle Annular Dark Field (HAADF)-STEM imaging.

Trace and minor element variations and sulfur isotopes in crystalline and colloform ZnS: Incorporation mechanisms and implications for their genesis

Various models have been proposed to explain the formation mechanism of colloform sphalerite, but the origin is still under debate. In order to decipher influences on trace element incorporation and sulfur isotope composition, crystalline and colloform sphalerite from the carbonate-hosted Mississippi-Valley Type (MVT) deposit near Wiesloch, SW Germany, were investigated and compared to sphalerite samples from 52 hydrothermal vein-type deposits in the Schwarzwald ore district, SW Germany to study the influence of different host rocks, formation mechanisms and fluid origin on trace element incorporation. Trace and minor element incorporation in sphalerite shows some correlation to their host rock and/or origin of fluid, gangue, paragenetic minerals and precipitation mechanisms (e.g., diagenetic processes, fluid cooling or fluid mixing). Furthermore, crystalline sphalerite is generally enriched in elements like Cd, Cu, Sb and Ag compared to colloform sphalerite that mainly incorporates elements like As, Pb and Tl. In addition, sulfur isotopes are characterized by positive values for crystalline and strongly negative values for colloform sphalerite. The combination of trace element contents, typical minerals associated with colloform sphalerite from Wiesloch, sulfur isotopes and thermodynamic considerations helped to evaluate the involvement of sulfate-reducing bacteria in water-filled karst cavities. Sulfate-reducing bacteria cause a sulfide-rich environment that leads in case of a metal-rich fluid supply to a sudden oversaturation of the fluid with respect to galena, sphalerite and pyrite. This, however, exactly coincides with the observed crystallization sequence of samples involving colloform sphalerite from the Wiesloch MVT deposit.

Hydrogen and minor element incorporation in synthetic rutile

AbstractThe solubility and incorporation mechanisms of H and various trivalent and divalent cations in synthetic rutile have been investigated. Experiments performed using different bulk Fe2O3 contents demonstrate that Fe3+ substitutes onto the main Ti site, charge-balanced by oxygen vacancies. Under more reducing conditions in Fe-poor systems, the concentration of Ti interstitials in rutile is increased, resulting in a decrease in H solubility. Variation in the solubility of different oxides in rutile as a function of ionic radius implies substitution onto the main Ti site, probably charge-balanced by oxygen vacancies. To a lesser degree, substitution of trivalent and divalent cations is locally charge-balanced by H incorporation. Variation in OH-stretching frequencies in infrared spectra as a function of composition implies that octahedral defects and structurally-incorporated H are coupled. However, in all samples, some of the H is also decoupled from substitutional impurities, as is evident from an OH-absorption band at 3279 cm–1. This band corresponds to the main OH band seen in spectra of many natural rutiles, implying that in most rutiles, H defects are decoupled from substitutional defects.

Early marine diagenesis of biogenic aragonite and Mg-calcite: new constraints from high-resolution STEM and AEM analyses of modern platform carbonates

Prior studies of porewater compositions from modern shallow marine carbonates have established that dissolution and recrystallization of metastable aragonite and Mg-calcites occur during the earliest stages of sedimentation. Yet, significant microstructural and mineralogical changes in sediment allochems have not been observed. In part, this is due to the high resolution required to identify diagenetic changes in these finely crystalline materials. Herein, we present ultrastructural and microgeochemical evidence for early marine diagenesis using scanning electron microscopy (SEM) and high resolution scanning transmission/analytical electron microscopy (STEM/AEM) methods to characterize original biogenic materials and sediment boxcores from modern deposits on the Floridan–Bahamas platform. We selected the two most common sediment producers, a Mg-calcite foraminifer, Peneropolid, and an aragonitic green algae, Halimeda, for study. Both are composed of submicron-sized crystallites, whose excess free energy could provide a thermodynamic driving force for recrystallization. STEM observations of grain mounts containing dispersed Mg-calcite and aragonite crystallites from gently disaggregated individual Peneropolids and Halimeda segments indicate that living specimens contain euhedral prismatic crystals with blunt terminations and smooth faces. Specimens from the sediment column show abundant evidence for alteration. The most dramatic changes involve crystallite dissolution and precipitation of thin overgrowths, causing the overall aspect ratio of crystallites to shorten and widen. Direct measurements of several hundred individual crystallites confirm that crystallite length and width distributions differ significantly between original and altered materials. These changes are consistent with crystal-growth processes that tend to minimize surface free energy by increasing overall crystal size and by producing more equant shape. The precipitated material has a composition and structure which is grossly similar to the host crystallite, as determined by AEM and electron diffraction analyses. AEM analyses of hundreds of Mg-calcite crystallites from living specimens of Peneropolid foraminifera indicate a composition of ∼13–14 mol% MgCO 3, whereas crystallites from altered tests are ∼1–3 mole% lower. Taken together, these data suggest that early diagenesis in shallow marine carbonates is indeed manifested in the solids as well as the sediment pore waters, producing significant changes in biogenic Mg-calcite and aragonite crystallite size and shape. This alteration would not produce changes in the mineral assemblage because dissolution is accompanied by precipitation of compositionally similar overgrowths, under major element conditions not drastically dissimilar from overlying seawater. However, crystallite recrystallization likely has implications for incorporation of minor elements which undergo cycling via organic matter decomposition or redox changes (e.g., uranium, cadmium, manganese, rare earth elements).

Isotopic and Elemental Tracers in Calcium Carbonate Fossils

Calcareous fossils are used for an incredible variety of purposes in paleoceanographi c research: elemental and isotopic temporal variability in the ocean and geochronology. Apart from the major components themselves - calcium, carbon and oxygen, the latter two of which provide opportunities for stable isotope studies all of these tracers are incorporated into calcareous fossils as minor elements substituting for calcium or carbonate ion. The basic structural principles of minor element incorporation into

Crystal-liquid partition coefficients for pyroxene, spinels, and melilite, in slags

AbstractThe chemistry of spinels, plagioclase, and iron-rich, normally zoned pyroxene and melilite from some metallurgical slags has been investigated by electron microprobe analysis. Minor element partition coefficients, some of which are composition-dependent, have been calculated from the analytical data. The pyroxene/liquid partition coefficients range from 4.03 to 0.03 in the order Ti/Zr, Al, Mn, Zn, Ba, Sn, Na, reflecting increasing incompatibility. The spinel/liquid partition coefficients range from 40.89 to 0.02 but in the order Ti, Ni, Mg/Zn, Al, Mn, Cu, Sn, Zr. However, Sn becomes very compatible when the pyroxene and spinels crystallize in association with cassiterite. Melilite greatly discriminates against the incorporation of minor elements into its crystal structure. The crystallization of these phases produces residual liquids enriched in Na, Mn, Zn, Sn, Ba, W, and Pb.