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.     

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.     

Cation field strength effects on high pressure aluminosilicate glass structure: Multinuclear NMR and La XAFS results

We examined aluminosilicate glasses containing a variety of network modifying to intermediate cations (Li, La, Sc, and Fe), quenched from melts at 1 atm to 8 GPa, to further investigate the role of cation field strength in Al coordination changes and densification. ²⁷Al Nuclear Magnetic Resonance Spectroscopy (NMR) reveals that the mean Al coordination increases with increasing pressure in the Li-containing glasses, which can be explained by a linear dependence of fractional change in Al coordination number on cation field strengths in similar K-, Na-, and Ca-containing aluminosilicate glasses (K < Na < Li < Ca). Measured recovered densities follow a similar linear trend. In contrast, the La-containing glasses have significantly lower mean Al coordination numbers at given pressures than the cation field strength of La and glass density would predict. La L₃ X-ray absorption fine structure (XAFS) spectroscopy results indicate a significant increase with pressure in average La-O bond distances, suggesting that La and Al may be “competing” for higher coordinated sites and hence that both play a significant role in the densification of these glasses, especially in the lower pressure range. However, in Na aluminosilicate glasses with small amounts of Sc, ⁴⁵Sc NMR reveals only modest Sc coordination changes, which do not seem to significantly affect the mean Al coordination values. For a Li aluminosilicate glass, ¹⁷O MAS and multiple quantum magic angle spinning (3QMAS) NMR data are consistent with generation of more highly coordinated Al at the expense of non-bridging oxygen (NBO), whereas La aluminosilicate glasses have roughly constant O environments, even up to 8 GPa. Finally, we demonstrate that useful ²³Na and ²⁷Al MAS NMR spectra can be collected for Ca-Na aluminosilicate glasses containing up to 5 wt.% Fe oxide. We discuss the types of structural changes that may accompany density increases with pressure and how these structural changes are affected by the presence of different cations.     

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.     

Ab initio calculation of H, O, Al and Si NMR parameters, vibrational frequencies and bonding energetics in hydrous silica and Na-aluminosilicate glasses

Ab initio, molecular orbital (MO) calculations were performed on model systems of SiO₂, NaAlSi₃O₈ (albite), H₂O-SiO₂ and H₂O-NaAlSi₃O₈ glasses. Model nuclear magnetic resonance (NMR) isotropic chemical shifts (δ_iso) for ¹H, ¹⁷O, ²⁷Al and ²⁹Si are consistent with experimental data for the SiO₂, NaAlSi₃O₈, H₂O-SiO₂ systems where structural interpretations of the NMR peak assignments are accepted. For H₂O-NaSi₃AlO₈ glass, controversy has surrounded the interpretation of NMR and infrared (IR) spectra. Calculated δ_iso¹H, δ_iso¹⁷O, δ_iso²⁷Al and δ_iso²⁹Si are consistent with the interpretation of Kohn et al. (1992) that Si-(OH)-Al linkages are responsible for the observed peaks in hydrous Na-aluminosilicate glasses. In addition, a theoretical vibrational frequency associated with the Kohn et al. (1992) model agrees well with the observed shoulder near 900 cm⁻¹ in the IR and Raman spectra of hydrous albite glasses. MO calculations suggest that breaking this Si-(OH)-Al linkage requires ∼+56 to +82 kJ/mol which is comparable to the activation energies for viscous flow in hydrous aluminosilicate melts.     

Dissolution mechanisms of water in depolymerized silicate melts: Constraints from H and Si NMR spectroscopy and ab initio calculations

Dissolution of water in magmas significantly affects phase relations and physical properties. To shed new light on the this issue, we have applied ¹H and ²⁹Si nuclear magnetic resonance (NMR) spectroscopic techniques to hydrous silicate glasses (quenched melts) in the CaO-MgO-SiO₂ (CMS), Na₂O-SiO₂, Na₂O-CaO-SiO₂ and Li₂O-SiO₂ systems. We have also carried out ab initio molecular orbital calculations on representative clusters to gain insight into the experimental results.The most prominent result is the identification of a major peak at ∼1.1 to 1.7 ppm in the ¹H MAS NMR spectra for all the hydrous CMS glasses. On the basis of experimental NMR data for crystalline phases and ab initio calculation results, this peak can be unambiguously attributed to (Ca,Mg)OH groups. Such OH groups, like free oxygens, are only linked to metal cations, but not part of the silicate network, and are thus referred to as free hydroxyls in the paper. This represents the first direct evidence for a substantial proportion (∼13∼29%) of the dissolved water as free hydroxyl groups in quenched hydrous silicate melts. We have found that free hydroxyls are favored by (1) more depolymerized melts and (2) network-modifying cations of higher field strength (Z/R²: Z: charge, R: cation-oxygen bond length) in the order Mg > Ca > Na. Their formation is expected to cause an increase in the melt polymerization, contrary to the effect of SiOH formation. The ²⁹Si MAS NMR results are consistent with such an interpretation. This water dissolution mechanism could be particularly important for ultramafic and mafic magmas.The ¹H MAS NMR spectra for glasses of all the studied compositions contain peaks in the 4 to 17 ppm region, attributable to SiOH of a range of strength of hydrogen bonding and molecular H₂O. The relative population of SiOH with strong hydrogen bonding grows with decreasing field strength of the network-modifying cations. Ab initio calculations confirmed that this trend largely reflects hydrogen bonding with nonbridging oxygens.     

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.     

Structure vs. composition: A solid-state H and Si NMR study of quenched glasses along the Na2O-SiO2-H2O join

A suite of six hydrous (7 wt.% H₂O) sodium silicate glasses spanning sodium octasilicate to sodium disilicate in composition were analyzed using ²⁹Si single pulse (SP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, ¹H-²⁹Si cross polarization (CP) MAS NMR, and fast MAS ¹H-NMR. From the ²⁹Si SPMAS data it is observed that at low sodium compositions dissolved water significantly depolymerizes the silicate network. At higher sodium contents, however, dissolved H₂O does not affect a significant increase in depolymerization over that predicted based on the Na/Si ratio alone. The fast MAS ¹H-NMR data reveal considerable complexity in proton environments in each of the glasses studied. The fast MAS ¹H-NMR spectra of the highest sodium concentration glasses do not exhibit evidence of signficantly greater fractions of dissolved water as molecular H₂O than the lower sodium concentration glasses requiring that the decrease in polymerization at high sodium contents involves a change in sodium solution mechanism. Variable contact time ¹H-²⁹Si cross polarization (CP) MAS NMR data reveal an increase in the rotating frame spin lattice relaxation rate constant (T_1ρ*) for various Qⁿ species with increasing sodium content that correlates with a reduction in the average ¹H-²⁹Si coupling strength. At the highest sodium concentration, however, T_1ρ* drops significantly, consistent with a change in the Na₂O solution mechanism.     

The distribution of sodium ions in aluminosilicate glasses: a high-field Na-23 MAS and 3Q MAS NMR study

The local configurations around sodium ions in silicate glasses and melts and their distributions have strong implications for the dynamic and static properties of melts and thus may play important roles in magmatic processes. The quantification of distributions among charge-balancing cations, including Na⁺ in aluminosilicate glasses and melts, however, remains a difficult problem that is relevant to high-temperature geochemistry as well as glass science.Here, we explore the local environment around Na⁺ in charge-balanced aluminosilicate glasses (the NaAlO₂-SiO₂ join) and its distribution using ²³Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy at varying magnetic fields of 9.4, 14.1, and 18.8 T, as well as triple-quantum (3Q)MAS NMR spectroscopy at 9.4 T, to achieve better understanding of the extent of disorder around this cation. We quantify the extent of this disorder in terms of changes in Na-O distance (d[Na-O]) distributions with composition and present a structural model favoring a somewhat ordered Na distribution, called a “perturbed” Na distribution model. The peak position in ²³Na MAS spectra of aluminosilicate glasses moves toward lower frequencies with increasing Si/Al ratios, implying that the average d(Na-O) increases with increasing R. The peak width is significantly reduced at higher fields (14.1 and 18.8 T) because of the reduced effect of second-order quadrupolar interaction, and ²³Na MAS NMR spectra thus provide relatively directly the Na chemical shift distribution and changes in atomic environment with composition. Chemical shift distributions obtained from ²³Na 3Q MAS spectra are consistent with MAS NMR data, in which deshielding decreases with R. The average distances between Na and the three types of bridging oxygens (BOs) (Na-{Al-O-Al}, Na-{Si-O-Al}, and Na-{Si-O-Si}) were obtained from the correlation between d(Na-O) and isotropic chemical shift. The calculated d(Na-{Al-O-Al}) of 2.52 Å is shorter than the d(Na-{Si-O-Si}) of 2.81 Å, and d(Na-{Al-O-Al}) shows a much narrower distribution than the other types of BOs. ²³Na chemical shifts in binary (Al-free) sodium silicate glasses are more deshielded and have ranges distinct from those of aluminosilicate glasses, implying that d(Na-NBO) (nonbridging oxygen) is shorter than d(Na-BO) and that d(Na-{Si-O-Si}) in binary silicates can be shorter than that in aluminosilicate glasses. The results given here demonstrate that high-field ²³Na NMR is an effective probe of the Na⁺ environment, providing not only average structural information but also chemically and topologically distinct chemical shift ranges (distributions) and their variation with composition and their effects on static and dynamic properties.     

A note on the Raman spectra of water-bearing albite glasses

The Raman spectra of albite glasses with 4.5 and 6.6 weight percent water have been obtained, and are compared with that of a dry sample. The hydrous glasses show bands near 3600 cm^?1 due to O-H stretching, and a previously unreported weak band near 1600 cm^?1 due to bending of molecular H₂O. Other weak spectral features are discussed, and the effect of dissolved water on the aluminosilicate framework vibrations is considered.     

Solubility mechanisms of fluorine in peralkaline and meta-aluminous silicate glasses and in melts to magmatic temperatures

Structural interaction between dissolved fluorine and silicate glass (25°C) and melt (to 1400°C) has been examined with ¹⁹F and ²⁹Si MAS NMR and with Raman spectroscopy in the system Na₂O-Al₂O₃-SiO₂ as a function of Al₂O₃ content. Approximately 3 mol.% F calculated as NaF dissolved in these glasses and melts. From ¹⁹F NMR spectroscopy, four different fluoride complexes were identified. These are (1) Na-F complexes (NF), (2) Na-Al-F complexes with Al in 4-fold coordination (NAF), (3) Na-Al-F complexes with Al in 6-fold coordination with F (CF), and (4) Al-F complexes with Al in 6-fold, and possibly also 4-fold coordination (TF). The latter three types of complexes may be linked to the aluminosilicate network via Al-O-Si bridges.The abundance of sodium fluoride complexes (NF) decreases with increasing Al/(Al + Si) of the glasses and melts. The NF complexes were not detected in meta-aluminosilicate glasses and melts. The NAF, CF, and TF complexes coexist in peralkaline and meta-aluminosilicate glasses and melts.From ²⁹Si-NMR spectra of glasses and Raman spectra of glasses and melts, the silicate structure of Al-free and Al-poor compositions becomes polymerized by dissolution of F because NF complexes scavenge network-modifying Na from the silicate. Solution of F in Al-rich peralkaline and meta-aluminous glasses and melts results in Al-F bonding and aluminosilicate depolymerization.Temperature (above that of the glass transition) affects the Qⁿ-speciation reaction in the melts, 2Q³ ⇔ Q⁴ + Q², in a manner similar to other alkali silicate and alkali aluminosilicate melts. Dissolved F at the concentration level used in this study does not affect the temperature-dependence of this speciation reaction.     

Nature of polymerization and properties of silicate melts and glasses at high pressure

The structures of sodium silicate and aluminosilicate glasses quenched from melts at high pressure (6-10 GPa) with varying degrees of polymerization (fractions of nonbridging oxygen) were explored using solid-state NMR [¹⁷O and ²⁷Al triple-quantum magic-angle spinning (3QMAS) NMR]. The bond connectivity in melts among four and highly coordinated network polyhedra, such as ^[4]Al, ^[5,6]Al, ^[4]Si, and ^[5,6]Si, at high pressure is shown to be significantly different from that at ambient pressure. In particular, in the silicate and aluminosilicate melts, the proportion of nonbridging oxygen (NBO) generally decreases with increasing pressure, leading to the formation of new oxygen clusters that include 5- and 6-coordinated Si and Al in addition to 4-coordinated Al and Si, such as ^[4]Si-O-^[5,6]Si, ^[4]Si-O-^[5,6]Al and Na-O-^[5,6]Si. While the fractions of ^[5,6]Al increase with pressure, the magnitude of this increase diminishes with increasing degrees of ambient-pressure polymerization under isobaric conditions. Incorporating the above structural information into models of melt properties reproduces the anomalous pressure-dependence of O²⁻ diffusivity and viscosity often observed in silicate melts.     

Water dissolution in albite melts:: Constraints from ab initio NMR calculations

Hartree-Fock and B3LYP NMR calculations were performed at the 6-311+G(2df,p) level on cluster models representing albite glasses using B3LYP/6 to 31G* optimized geometries. Calculation results on several well-known crystalline materials, such as low albite and KHSi₂O_5, were used to check the accuracy of the calculation methods.Calculated ²⁹Si-NMR results on clusters that model protonation of Al-O-Si linkages and the replacement of Na⁺ by H⁺ indicate a major increase in Si-O(H) bond length and a 5 ppm difference in δ_iso for ²⁹Si compared to that for anhydrous albite glass. The calculated δ_iso of ²⁷Al in such linkages agrees with the experimental data, but shows an increase in C_q that cannot be fully diminished by H-bonding to additional water molecules. This protonation model is consistent with both experimental ¹⁷O NMR data and the major peak of ¹H-NMR spectra. It cannot readily explain the existence of the small peak in the experimental ¹H spectra around 1.5 ppm. Production of the depolymerized units Al [Q³]-O-H upon the dissolution of water is not consistent with ²⁷Al, ¹H, or ¹⁷O NMR experimental results. Production of Si [Q³]-O-H is consistent with all of the experimental ¹⁷O and ¹H-NMR data; such units can produce both the major peak at 3.5 ppm and the small peak at 1.5 ppm in ¹H spectra, either with or without hydrogen bonding. This species, however, cannot produce the main features of ²⁹Si spectra.It is concluded that although neither protonation nor the production of Si [Q³]-O-H alone is consistent with the available experimental data, the combination of these two processes is consistent with available experimental NMR data.     

Si,Al ordering in leucite by high-resolution 27Al MAS NMR spectroscopy

High-resolution ²⁷Al MAS NMR spectra of natural leucite recorded at H ₀=11.7T contain three resolvable resonances at ²⁷Al δ _i = 69.2, 64.7, and 61.0±0.5 ppm. These three resonances are assigned to the three inequivalent framework positions of leucite: T3, T2, and T1, respectively. Fitting the observed spectra yields a Si,Al distribution for leucite in which approximately one-half of the Al is in T1 and one-quarter in each of T2 and T3. This Si,Al distribution differs substantially from those obtained by previous workers using ²⁹Si NMR spectroscopy and X-ray diffraction. New ²⁹Si NMR spectra and revision of previously reported ²⁹Si NMR peak assignments, however, make the ²⁷Al and ²⁹Si NMR results consistent. The ²⁷Al δ _i correlate linearly with the mean T-O-T′ bond angles of the average structure, which allows the peak assignments to be made. However, this correlation lies distinctly toward higher frequency and larger bond angles than correlations for Si,Al ordered aluminosilicates, suggesting that the mean T(Al)-O-T′(Si) bond angle for each site in leucite is smaller than the mean bond angle of the average structure, which is averaged over T(Al)-O-T′(Si) and T(Si)-OT′(Si,Al) angles.     

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.     

Temperature-induced changes in the NIR spectra of hydrous albitic and rhyolitic glasses between 300 and 100 K

Near-infrared (NIR) absorption bands related to total water (4000 and 7050 cm⁻¹), OH groups (4500 cm⁻¹) and molecular H₂O (5200 cm⁻¹) were studied in two polymerised glasses, a synthetic albitic composition and a natural obsidian. The water contents of the glasses were determined using Karl Fischer titration. Molar absorption coefficients were calculated for each of the bands using albitic glasses containing between 0.54 and 9.16 wt.% H₂O and rhyolitic glasses containing between 0.97 and 9.20 wt.% H₂O. Different combinations of baseline type and intensity measure (peak height/area) for the combination bands at 4500 and 5200 cm⁻¹ were used to investigate the effect of evaluation procedure on calculated hydrous species concentrations. Total water contents calculated using each of the baseline/molar absorption coefficient combinations agree to within 5.8% relative for rhyolitic and 6.5% relative for albitic glasses (maximum absolute differences of 0.08 and 0.15 wt.% H₂O, respectively). In glasses with water contents >1 wt.%, calculated hydrous species concentrations vary by up to 17% relative for OH and 11% relative for H₂O (maximum absolute differences of 0.33 and 0.43 wt.% H₂O, respectively). This variation in calculated species concentrations is typically greater in rhyolitic glasses than albitic. In situ, micro-FTIR analysis at 300 and 100 K was used to investigate the effect of varying temperature on the NIR spectra of the glasses. The linear and integral molar absorption coefficients for each of the bands were recalculated from the 100 K spectra, and were found to vary systematically from the 300 K values. Linear molar absorption coefficients for the 4000 and 7050 cm⁻¹ bands decrease by 16–20% and integral molar absorption coefficients by up to 30%. Depending on glass composition and baseline type, the integral molar absorption coefficients for the absorption bands related to OH groups and molecular H₂O change by up to −5.8 and +7.4%, respectively, while linear molar absorption coefficients show less variation, with a maximum change of ∼4%. Using the new molar absorption coefficients for the combination bands to calculate species concentrations at 100 K, the maximum change in species concentration is 0.08 wt.% H₂O, compared with 0.39 wt.% which would be calculated if constant values were assumed for the combination band molar absorption coefficients. Almost all the changes in the spectra can therefore be interpreted in terms of changing molar absorption coefficient, rather than interconversion between hydrous species. Received: 17 December 1998 / Revised, accepted 8 July 1999     

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.     

Association of dissolved aluminum with silica: Connecting molecular structure to surface reactivity using NMR

We studied uptake mechanisms for dissolved Al on amorphous silica by combining bulk-solution chemistry experiments with solid-state Nuclear Magnetic Resonance techniques (²⁷Al magic-angle spinning (MAS) NMR, ²⁷Al{¹H} cross-polarization (CP) MAS NMR and ²⁹Si{¹H} CP-MAS NMR). We find that reaction of Al (1 mM) with amorphous silica consists of at least three reaction pathways; (1) adsorption of Al to surface silanol sites, (2) surface-enhanced precipitation of an aluminum hydroxide, and (3) bulk precipitation of an aluminosilicate phase. From the NMR speciation and water chemistry data, we calculate that 0.20 (±0.04) tetrahedral Al atoms nm⁻² sorb to the silica surface. Once the surface has sorbed roughly half of the total dissolved Al (∼8% site coverage), aluminum hydroxides and aluminosilicates precipitate from solution. These precipitation reactions are dependent upon solution pH and total dissolved silica concentration. We find that the Si:Al stoichiometry of the aluminosilicate precipitate is roughly 1:1 and suggest a chemical formula of NaAlSiO₄ in which Na⁺ acts as the charge compensating cation. For the adsorption of Al, we propose a surface-controlled reaction mechanism where Al sorbs as an inner-sphere coordination complex at the silica surface. Analogous to the hydrolysis of , we suggest that rapid deprotonation by surface hydroxyls followed by dehydration of ligated waters results in four-coordinate (>SiOH)₂Al(OH)₂ sites at the surface of amorphous silica.     

Polymerization of silicate and aluminate tetrahedra in glasses,melts, and aqueous solutions—V. The polymeric structure of silica in albite and anorthite composition glass and the devitrification of amorphous anorthite

²⁹Si MAS NMR experiments have been carried out to determine the silica species distribution (Q distribution) in albite, NaAlSi₃O₈, and anorthite, CaAl₂Si₂O₈, composition glasses (designated albite and anorthite glass). Our results indicate that the Q distribution of albite glass contains all five possible silica species and shows a tendency towards high Q₃ and Q₄ concentrations, whereas anorthite glass does not contain Q₄ and has a high Q₀ concentration. Rationalizations are made in terms of the observed Q distributions to explain differences in devitrification behavior of these two glasses. ²⁷Al MAS NMR data for these glasses suggest that differences in devitrification behavior between these two glasses should be ascribed to small growth rates rather than small nucleation rates of crystalline albite from albite glass.     

Quantum mechanical calculation of 23Na NMR shieldings in silicates and aluminosilicates

To assist in the assignment and interpretation of ²³Na NMR spectra in silicate and aluminosilicate minerals and glasses we have calculated the ²³Na NMR shieldings and the electric field gradients (EFG) at the Na for a number of Na-containing species. Included are Na(OH₂) _n ⁺, n = 1, 2, 4, 5, 6 and 8, and Na⁺ complexes with SiH₃OH, SiH₃ONa and O(SiH₃)₂. We have also evaluated shieldings and EFGs for Na-centered clusters extracted from crystalline Na₂SiO₃ and anhydrous sodalite, Na₆[AlSiO₄]₆. Using 6-31G* SCF optimized geometries and the GIAO method with a 6-31G* basis set [and 6-311(2d,p) bases for the smaller clusters] we find a calculated increase in shielding with coordination number (CN) for the Na(OH₂) _n ⁺, n = 4, 6, 8 series that agrees reasonably well with experimental trends. Calculated changes in the Na shielding as water is replaced by bridging or nonbridging silicate O atoms are also consistent with experimental observations. The deshielding of Na (with respect to gas-phase Na⁺) which is produced by an O-containing ligand is a strongly decreasing function of the R(Na–O) and a weakly decreasing function of the underbonding or free valence of the O. Deshielding contributions to the isotropic shielding from different ligands are additive to good approximation for low CN species, so that the total deshielding can be calculated accurately by summing the contributions from the individual ligands. However for the larger CN species the directly calculated deshieldings are substantially smaller than those obtained using such an additivity approximation. We further test this approximation by calculating the deshieldings for Na in 12 different sites in silicate and aluminosilicate minerals which have recently been studied experimentally, using our calculated deshielding contributions for individual O-containing ligands and experimental values for the Na–O distances. Correlation coefficients between the experimental shifts and the calculated deshieldings are around 0.9 and the slope of the correlation is almost 1.0 . Calculations on large Na-centered clusters extracted from the crystal structures of Na₂SiO₃ and anhydrous sodalite reproduce the experimental values for both NMR shieldings and electric field gradients but at considerable computational cost. Comparison with recent ²³Na NMR studies on hydrous albite glasses indicates that coordination of either H₂O or OH⁻ to the Na could give the magnitude of deshielding observed, depending upon the detailed Na–O distances within the hydrous glass. Received: 31 December 1998 / Revised, accepted: 11 May 1999