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1.
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.  相似文献   

2.
Micro-Raman spectroscopy, even though a very promising technique, is not still routinely applied to analyse H2O in silicate glasses. The accuracy of Raman water determinations critically depends on the capability to predict and take into account both the matrix effects (bulk glass composition) and the analytical conditions on band intensities. On the other hand, micro-Fourier transform infrared spectroscopy is commonly used to measure the hydrous absorbing species (e.g., hydroxyl OH− and molecular H2O) in natural glasses, but requires critical assumptions for the study of crystal-hosted glasses. Here, we quantify for the first time the matrix effect of Raman external calibration procedures for the quantification of the total H2O content (H2OT = OH− + H2Om) in natural silicate glasses. The procedures are based on the calibration of either the absolute (external calibration) or scaled (parameterisation) intensity of the 3550 cm−1 band. A total of 67 mafic (basanite, basalt) and intermediate (andesite) glasses hosted in olivines, having between 0.2 and 4.8 wt% of H2O, was analysed. Our new dataset demonstrates, for given water content, the height (intensity) of Raman H2OT band depends on glass density, reflectance and water environment. Hence this matrix effect must be considered in the quantification of H2O by Raman spectroscopy irrespective of the procedure, whereas the parameterisation mainly helps to predict and verify the self-consistency of the Raman results. In addition, to validate the capability of the micro-Raman to accurately determine the H2O content of multicomponent aluminosilicate glasses, a subset of 23 glasses was analysed by both micro-Raman and micro-FTIR spectroscopy using the band at 3550 cm−1. We provide new FTIR absorptivity coefficients (ε3550) for basalt (62.80 ± 0.8 L mol−1 cm−1) and basanite (43.96 ± 0.6 L mol−1 cm−1). These values, together with an exhaustive review of literature data, confirm the non-linear decline of the FTIR absorptivity coefficient (ε3550) as the glass depolymerisation increases. We demonstrate the good agreement between micro-FTIR and micro-Raman determination of H2O in silicate glasses when the matrix effects are properly considered.  相似文献   

3.
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 (AlIV), but a small amount of five-coordinate Al (AlV) is also observed in all the anhydrous and hydrous glasses. Furthermore, six-coordinate Al (AlVI) is also present in most of the hydrous glasses. As Al of higher coordinations are favored by high pressure, AlVIOH and AlVOH may become major water species at higher pressures corresponding to those of the Earth’s mantle.  相似文献   

4.
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 (δiso) 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 δiso1H, δiso17O, δiso27Al and δiso29Si 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.  相似文献   

5.
A review of published and newly measured densities for 40 hydrous silicate glasses indicates that the room-temperature partial molar volume of water is 12.0 ± 0.5 cm3/mol. This value holds for simple or mineral compositions as well as for complex natural glasses, from rhyolite to tephrite compositions, prepared up to 10–20 kbar pressures and containing up to 7 wt% H2O. This volume does not vary either with the molar volume of the water-free silicate phase, with its degree of polymerization or with water speciation. Over a wide range of compositions, this constant value implies that the volume change for the reaction between hydroxyl ions and molecular water is zero and that, at least in glasses, speciation does not depend on pressure. Consistent with data from Ochs and Lange (1997, 1999), systematics in volume expansion for SiO2–M2O systems (M=H, Li, Na, K) suggests that the partial molar thermal expansion coefficient of H2O is about 4 × 10−5 K−1 in silicate glasses. Received: 30 June 1999 / Accepted: 5 November 1999  相似文献   

6.
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.  相似文献   

7.
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.  相似文献   

8.
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 NaAlSi7.7O17.4 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.  相似文献   

9.
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 (T1ρ*) 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, T1ρ* drops significantly, consistent with a change in the Na2O solution mechanism.  相似文献   

10.
Liquidus phase relationships have been determined for a high-MgO basalt (STV301: MgO=12.5 wt%, Ni=250 ppm, Cr=728 ppm) from Black Point, St Vincent (Lesser Antilles arc). Piston-cylinder experiments were conducted between 7.5 and 20 kbar under both hydrous and oxidizing conditions. AuPd capsules were used as containers. Compositions of supraliquidus glasses and mass-balance calculations show that Fe loss is < 10% in the majority of experiments. Two series of water concentrations in melt were investigated: (i) 1.5 wt% and (ii) 4.5 wt% H2O, as determined by SIMS analyses on quenched glasses and with the by difference technique. The Fe3+/Fe2+ partitioning between Cr-Al spinel and melt and olivine-spinel equilibria show that oxidizing fO2 were imposed (NNO + 1.5 for the 1.5 wt% H2O series, NNO + 2.3 for the 4.5 wt% H2O series). For both series of water concentrations, the liquid is multiply-saturated with a spinel lherzolite phase assemblage on its liquidus, at 1235°C, 11.5 kbar (1.5 wt% H2O) and 1185°C, 16 kbar (4.5 wt% H2O). Liquidus phases are homogeneous and comparable to typical mantle compositions. Mineral-melt partition coefficients are generally identical to values under anhydrous conditions. The modal proportion cpx/opx on the liquidus decreases from the 1.5 wt% to the 4.5 wt% H2O series. The experimental data are consistent with STV301 being a product of partial melting of lherzolitic mantle. Conditions of multiple saturation progressively evolve toward lower temperatures and higher pressures with increasing melt H2O concentration. Phase equilibria constraints, i.e., the necessity of preserving the mantle signature seen in high-MgO and picritic arc basalts, and glass inclusion data suggest that STV301 was extracted relatively dry (∼ 2 wt% H2O) from its mantle source. However, not all primary arc basalts are extracted under similarly dry conditions because more hydrous melts will crystallize during ascent and will not be present unmodified at the surface. From degrees of melting calculated from experiments on KLB-1, extraction of a 12.5 wt% MgO melt with ∼ 2 wt% H2O would require a H2O concentration of 0.3 wt% in the sub-arc mantle. For mantle sources fluxed with a slab-derived hydrous component, extracted melts may contain up to ∼ 5.5 wt% H2O.  相似文献   

11.
The investigation of hydrous boro(alumino)silicate melts and glasses with near infrared (NIR) spectroscopy revealed an important effect of boron on the water speciation. In the NIR spectra of B-bearing glasses new hydroxyl-related bands develop at the high frequency side of the 4500 cm−1 peak. In NaAlSi3O8 + B2O3 glasses this new peak is present as a shoulder at 4650 cm−1, and in NaAlSi3O8-NaBSi3O8 (Ab-Rd) glasses it appears as a resolved peak at 4710 cm−1. These bands increase with increasing boron concentration, suggesting that they are due to B-OH complexes. Furthermore, the variations in the NIR spectra indicate that with increasing B-content, but constant total water concentration, the amount of structurally bonded hydroxyl groups increases at the expense of molecular H2O. For example, at a total water concentration of 4 wt.%, pure Rd-glass contains ∼50% more water as hydroxyl groups than pure Ab-glass.In-situ NIR spectroscopy at high P and T using a hydrothermal diamond-anvil cell was used to gain information about the temperature dependence of the water speciation in NaBSi3O8 melts. The data demonstrate the conversion of molecular H2O to hydroxyl groups with increasing temperature. However, a fully quantitative evaluation of the high T spectra was hampered by problems with defining the correct baseline in the spectra. As an alternative approach annealing experiments on a Rd-glass containing 2.8 wt.% water were performed. The results confirm the conversion of H2O to OH groups with increasing T, but also suggest that the OH groups represented by the 4710 cm−1 peak (B-OH) participate much less in the conversion reaction compared to X-OH, represented by the 4500 cm−1 peak.  相似文献   

12.
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 δiso) 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.  相似文献   

13.
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  相似文献   

14.
Hydrous silica glasses containing 2.5 wt.% and 8.7 wt.% H2O were prepared by quenching melts from high pressures and temperatures, and investigated by 29Si NMR. Cross-polarisation magic angle spinning NMR experiments are described and used to identify silicon in different structural units within the glasses. The relative amounts of each structural unit are obtained by quantitative single pulse MASNMR and used together with the known water content to give the extent of hydroxyl formation. The hydroxyl concentration appears to be lower than that suggested by most previous work on hydrous silicate glasses using other experimental techniques.  相似文献   

15.
To interpret the degassing of F-bearing felsic magmas, the solubilities of H2O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C. Chloride solubility in these liquids increases with decreasing H2O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl− exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate liquids containing as little as 1 wt% H2O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H2O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl− in H2O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H2O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H2O versus Cl− in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H2O and Cl− concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive H2O and Cl− concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior to eruption. Received: 6 November 1995 / Accepted: 29 January 1998  相似文献   

16.
We determined total CO2 solubilities in andesite melts with a range of compositions. Melts were equilibrated with excess C-O(-H) fluid at 1 GPa and 1300°C then quenched to glasses. Samples were analyzed using an electron microprobe for major elements, ion microprobe for C-O-H volatiles, and Fourier transform infrared spectroscopy for molecular H2O, OH−, molecular CO2, and CO32−. CO2 solubility was determined in hydrous andesite glasses and we found that H2O content has a strong influence on C-O speciation and total CO2 solubility. In anhydrous andesite melts with ∼60 wt.% SiO2, total CO2 solubility is ∼0.3 wt.% at 1300°C and 1 GPa and total CO2 solubility increases by about 0.06 wt.% per wt.% of total H2O. As total H2O increases from ∼0 to ∼3.4 wt.%, molecular CO2 decreases (from 0.07 ± 0.01 wt.% to ∼0.01 wt.%) and CO32− increases (from 0.24 ± 0.04 wt.% to 0.57 ± 0.09 wt.%). Molecular CO2 increases as the calculated mole fraction of CO2 in the fluid increases, showing Henrian behavior. In contrast, CO32− decreases as the calculated mole fraction of CO2 in the fluid increases, indicating that CO32− solubility is strongly dependent on the availability of reactive oxygens in the melt. These findings have implications for CO2 degassing. If substantial H2O is present, total CO2 solubility is higher and CO2 will degas at relatively shallow levels compared to a drier melt. Total CO2 solubility was also examined in andesitic glasses with additional Ca, K, or Mg and low H2O contents (<1 wt.%). We found that total CO2 solubility is negatively correlated with (Si + Al) cation mole fraction and positively correlated with cations with large Gibbs free energy of decarbonation or high charge-to-radius ratios (e.g., Ca). Combining our andesite data with data from the literature, we find that molecular CO2 is more abundant in highly polymerized melts with high ionic porosities (>∼48.3%), and low nonbridging oxygen/tetrahedral oxygen (<∼0.3). Carbonate dominates most silicate melts and is most abundant in depolymerized melts with low ionic porosities, high nonbridging oxygen/tetrahedral oxygen (>∼0.3), and abundant cations with large Gibbs free energy of decarbonation or high charge-to-radius ratio. In natural silicate melt, the oxygens in the carbonate are likely associated with tetrahedral and network-modifying cations (including Ca, H, or H-bonds) or a combinations of those cations.  相似文献   

17.
Water speciation in rhyolitic melts with dissolved water ranging from 0.8 to 4 wt% under high pressure was investigated. Samples were heated in a piston-cylinder apparatus at 624-1027 K and 0.94-2.83 GPa for sufficient time to equilibrate hydrous species (molecular H2O and hydroxyl group, H2Om + O ? 2OH) in the melts and then quenched roughly isobarically. The concentrations of both hydrous species in the quenched glasses were measured with Fourier transform infrared (FTIR) spectroscopy. For the samples with total water content less than 2.7 wt%, the equilibrium constant (K) is independent of total H2O concentration. Incorporating samples with higher water contents, the equilibrium constant depends on total H2O content, and a regular solution model is used to describe the dependence. K changes with pressure nonmonotonically for samples with a given water content at a given temperature. The equilibrium constant does not change much from ambient pressure to 1 GPa, but it increases significantly from 1 to 3 GPa. In other words, more molecular H2O reacts to form hydroxyl groups as pressure increases from 1 GPa, which is consistent with breakage of tetrahedral aluminosilicate units due to compression of the melt induced by high pressure. The effect of 1.9 GPa (from 0.94 to 2.83 GPa) on the equilibrium constant at 873 K is equivalent to a temperature effect of 49 K (from 873 K to 922 K) at 0.94 GPa. The results can be used to evaluate the role of speciation in water diffusion, to estimate the apparent equilibrium temperature, and to infer viscosity of hydrous rhyolitic melts under high pressure.  相似文献   

18.
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.  相似文献   

19.
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 SiO4-nSn (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.  相似文献   

20.
The structural environments of Mn2+ and Sr2+ at concentrations of 0.2–0.8 wt% in dry and hydrous silicate glasses have been studied using X-ray absorption spectroscopy (EXAFS and XANES). The environment of Mn in hydrous silica glasses containing 4.5 and 6.0 wt% H2O appears to be close to an undistorted octahedral site, whereas Mn in other compositions, both dry and hydrous, occupies either a distribution of octahedral and tetrahedral sites, or distorted, non-centrosymmetric sites. The fraction of non-centrosymmetric sites (including tetrahedral) or the average degree of distortion of the sites decreases with both increasing dissolved water concentration and number of non-bridging oxygens in the glass. For Sr the changes as a function of polymerisation and water concentration are less clear, but it appears that the average Sr-O distances are shorter than in crystalline silicates and decrease further with decreasing polymerisation. The structural data obtained in this study are not readily integrated with previous partitioning data for Mn, suggesting that changes in the first-shell coordination geometry of Mn do not play a direct role in controlling the melt compositional dependence of trace element partition coefficients. It is tentatively suggested that distortions of the framework resulting from the incorporation of Mn (i.e. a second shell effect) control the partitioning behaviour.  相似文献   

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