Showing papers on "Silicate minerals published in 1996"

Pore wall of a mesoporous molecular sieve derived from kanemite

Abstract: The pore diameters and pore wall thicknesses of the mesoporous molecular sieves FSM-16 derived from a layered silicate (kanemite) were estimated by X-ray diffraction, physisorption, transmission electron micrographs, 29Si MAS NMR, and modeling and simulation. The XRD simulation with a simple model of a hexagonal array of cylinders gave reasonable values coinciding with those obtained by TEM and nitrogen adsorption, with some corrections. As-synthesized FSM-16 material had pore walls 0.4 nm thick, corresponding to the slightly less wrinkled single SiO4 tetrahedral layer observed in the crystalline layered silicate minerals, and an apparent pore diameter of 4.2 nm. Surfactant-free FSM-16, formed by calcination and exchange for H+, had thicker pore walls of 0.8 and 0.9 nm corresponding to double SiO4 tetrahedral layers and pore diameters of 3.4 and 3.7 nm.

Structural characterization of pentacoordinate silicon in a calcium silicate

Abstract: IN silicate minerals formed at pressures typical of the Earth's crust, the silicon is usually coordinated by four oxygen atoms. In contrast, silicates formed at higher pressures, typical of the Earth's transition zone and lower mantle, contain predominantly six-coordinated silicon. Silicon coordinated by five oxygen atoms is not normally found as a structural element in crystalline phases, but is nevertheless believed to play a central role in many dynamic processes that occur in silicates. For example, pentacoordinate silicon is probably a component of aluminosili-cate melts and glasses at mantle temperatures and pressures1,2, where it will dominate their transport properties3–6; it is also believe to act as an intermediate activated state during oxygen diffusion in silicate minerals7,8. Here we report the complete structure determination of an inorganic crystalline silicate—CaSi2O5—containing SiO5 groups. Our results confirm the previous attribution1,2,9 of peaks in the 29Si NMR spectrum of this material to the presence of pentacoordinate silicon, and the detailed geometry that we determine for the SiO5 group should provide a firm basis for characterizing and quantifying the role of pentacoordinate silicon in silicate melts and glasses.

Geological, mineralogical and geochemical characteristics of zeolite deposits associated with borates in the Bigadiç, Emet and Kirka Neogene lacustrine basins, western Turkey

Abstract: The Bigadic, Emet and Kirka lacustrine basins of western Turkey may be considered as Tibet-type graben structures that were developed during the Miocene within the Izmir-Ankara suture zone complex. The volcanic-sedimentary successions of these basins are made up of mudstone, carbonate (limestone and dolomite) and detrital rocks, and also of crystal or vitric tuffs about 135 to 200 m thick. The Degirmenli (Bigadic), Emirler (Bigadic) Kopenez (Emet) and Karaoren (Kirka) tuffs constituting the zeolite deposits are situated beneath four borate deposits (colemanite, ulexite, borax). The most abundant diagenetic silicate minerals are K- and Ca-clinoptilolites in the zeolite deposits, and Li-rich trioctahedral smectites (stevensite, saponite and hectorite) and K-feldspar in the borate deposits. In the Degirmenli, Emirler. Kopenez and Karaoren deposits, the following diagenetic facies were developed from rhyolitic glasses rich in K and poor in Na: (glass + smectite), (K-clinoptilolite + opal-CT), (Caclinoptilolite + K-feldspar ± analcime ± quartz) and (K-feldspar+analcime+quartz). K-feldspar which is also rarely associated with phillipsite (Karaoren) and heulandite (Degirmenli and Karaoren), succeeds clinoptilolite and precedes analcime in these diagenetic facies where dioctahedral smectites, opal-CT and quartz are the latest minerals. No diagenetic transformations exist between clinoptilolite, K-feldspar and analcime that were formed directly from glass. The lateral facies distributions resulted from the differences in salinity and pH of pore water trapped during deposition of the tuffs, but vertical distributions in vitric tuffs seem to have been controlled by the glass/liquid ratio of the reacting system and the permeability or diffusion rate of alkali elements. The Bigadic, Emet and Kirka zeolite deposits which were formed in saline basins rich in Ca and Mg ions, show similar chemical changes, i.e. loss of alkalis and gain in alkaline-earth elements that have taken place during the diagenetic transformation of rhyolitic glasses to dioctahedral smectites or clinoptilolite. The absence of sodic zeolites such as mordenite, erionite, chabazite and silica-rich phillipsite is mainly due to the very high K/Na ratio of the starting materials rather than initial alkaline conditions or high Na content in lake waters.

Carbon isotope exchange during polymetamorphism in the Panamint Mountains, California, USA

Abstract: Carbon isotope fractionations between calcite and graphite in the Panamint Mountains, California, USA, demonstrate the importance of mass balance on carbon isotope values in metamorphosed carbon-bearing minerals while recording the thermal conditions during peak regional metamorphism. Interbedded graphitic marbles and graphitic calcareous schists in the Kingston Peak Formation define distinct populations on a δ13C(gr)–δ13C(cc) diagram. The δ13C values of both graphite and calcite in the marbles are higher than the values of the respective minerals in the schists. δ13C values in both rock types were controlled by the relative proportions of the carbon-bearing minerals: calcite, the dominant carbon reservoir in the marble, largely controlled the δ13C values in this lithology, whereas the δ13C values in the schists were largely controlled by the dominant graphite. This is in contrast to graphite-poor calcsilicate systems where carbon isotope shifts in carbonate minerals are controlled by decarbonation reactions. The marbles record a peak temperature of 531±30 °C of a Jurassic low-pressure regional metamorphic event above the tremolite isograd. In the schists there is a much wider range of recorded temperatures. However, there is a mode of temperatures at c. 435 °C, which approximately corresponds to the temperatures of the principal decarbonation metamorphic reactions in the schists, suggesting that the carbon exchange was set by loss of calcite and armouring of graphite by newly formed silicate minerals. The armouring may explain the relatively large spread of apparent temperatures. Although the modal temperature also corresponds to the approximate temperature of the Cretaceous retrograde event, retrograde exchange is thought less likely due to very slow exchange rates involving well-crystallized graphite, armouring of graphite by silicates during the earlier event, and because of other barriers to retrograde carbon exchange. Thus, only the calcite–graphite carbon isotope fractionations recorded by the marbles demonstrate the high-temperature conditions of the low-pressure Jurassic metamorphic event that was associated with the emplacement of granitic plutons to the west of the Panamint Mountains.

Intrinsic and external controls on the incorporation of rare-earth elements in calc-silicate minerals

Abstract: The rare-earth elements (REE) in twenty nine calc-silicate minerals, mostly of metamorphic or hydrothermal origin, and catapleiite have been analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). Using pure mineral separates, very low detection limits for REE in minerals (3 to 54 ppb) are achieved, and REE patterns for some minerals with low abundances of REE are reported for the first time. The chondrite-normalized REE patterns of the calc-silicate minerals vary significantly. Although external geochemical factors are important in determining the REE characteristics of metamorphic and hydrothermal calc-silicate minerals, crystal-chemical effects are resolved also. A consideration of stereochemical environments of the Ca site(s) allows the formulation of qualitative rules for selectivity of REE and site occupancy in calc-silicate minerals. The selectivity of REE is controlled largely by the size of the Ca position: in calc-silicate minerals with multiple Ca sites, bond-valence calculation is a powerful first-approximation technique for predicting the site preference of REE.

Surface Chemistry and Biological Pathogenicity of Silicates: An X-Ray Photoelectron Spectroscopic Study

Abstract: We extend our electron spectroscopy for chemical analysis studies of the chemistry of silicates to provide direct surface chemical information on the interactions involved in silicate-induced lung and tissue pathology. A total of five fibrous and non-fibrous silicate minerals, primarily amphiboles, have been studied: anthophyllite, tremolite, cummingtonite, hornblende and actinolite. We have followed the `inlattice9 surface chemistry of these materials and monitored features such as the simultaneous presence of four- and six-coordinate (with respect to oxygen) structural aluminium, and the presence of iron in the M4 octahedral positions. In vitro experiments involving contact of the silicate with cultured murine Ehrlich cells have identified modifications in the surface chemistry of Al, Mg and Fe in the silicates and changes in cellular iron content.

Aqueous chemistry of major ions and trace metals in the Clutha River, New Zealand

Abstract: Concentrations of the major ions Na+, K+, Mg2+, Ca2+, HCO32-, SO42- and Cl-, of silicate, and of both total and dissolved trace metals Zn, Cu, Pb and Cd were determined at sites along the Clutha River. The major ions exhibited differing behaviour both spatially and temporally. A large proportion of Cl- and Na+ throughout the catchment could be attributed to atmospheric input of sea salt. Contributions from this source for the rest of the major elements were minimal, except perhaps near the mouth of the river (K+, Mg2+). The water composition was dominated by the weathering of carbonate rather than of silicate minerals. Increases in the concentrations of Mg and Na + K indicate the weathering of Mg, K and Na from rock, minerals and clays within the catchment. The concentrations of major ions and Si were relatively low compared with other rivers in New Zealand and elsewhere. Concentrations of dissolved Zn, Cu, Pb and Cd all increased consistently with distance down river, and between 40% and 100%, for a given metal, was found in the dissolved fraction. High correlation among the dissolved trace metals, with essentially constant Cu : Zn : Pb : Cd ratios of 755 : 716 : 223 : 1 (molar basis), indicates that the same weathering process or source may be occurring throughout the catchment.

X‐ray Photoelectron Spectroscopic Study of the Possible Interactions of Biocells with the Surfaces of Select Silicates

Abstract: Silicate minerals are the dominant materials in the Earth's crust, and thus literally define the term 'geochemistry', but, with the exception of glass chemistry and some areas of catalysis, silicates have, in the past, received scant attention in many areas of chemistry, physics and materials science. In fact, many chemists are largely uninformed about the broad diversity of silicate structures and bondings. Recent explorations by biochemists in the pathogenesis of certain diseases have, however, resulted in a new-found, much broader interest in silicates. This paper extends our studies of the chemistry of mineral silicates using primarily x-ray photoelectron spectroscopy (XPS or ESCA). Following a short consideration of the sometimes contradictory arguments concerning the possible pathogenicity of these materials, we will outline our rapidly expanding unique studies of the interaction of certain biocellular bodies with select silicate systems. The latter studies feature the first 'before, during and after' analyses of the silicates (as well as the cells), with the implementation of unique methods for cell-silicate separation and also the freeze drying and surface analysis of the combined systems. In this manner we have been able to identify cell-induced modifications in the surface chemistry of the silicate components, along with alterations in the cellular species. Many of these analyses were facilitated by the recent clarifications in bioorganic ESCA.

The Solubility of Some Alkali-Bearing Zr Minerals in Hydrothermal Solutions

Abstract: The hydrolytic dissolution of some Zr-bearing minerals has been measured in aqueous KCl and KF solutions. Depending on the mineral being investigated, Zr concentrations varied from about 9 ppb to nearly 40 ppm in equilibrated solutions; this validates the notion of Zr mobility in some crustal fluids. At 50 C, {Delta}{sub f}G were determined (using the measured solubility products) for Na catapleiite, elpidite, vlasovite and weloganite to be {minus}4,654.2 {+-} 5.6, {minus}7,391.0 {+-} 11.6, {minus}5,181.0 {+-} 7.3 and {minus}7,130.5 {+-} 1.0 kJ/mol, respectively. Calculated phase relationships among the zirconosilicates found at the Strange Lake peralkaline complex indicate that elpidite and gittinsite cannot coexist stably implying a two-stage hydrothermal mineralization process. The relative stability of Zr-bearing minerals and the aqueous geochemistry of Zr has been attracting some recent interest owing to their relevance to the problem of radioactive waste management, hydrothermal Zr mineralization, the geochemical cycle of zirconium and the role of zirconium as a strategic metal.

A Model and Simulation of Branching Mineral Growth from Cooling Contacts and Glasses

Abstract: Silicate minerals grown from glasses, and rapidly cooled melts, often have non-compact branching or ‘spherulitic’ morphology. The branching patterns are observed in volcanic rocks, glasses, meteorites, slags and sometimes in shallow level intrusive rocks. Experiments, observations, theory and simulations all support the concept that the crystal morphology is the result of growth under diffusion limited conditions. We show that in a silicate melt under appropriate conditions the equations for heat transfer and chemical-diffusion reduce to the Laplace equation. This means that the temperature or chemical gradient is a steady state field. Interaction between this field and a random variable (Brownian motion of growth species) is modelled and yields complex branching objects. The growing cluster affects the field such that an in-filled structure cannot be formed. The branching structures of the model crystal are remarkably similar to those formed in nature, and to those produced in laboratory experiments, implying that the model captures the essence of the branching-growth process.

Treatment of fillers with oxa-silacycloalkanes

Abstract: A method of hydrophobing fillers such as inorganic pigments, metal oxides, metal hydroxides and silicate minerals, involves contacting a filler with an oxa-silacycloalkane, until hydroxyl functionality on the filler is replaced with carbinol functionality (i.e., ≡C-OH). The preferred oxa-silacycloalkane is 2,2,4-trimethyl-1-oxa-2-silacyclopentane.

Uraninite-Water Interactions in an Oxidizing Environment

Abstract: Exceptionally low jg 18O values of primary uraninite and pitchblende (i.e. -32 per mil to -19.5 per mil) from Proterozoic unconformity-type uranium deposits in Saskatchewan, Canada, in conjunction with theoretical uraninite-water oxygen isotope fractionation factors suggest that primary uranium mineralization is not in oxygen isotopie equilibrium with clays and silicates. The low jg 18O values have been interpreted to have resulted from the recrystallization of primary uranium mineralization in the presence of modern meteoric fluids having low jg 18O values of ca. -18 per mil. The absence of apparent alteration in many of the uraninite and pitchblende samples requires that the uranium minerals exchange oxygen isotopes with fluids, with only minor disturbances to their original chemical compositions and textures. However, experiments on the interaction between water and natural uraninites, from these deposits, and detailed electron micro-probe analyses of natural uraninite and pitchblende indicate that, in the presence of water, old uraninite rapidly alters to curite (Pb2U5O174H2O). The hydration of uraninite to curite releases uranium and calcium into solution and becquerelite (Ca(UO2)6O4(OH)6H2O) is precipitated. In the presence of Si-saturated waters, uranium silicate minerals, soddyite ((UO2)2(SiO4)2H2O) and kasolite (Pb(UO2)SiO4H2O are precipitated in addition to, curite and schoepite ((UO2)8O2(OH)12(H2O)12). The mineral paragenesis observed in these experiments is similar to sequences observed in oxidized zones in uranium deposits and UO2-water experiments. Therefore, it is unlikely that natural uraninite and pitchblende can simply exchange oxygen with an oxidizing fluid without a concomitant change in phase chemistry or structure, nor will oxidation of uraninite lead to the formation of U3O7. as predicted by theoretical calculations used in natural analogue studies for the disposal of high level nuclear waste.

Entropy of calcium and magnesium aluminosilicate glasses

Abstract: The heat capacity of a magnesium and two calcium aluminosilicate glasses has been measured adiabatically at low temperatures. The residual entropy of quenched Mg,Al,Si,O,, glass is 94 f 13 J mol-’ K-‘. Above - 150 K, available data show that C, is an additive function of composition to within _t 0.5% throughout the investigated glass-forming part of the system CaO-MgO-Al,O,-SiO,. The breakdown of these linear variations observed at the lowest temperatures does not affect significantly the additive nature of room-temperature vibrational entropies. Excess heat capacities with respect to Debye’s mode1 are not a special feature of glasses. Also observed for crystals, they are the strongest for framework silicate minerals and determine the entropy difference between silicate glasses and crystals. Comparisons between entropy data for glasses and crystals confirm that Ca and Mg depart little from octahedra1 coordination in alkaline-earth aluminosilicate glasses.

The thermochemical reactivity of silicate minerals in hydrogen and methane

Abstract: The thermochemical reduction of transition metal silicates, i.e. garnierite (Ni,Mg)6[(OH)8(Si2O7)], chrysocolla (Cu,Al)2H2Si2O5(OH4)·nH2O, dioptase CuSiO3·H2O, willemite Zn2SiO4, and hemimorphite Zn4Si2O7(OH)2·H2O has been studied. By means of combined thermogravimetric/mass spectrometric measurements, X-ray diffraction and analytical scanning electron microscopy it is shown that in a 5% H2/95% N2 or in a methane atmosphere the transition metals are selectively reduced at temperatures qualitatively corresponding to their electrochemical potential. Mixtures of elemental transition metals and quartz, SiO2, are obtained as solid products. Depending on the nature of the parent mineral, different mixtures of volatile products are obtained. Principal volatile product, however, is water vapour. The reduction in methane leads to the formation of syngas.

Layered mineral and composition containing it

Abstract: Novel silicate minerals that have undergone a cation exhange with at least one heteroaromatic cation comprising a positively charged organo-substituted heteroatom and/or at least one positively charged heteroatom not part of an aromatic ring with at least one bond having a bond order greater than one and compositions comprising the same are described.

The Effect of the Adsorption of Metal Ions on the Interfacial Behavior of Silicate Minerals

Abstract: Because of its importance in the fields of environmental and mineral engineering, the adsorption mechanism of metal ions on oxides has been studied rather extensively using such techniques as titration, adsorption isotherm determination, flotation and spectroscopy. However, limited work has been done to compare the behavior of hydrophilic silicates with those that are naturally hydrophobic. Our investigation was undertaken to provide a better understanding of the adsorption mechanism of selected metal ions at the solid/liquid interface through simultaneous measurements of zeta potentials, adsorption densities and turbidity on colloidal suspensions of two silicate minerals, quartz and talc, as a function of pH, to delineate the effect of hydrophobicity and surface heterogeniety on these processes.

One – Silicate structures

Abstract: Publisher Summary This chapter focuses on silicate structures, which can be considered inorganic polymers based on two basic “monomer” structures: the tetrahedron and the octahedron. Many of the silicates can be pictured as configurations made by the joining of such tetrahedra and octahedra to themselves and to each other in three dimensions. These involve the sharing of corners, edges, and faces in numerous conformations. The possible geometric permutations are further modified by chemical substitutions within the structure, which usually depend on how well a metal ion will fit among close-packed oxygen ions. The fundamental structural unit of industrial silicate minerals is silica tetrahedron. Extended in three dimensions, this structure provides the characteristic hardness and inertness of quartz. The various forms of crystalline silica—most commonly quartz, cristobalite, and tridymite—differ mainly in the relative orientation of adjacent tetrahedra and the shape of voids created within a given plane. Among the industrial minerals, high terahedra density and consequent hardness are also found in chain silicates. Wollastonite is characterized by a repeating, twisted, three-tetrahedra unit, where the chains formed by these silica tetrahedra are connected by calcium in octahedral coordination. Because of this chain structure, wollastonite can occur as acicular crystals in some cases of macroscopic dimensions. Phyllosilicates are characterized in part by an indefinitely extended sheet of rings, with three of the tetrahedral oxygens shared and the fourth (apical) oxygen, in each case pointing in the same direction. Phyllosilicates are often differentiated as dioctahedral or trioctahedral based on octahedral occupancy.