Water solubility mechanism in hydrous aluminosilicate glasses: information from Al MAS and MQMAS NMR

New ²⁷Al NMR data are presented in order to clarify the discrepancies in the interpretation of the previous ²⁷Al Magic Angle Spinning (MAS) spectra from hydrous aluminosilicate glasses. The ²⁷Al MAS data have been collected at much higher magnetic field (14.1 and 17.6 T) than hitherto, and in addition, multiple quantum (MQ) MAS NMR data are presented for dry and hydrous nepheline glasses and NaAlSi_7.7O_17.4 glass that, according to the model of Zeng et al. (Zeng Q., Nekvasil H., and Grey C. P. 2000. In support of a depolymerisation model for water in sodium aluminosilicate glasses: Information from NMR spectroscopy. Geochim. Cosmochim. Acta64, 883-896), should produce a high fraction (up to 30%) of Al in Al Q³-OH on hydration. Although small differences in the MAS spectra of anhydrous and hydrous nepheline glasses are observed, there is no evidence for the existence of significant (>∼2%) amounts of Q³ Al-OH in these glasses in either the MAS or MQMAS data.     

The speciation of carbon dioxide in sodium aluminosilicate glasses

Infrared spectroscopy has been used to study the speciation of CO₂ in glasses near the NaAlO₂-SiO₂ join quenched from melts held at high temperatures and pressures. Absorption bands resulting from the antisymmetric stretches of both molecular CO₂ (2,352 cm^–1) and CO ₃ ^2– (1,610 cm^–1 and 1,375 cm^–1) are observed in these glasses. The latter are attributed to distorted Na-carbonate ionic-complexes. Molar absorptivities of 945 liters/mole-cm for the molecular CO₂ band, 200 liters/mole-cm for the 1,610 cm^–1 band, and 235 liters/mole-cm for the 1,375 cm^–1 band have been determined. These molar absorptivities allow the quantitative determination of species concentrations in the glasses with a precision on the order of several percent of the amount present. The accuracy of the method is estimated to be ±15–20% at present.The ratio of molecular CO₂ to CO ₃ ^2– in sodium aluminosilicate glasses varies little for each silicate composition over the range of total dissolved CO₂ content (0–2%), pressure (15–33 kbar) and temperature (1,400–1,560° C) that we have studied. This ratio is, however, a strong function of silicate composition, increasing both with decreasing Na₂O content along the NaAlO₂-SiO₂ join and with decreasing Na₂O content in peraluminous compositions off the join.Infrared spectroscopic measurements of species concentrations in glasses provide insights into the molecular level processes accompanying CO₂ solution in melts and can be used to test and constrain thermodynamic models of CO₂-bearing melts. CO₂ speciation in silicate melts can be modelled by equilibria between molecular CO₂, CO ₃ ^2– , and oxygen species in the melts. Consideration of the thermodynamics of such equilibria can account for the observed linear relationship between molecular CO₂ and carbonate concentrations in glasses, the proposed linear relationship between total dissolved CO₂ content and the activity of CO₂ in melts, and observed variations in CO₂ solubility in melts.     

Dynamics of Na in sodium aluminosilicate glasses and liquids

²³Na NMR measurements on Na₂Si₃O₇, Na₃AlSi₆O₁₅, and NaAlSi₃O₈ glasses from room temperature to 1200°C show that the dynamics and local structure of sodium in silicate/aluminosilicate glasses and melts vary with composition and temperature.The peak positions decrease in frequency between room temperature and 200°C indicating that the Na sees a larger average site as temperature is increased. Between 200°–300° and 700°C, line widths, nutation frequencies and peak positions are consistent with motional averaging of quadrupolar satellites. Above 700°C there is little or no change in the peak positions with temperature. Chemical shifts of the materials at 1000°C (Na₂Si₃O₇: 3.6; Na₃AlSi₆O₁₅:-1.3; NaAlSi₃O₈:-6.4 ppm) indicate a slight change in the average Na coordination number from 6–7 for the silicate to 7–8 for the aluminosilicates.     

Effects of the degree of polymerization on the structure of sodium silicate and aluminosilicate glasses and melts: An O NMR study

Revealing the atomic structure and disorder in oxide glasses, including sodium silicates and aluminosilicates, with varying degrees of polymerization, is a challenging problem in high-temperature geochemistry as well as glass science. Here, we report ¹⁷O MAS and 3QMAS NMR spectra for binary sodium silicate and ternary sodium aluminosilicate glasses with varying degrees of polymerization (Na₂O/SiO₂ ratio and Na₂O/Al₂O₃ ratio), revealing in detail the extent of disorder (network connectivity and topological disorder) and variations of NMR parameters with the glass composition. In binary sodium silicate glasses [Na₂O-k(SiO₂)], the fraction of non-bridging oxygens (NBOs, Na-O-Si) increases with the Na₂O/SiO₂ ratio (k), as predicted from the composition. The ¹⁷O isotropic chemical shifts (¹⁷O δ_iso) for both bridging oxygen (BO) and NBO increase by about 10-15 ppm with the SiO₂ content (for k = 1-3). The quadrupolar coupling products of BOs and NBOs also increase with the SiO₂ content. These trends suggest that both NBOs and BOs strongly interact with Na; therefore, the Na distributions around BOs and NBOs are likely to be relatively homogenous for the glass compositions studied here, placing some qualitative limits on the extent of segregation of alkali channels from silica-enriched regions as suggested by modified random-network models. The peak width (in the isotropic dimension) and thus bond angle and length distributions of Si-O-Si and Na-O-Si increase with the SiO₂ content, indicating an increase in the topological disorder with the degree of polymerization. In the ternary aluminosilicate glasses [Na₂O]_x[Al₂O₃]_1−xSiO₂, the NBO fraction decreases while the Al-O-Si and Al-O-Al fractions apparently increase with increasing Al₂O₃ content. The variation of oxygen cluster populations suggests that deviation from “Al avoidance” is more apparent near the charge-balanced join (Na/Al = 1). The Si-O-Si fraction, which is closely related to the activity coefficient of silica, would decrease with increasing Al₂O₃ content at a constant mole fraction of SiO₂. Therefore, the activity of silica may decrease from depolymerized binary silicates to fully polymerized sodium aluminosilicate glasses at a constant mole fraction of SiO₂.     

Coordination of sodium ions in NaAlO2–SiO2 melts: a high temperature 23Na NMR study

The coordination environment of the sodium ion in the melts of several simple ionic liquids and an Na₂O–Al₂O₃–SiO₂ mixture has been investigated by high temperature ²³Na NMR measurements. A new high temperature NMR probe was utilized for the measurements of the compositional and temperature dependence of the ²³Na NMR chemical shift at temperatures up to 1600?°C. ²³Na NMR spectra of ionic liquids, NaCl, NaBr and NaNO₃, show two peaks at their solid to liquid transition, corresponding to the solid and liquid state, respectively. The ²³Na NMR peak shift in passing from the liquid to the solid is positive. This suggests a decrease in the coordination number for the molten state compared to the crystalline state. The ²³Na peak position for the Na₂O–Al₂O₃–SiO₂ melts of the composition range Na/Al≥1 shifted almost linearly in the positive direction as a function of both the increased degree of depolymerization, NBO/T, and [Al]/([Al]+[Si]). ²³Na MAS-NMR measurement for crystalline silicate compounds of known structure provided a revised relationship between the mean Na–O distances and ²³Na chemical shifts. Comparison of the ²³Na chemical shift of the melts with that of crystalline silicate compounds suggests that the coordination number of Na in those melts is around 6–8 with little compositional dependence. The ²³Na peak position shifted in the negative direction with increasing temperature for sodium silicates, whereas that of aluminosilicates did not show any temperature dependence. The activation energy from the temperature dependence of the ²³Na line width shows little compositional dependence, and the value (51～58?kJ/mol) was close to that of the trace Na ion diffusion in NaAlSi₃O₈ glass.     

Structure of hydrous aluminosilicate glasses along the diopside-anorthite join: A comprehensive one- and two-dimensional H and Al NMR study

We have taken a systematic approach utilizing advanced solid-state NMR techniques to gain new insights into the controversial issue concerning the dissolution mechanisms of water in aluminosilicate melts (glasses). A series of quenched anhydrous and hydrous (∼2 wt% H₂O) glass samples along the diopside (Di, CaMgSi₂O₆)—anorthite (An, CaAl₂Si₂O₈) join with varying An components (0, 20, 38, 60, 80, and 100 mol %) have been studied. A variety of NMR techniques, including one-dimensional (1D) ¹H and ²⁷Al MAS NMR, and ²⁷Al → ¹H cross-polarization (CP) MAS NMR, as well as two-dimensional (2D) ¹H double-quantum (DQ) MAS NMR, ²⁷Al triple-quantum (3Q) MAS NMR, and ²⁷Al → ¹H heteronuclear correlation NMR (HETCOR) and 3QMAS/HETCOR NMR, have been applied. These data revealed the presence of SiOH, free OH ((Ca,Mg)OH) and AlOH species in the hydrous glasses, with the last mostly interconnected with Si and residing in the more polymerized parts of the structure. Thus, there are no fundamental differences in water dissolution mechanisms for Al-free and Al-bearing silicate melts (glasses), both involving two competing processes: the formation of SiOH/AlOH that is accompanied by the depolymerization of the network structure, and the formation of free OH that has an opposite effect. The latter is more important for depolymerized compositions corresponding to mafic and ultramafic magmas.Aluminum is dominantly present in four coordination (Al^IV), but a small amount of five-coordinate Al (Al^V) is also observed in all the anhydrous and hydrous glasses. Furthermore, six-coordinate Al (Al^VI) is also present in most of the hydrous glasses. As Al of higher coordinations are favored by high pressure, Al^VIOH and Al^VOH may become major water species at higher pressures corresponding to those of the Earth’s mantle.     

Structure of Cl-containing silicate and aluminosilicate glasses: A Cl MAS-NMR study

Chlorine-35 magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were collected at 14.1 and 18.8 Tesla fields to determine the atomic scale structural environments of the chloride ions in anhydrous and hydrous silicate and aluminosilicate glasses containing 0.2 to 0.7 wt% Cl. NMR peaks are broad and featureless, but are much narrower than the total chemical shift range for the nuclide in inorganic chlorides. Peak widths are primarily due to quadrupole interactions and to a lesser extent to chemical shift distributions. Peak positions are quite different for the Na- and Ca-containing glasses, suggesting that most Cl⁻ coordination environments contain network modifier cations. Comparison of peak positions and shapes for silicate and aluminosilicate glasses containing either Na or Ca suggests that there is no obvious contribution from Cl⁻ bonded to Al, and relative quantitation of peak areas indicates that there is no systematic undercounting of ³⁵Cl spins in the aluminous vs. the Al-free samples. In Ca-Na silicate glasses with varying Ca/(Ca + Na), the mixed-cation glasses have intermediate chemical shifts between those of the end members, implying that there is not a strong preference of either Ca²⁺ or of Na⁺ around Cl⁻. Hydrous Na-aluminosilicate glasses with H₂O contents up to 5.9 wt% show a shift to higher frequency NMR signal with increasing H₂O content, while the quadrupole coupling constant (C_Q) remains constant at ∼3.3 MHz. However, the change in frequency is much smaller than that expected if H₂O systematically replaced Na⁺ in the first-neighbor coordination shell around Cl⁻. A series of hydrous Ca-aluminosilicate glasses with H₂O contents up to 5.5 wt% show no shift in NMR signal with increasing H₂O content. The C_Q remains constant at ∼4.4 MHz, again suggesting no direct interaction between Cl⁻ and H₂O in these samples.     

Disorder and the extent of polymerization in calcium silicate and aluminosilicate glasses: O-17 NMR results and quantum chemical molecular orbital calculations

Estimation of the framework connectivity and the atomic structure of depolymerized silicate melts and glasses (NBO/T > 0) remains a difficult question in high-temperature geochemistry relevant to magmatic processes and glass science. Here, we explore the extent of disorder and the nature of polymerization in binary Ca-silicate and ternary Ca-aluminosilicate glasses with varying NBO/T (from 0 to 2.67) using O-17 NMR at two different magnetic fields of 9.4 and 14.1 T in conjunction with quantum chemical calculations. Non-random distributions among framework cations (Si and Al) are demonstrated in the variation of relative populations of oxygen sites with NBO/T. The proportion of non-bridging oxygen (NBO, Ca-O-Si) in the binary and ternary aluminosilicate glasses increases with NBO/T. While the trend is consistent with predictions from composition, the detailed fractions apparently deviate from the predicted values, suggesting further complications in the nature of polymerization. The proportion of each bridging oxygen in the glasses also varies with NBO/T. The fractions of Al-O-Si and Al-O-Al increase with increasing polymerization as CaO is replaced with Al₂O₃, while that of Si-O-Si seems to decrease, implying that activity of silica may decrease from calcium silicate to polymerized aluminosilicates (X_SiO2=constant). Quantum chemical molecular orbital calculations based on density functional theory show that a silicate chain with Al-NBO (Ca-O-Al) has an energy penalty (calculated cluster energy difference) of about 108 kJ/mol compared with the cluster with Ca-O-Si, consistent with preferential depolymerization of Si-networks, reported in an earlier O-17 NMR study [Allwardt, J., Lee, S.K., Stebbins, J.F., 2003. Bonding preferences of non-bridging oxygens in calcium aluminosilicate glass: Evidence from O-17 MAS and 3QMAS NMR on calcium aluminate glass. Am. Mineral.88, 949-954]. These prominent types of non-randomness in the distributions suggest significant chemical order in silicate glasses that leads to a decrease in silica activity coefficient and will be useful in modeling transport properties of melts.     

Sulfur speciation and network structural changes in sodium silicate glasses: Constraints from NMR and Raman spectroscopy

Information about the state of sulfur in silicate melts and glasses is important in both earth sciences and materials sciences. Because of its variety of valence states from S²⁻ (sulfide) to S⁶⁺ (sulfate), the speciation of sulfur dissolved in silicate melts and glasses is expected to be highly dependent on the oxygen fugacity. To place new constraint on this issue, we have synthesized sulfur-bearing sodium silicate glasses (quenched melts) from starting materials containing sulfur of different valence states (Na₂SO₄, Na₂SO₃, Na₂S₂O₃ and native S) using an internally heated gas pressure vessel, and have applied electron-induced SKα X-ray fluorescence, micro-Raman and NMR spectroscopic techniques to probe their structure. The wavelength shift of SKα X-rays revealed that the differences in the valence state of sulfur in the starting compounds are largely retained in the synthesized sulfur-bearing glasses, with a small reduction for more oxidized samples. The ²⁹Si MAS NMR spectra of all the glasses contain no peaks attributable to the SiO_4-nS_n (with n > 0) linkages. The Raman spectra are consistent with the coexistence of sodium sulfate (Na₂SO₄) species and one or more types of more reduced sulfur species containing S-S linkages in all the sulfur-bearing silicate glasses, with the former dominant in glasses produced from Na₂SO₄-doped starting materials, and the latter more abundant in more reduced glasses. The ²⁹Si MAS NMR and Raman spectra also revealed changes in the silicate network structure of the sulfur-bearing glasses, which can be interpreted in terms of changes in the chemical composition and sulfur speciation.     

27Al, 29Si,and 23Na MAS NMR study of an Al,Si ordered alkali feldspar solid solution series

Solid-state ²⁷Al, ²⁹Si and ²³Na MAS NMR spectra have been obtained for an Al,Si ordered low albite to low microcline ion exchange series for which unit-cell parameters and ²⁹Si NMR data have previously been reported. ²⁷Al δ_i vary continuously with composition from 63.4 (±0.5) ppm for albite to 58.9 (±0.5) ppm for microcline, and parallel the ²⁹Si chemical shifts assigned to the T2m-site. The ²⁷Al and ²⁹Si chemical shifts for this series correlate well with composition-dependent lattice parameters, most notably cell volume and the angle [201]¹b. The linewidths of the ²⁹Si and ²⁷Al resonances indicate a significant amount of structural disorder in the intermediate compositions due to Na, K substitution. The 1 σ width of the distribution of average Si-O-T angles for each T-site is estimated to be about 1° for the Or33 sample. The average ²³Na δ_i varies monotonically from -8.5 (±1) ppm for albite to -24.3 (±1)ppm for Or83. Similarly, the average ²³Na nuclear quadrupole coupling constant decreases from 2.60 to 1.15 (±0.05) MHz and the asymmetry parameter of the electric field gradient increases from 0.25 to 0.6 with increasing K-content from albite to Or83. The observed variations in the quadrupole coupling parameters are consistent with simple electrostatic calculations. Higher resolution ²³Na spectra of the intermediate compositions obtained at 11.7 T indicate the presence of an inhomogeneous linebroadening which is related to the distribution of Na-environments. A model based on a random distribution of local compositions does not simulate the spectra, suggesting that the distribution of Na is skewed toward Na-rich clusters. Observation of the ²³Na NMR lineshape of Or49 after short periods of heat treatment indicate that ²³Na NMR is very sensitive to the changes in the Na, K distribution accompanying the early stages of exsolution. Reversible changes occur after heating at 530° C for 3 h, whereas heating at 600° C produces no changes, possibly bracketing the position of the coherent spinodal for Al, Si ordered alkali feldspars at this composition.     

23Na NMR chemical shifts and local Na coordination environments in silicate crystals,melts and glasses

In order to decipher information about the local coordination environments of Na in anhydrous silicates from ²³Na nuclear magnetic resonance spectroscopy (NMR), we have collected ²³Na magic angle spinning (MAS) NMR spectra on several sodium-bearing silicate and aluminosilicate crystals with known structures. These data, together with those from the literature, suggest that the ²³Na isotropic chemical shift correlates well with both the Na coordination and the degree of polymerization (characterized by NBO/T) of the material. The presence of a dissimilar network modifier also affects the ²³Na isotropic chemical shift. From these relations, we found that the average Na coordinations in sodium silicate and aluminosilicate liquids of a range of compositions at 1 bar are nearly constant at around 6–7. The average Na coordinations in glasses of similar compositions also vary little with Na content (degree of polymerization). However, limited data on ternary alkali silicate and aluminosilicate glasses seem to suggest that the introduction of another network-modifier, such as K or Cs, does cause variations in the average local Na coordination. Thus it appears that the average Na coordination environments in silicate glasses are more sensitive to the presence of other network-modifiers than to the variations in the topology of the silicate tetrahedral network. Further studies on silicate glasses containing mixed cations are necessary to confirm this conclusion.     

The coordination number of ferrous ions in silicate glasses

Mössbauer and optical absorption measurements on a number of silicate glasses show that most of the ferrous ions in these glasses are distributed over two types of octahedral sites. Less than 0.5% of all ferrous ions occupy tetrahedral sites. It is shown that the Bernal liquid model of random close packing does not apply to silicate glasses. Attention is drawn to the fact that measurements on silicate glasses cannot provide reliable information about coordination numbers in silicate liquids.     

Al speciation in tropical podzols of the upper Amazon Basin: A solid-state Al MAS and MQMAS NMR study

In the upper Amazon Basin, aluminum previously accumulated in lateritic formations is massively remobilised in soils by podzolization and exported in waters. We have investigated the speciation of aluminum in the clay-size fractions of eight horizons of waterlogged podzols lying in a depression of a plateau. The horizons illustrate the main steps involved in the podzolization of laterites. They belong to eluviated topsoil A horizons and illuviated subsoil Bhs, Bh and 2BCs horizons of weakly and better-expressed podzols located at the margin and centre of the depression. For the first time, aluminum speciation is quantitatively assessed in soils by spectroscopic methods, namely FTIR, ²⁷Al magic angle spinning (MAS) and multiple-quantum magic angle spinning (MQMAS), nuclear magnetic resonance (NMR). The results thus obtained are compared to chemical extraction data.Solid-state ²⁷Al MAS NMR spectra enable to distinguish Al bound to organic compounds from that incorporated in secondary mineral phases detected by FTIR. MQMAS experiments additionally show that both chemical shifts and quadrupolar constants are distributed for Al nuclei linked with organic compounds. Similar amounts of chelated Al are obtained from NMR spectra and chemical extractions. The study enables to highlight three major steps in the fate of aluminum. (i) Aluminum is first released by mineral weathering, feeds complexing sites of organic matter and accumulates in subsurface Bhs horizons of weakly expressed podzols (acidocomplexolysis). (ii) Complexes of aluminum with organic matter (Al-OM) then migrate downwards in sandy horizons of better-expressed podzols and accumulate at depth in less permeable 2BCs horizons. (iii) The minor amounts of aluminum present in the 2BCs horizon of the downslope podzol show that aluminum is eventually exported towards the river network, either complexed with organic matter or as Al³⁺ ions after desorption from organic compounds, due to decreasing pH or biodegradation of organic ligands. The direct spectroscopic determination of Al-speciation during the formation of podzolic soils opens new perspectives to trace metal loads in the rivers of the upper Amazon Basin.     

Oxygen sites in hydrous aluminosilicate glasses: the role of Al-O-Al and H2O

We describe here high-field ¹⁷O magic-angle-spinning (MAS) and triple-quantum MAS (3QMAS) NMR spectra for several alkali silicate and Na, K, and Ca aluminosilicate glasses containing up to 10 wt.% water. The H₂O site appears to have a large quadrupolar coupling constant, and its chemical shift increases from Na- to K- glasses, suggesting significant cation-H₂O interactions. In ¹⁷O one-pulse MAS and 3QMAS and ²⁷Al one-pulse NMR experiments, major differences were seen between spectra for anhydrous and hydrous calcium aluminosilicate glasses. The changes in the ¹⁷O MAS spectra can be explained by the addition of an H₂O peak and to the disappearance of an Al-O-Al peak from the ¹⁷O NMR spectrum for the hydrous glass. The ²⁷Al results are consistent with this interpretation.     

Time-resolved luminescence of Fe3+ and Mn2+ ions in hydrous volcanic glasses

 Time-resolved luminescence spectra of natural and synthetic hydrous volcanic glasses with different colors and different Fe, Mn, and H₂O content were measured, and the implications for the glass structure are discussed. Three luminescence ranges are observed at about 380–460, 500–560, and 700–760 nm. The very short-living (lifetimes less than 40 ns) blue band (380–460 nm) is most probably due to the ⁴T₂(⁴D) →⁶A₁(⁶S) and ⁴A₁(⁴G) →⁶A₁(⁶S) ligand field transitions of Fe³⁺. The green luminescence (500–560 nm) arises from the Mn²⁺ transition ⁴T₁(⁴G) →⁶A₁(⁶S). It shows weak vibronic structure, short lifetimes less than 250 μs, and indicates that Mn²⁺ is tetrahedrally coordinated, occupying sites with similar distortions and ion–oxygen interactions in all samples studied. The red luminescence (700–760 nm) arising from the ⁴T₁(⁴G) →⁶A₁(⁶S) transition of Fe³⁺ has much longer lifetimes of the order of several ms, and indicates that ferric iron is also mainly tetrahedrally coordinated. Increasing the total water content of the glasses leads to quenching of the red luminescence and decrease of the distortions of the Fe³⁺ polyhedra. Received: 30 July 2001 / Accepted: 15 November 2001     

The distribution of mantle and atmospheric argon in oceanic basalt glasses

Argon analyses by both high-resolution stepheating and stepcrushing of MORB and Loihi basalt glasses were performed to separate pristine mantle-derived Ar and contaminating atmospheric Ar. In high-vesicularity glasses (> 0.8% vesicles), most of the mantle argon resides in vesicles, from which it is released by crushing or stepheating between 600 and 900 °C. By contrast, in low vesicularity glasses (< permil vesicularity), most mantle argon is dissolved in the glass matrix, as inferred from the correlation with neutron-induced, glass-dissolved argon isotopes (³⁹Ar, ³⁷Ar, ³⁸Ar from K, Ca, Cl). The distribution of mantle Ar between vesicles and glass matrix is well explained by melt-gas equilibrium partitioning at eruption according to Henry’s law, which is compatible with previously determined Henry constants of ∼(5-10) × 10⁻⁵ccSTP ⁴⁰Ar _mantle/g bar. Atmospheric Ar is heterogeneously distributed in all samples. Only a very minor part is dissolved in the glass matrix; a significant part correlates with vesicularity and is released by crushing, most probably from a rather small fraction of vesicles or microcracks that equilibrated with unfractionated air. Other carriers of atmospheric argon are pyroxene microlites and minor phases decomposing at intermediate temperatures that were probably contaminated upon eruption by fractionated atmospheric rare gases. Our high-resolution stepheating and stepcrushing analyses of low vesicularity samples with extraordinary high solar-like ²⁰Ne/²²Ne indicate successful discrimination of unfractionated air as a contamination source and suggest an upper mantle ⁴⁰Ar/³⁶Ar of 32,000 ± 4000 and a Hawaiian mantle plume source ⁴⁰Ar/³⁶Ar ratio close to 8000.     

The nature of hydroxyl groups in aluminosilicate glasses: Quantifying Si-OH and Al-OH abundances along the SiO2-NaAlSiO4 join by H, Al-H and Si-H NMR spectroscopy

The combined results of ²⁷Al-¹H and ¹H-²⁹Si-¹H cross polarization NMR experiments for hydrous glasses (containing 0.5-2 wt% water) along the SiO₂-NaAlSiO₄ join confirm that the dissolution mechanism of water in aluminosilicate glasses is fundamentally the same as for Al-free systems, i.e. the dissolved water ruptures oxygen bridges and creates Si-OH and Al-OH groups, in addition to forming molecular water (H₂O_mol). The fraction of Al-OH increases non-linearly as the Al content increases with up to half of the OH groups as Al-OH for compositions close to NaAlSiO₄. The relative abundances of the different species are controlled by the degree of Al-avoidance and the relative tendency of hydrolysis of the different types of oxygen bridges, Si-O-Si, Si-O-Al and Al-O-Al. A set of homogeneous reactions is derived to model the measured Al-OH/Si-OH speciation, and the obtained equilibrium constants are in agreement with literature data on the degree of Al-avoidance. With these equilibrium constants, the abundance of the different oxygen species, i.e. Si-O-Si, Si-O-Al, Al-O-Al, Si-OH, Al-OH and H₂O_mol, can be predicted for the entire range of water and Al contents.     

An experimental study of biotite in Na-, F-, and Li-rich pneumatolyto-hydrothermal solutions

Methodology is described in this paper for investigating biotite reactions in pneumatolyto-hydrothermal solutions rich in Na, F and Li by using tow buffer systems, NNO and CFG. The results show: (1) Under the experimental conditions biotite is unstable and different new stable phases will be formed, depending upon the fugacity of oxygen. Na-rich minerals, such as aegirine and albite, will be produced at low oxygen fugacity, while Li-rich micas are found stable at high oxygen fugacity. This agrees with the field observation that albitization occurs in general at a lower position than that of Li-rich micas, suggesting that alkalimetasomatism of biotite may provide the necessary components for subsequent reactions. (2) The stability of Li-rich micas is dependent on Mg²⁺ concentration in the medium, which in turn is determined by Mg²⁺ content in the starting biotite. Li-rich micas are favored by metasomatism of only those biotites that are poor in Mg²⁺. (3) Unstability of biotite would favor the incorporation of ore-forming elements contained in it as isomorphous impurities into solutions.     

The coprecipitation of strontium,magnesium, sodium,potassium and chloride ions with gypsum. An experimental study

The coprecipitation of Sr²⁺, Mg²⁺, Na⁺, K⁺ and Cl^? into gypsum was studied as a function of temperature, brine concentration and growth rate. The concentrations of the studied cations in the gypsum increase with growth rate (kinetic effect), with a tendency to reach a limiting value at high growth rates. The partition coefficients of Sr tend to increase with brine concentration and decrease with temperature. The partition coefficients of the other cations also decrease with temperature but depend only very slightly on brine concentration. The concentrations of coprecipitated chloride are negligibly small.The coprecipitation behavior is explained in terms of the relation between the rate of desorption of the coprecipitating ions from the surface of the growing crystal, and the rate of growth. The studied cations may substitute for Ca²⁺ in its normal lattice sites and/or reside in interstitial positions among the structural water molecules. The relative amount of foreign cations occupying interstitial positions increases with increasing growth rate.The elucidation of the behavior of coprecipitated ions in gypsum given here forms a basis for the utilization of these ions as geochemical indicators for the environment of deposition of gypsum. These indicators may help in reconstructing important parameters such as temperature, brine concentration and growth rate.