Rare earth partitioning between immiscible carbonate and silicate liquids and CO2 vapor: Results and implications for the formation of light rare earth-enriched rocks

Melting relations at 5 and 20 kbar on the composition join sanidine-potassium carbonate are dominated by a two-liquid region that covers over 60% of the join at 1,300 ° C. At this temperature, the silicate melt contains approximately 19 wt% carbonate component at 5 kbar and 32 wt% carbonate component at 20 kbar. The conjugate carbonate melt contains less than 5 wt% silicate component, and it varies less as a function of temperature than does the silicate melt.Partition coefficients for Ce, Sm, and Tm between the immiscible carbonate and silicate melts at 1,200 ° and 1,300 ° C at 5 and 20 kbar are in favor of the carbonate melt by a factor of 2–3 for light REE and 5–8 for heavy REE. The effect of pressure on partitioning cannot be evaluated independently because of complementary changes in melt compositions.Minimum REE partition coefficients for CO₂ vapor/carbonate melt and CO₂ vapor/silicate melt can be calculated from the carbonate melt/silicate melt partition coefficients, the known proportions of melt, and maximum estimates of the proportion of CO₂ vapor. The vapor phase is enriched in light REE relative to both melts at 20 kbar and enriched in all REE, especially the light elements, at 5 kbar. The enrichment of REE in CO₂ vapor relative to both melts is 3–4 orders of magnitude in excess of that in water vapor (Mysen, 1979) at 5 kbar and is approximately the same as that in water vapor at 20 kbar.Mantle metasomatism by a CO₂-rich vapor enriched in light REE, occurring as a precursor to magma genesis, may explain the enhanced REE contents and light REE enrichment of carbonatites, alkali-rich silicate melts, and kimberlites. Light REE enrichment in fenites and the granular suite of nodules from kimberlites attests to the mobility of REE in CO₂-rich fluids under both mantle and crustal conditions.     

Characterization of water in synthetic rhyolitic glasses and natural melt inclusions by Raman spectroscopy

Raman spectroscopy was used to analyze quantitatively water in silicate glasses and melt inclusions and to monitor H₂O–OH speciation. Calibration is based on synthetic glasses with various water contents (0.02–7.67% H₂O); water determination and OH–H₂O differentiation on the area of the Si–O broad band at 468 cm^–1 and the asymmetric O–H band at 3,550 cm^–1. Each Raman spectrum has been decomposed into four Gaussian + Lorentzian components centered at 3,330, 3,458, 3,560, and 3,626 cm^–1 using the Levenberg–Marquardt algorithm. These components are interpreted to be two different types of H₂O molecule sites. The influence of the temperature on the loss of water is more important for molecular water than for the hydroxyl groups. The H₂O–OH partition confirms the typical evolution of water speciation in rhyolitic glasses as a function of the bulk water content. Method limitations have been studied for the application to natural melt inclusions.Editorial responsibility: T.L Grove     

CO2 solubility and solubility mechanisms in silicate melts at high pressures

CO₂ solubility has a slight negative temperature dependence in olivine melilitite at 30 kb with 9% CO₂ dissolved at 1,450 °C, 8.5% at 1,550 °C and 8.3% at 1,650° C. CO₂ is dissolved as the carbonate molecule (CO ₃ ^2– ) only. Feldspar melts (albite-anorthite) dissolve much less CO₂ at 30 kb (around 2%) with a slight increase with increasing anorthite content. A CO₂ absorption peak in infrared spectra of albite-rich glasses diappears in favour of the CO ₃ ^2– peak with increasing anorthite content. It is inferred that CO₂ was present as CO ₃ ^2– in albite-rich melts also, but reverts to CO₂ during quenching because of bonding differences related to Ca²⁺ and Na⁺ in the melts.     

New density measurements on carbonate liquids and the partial molar volume of the CaCO<Subscript>3</Subscript> component

Density measurements on nine liquids in the CaCO₃–Li₂CO₃–Na₂CO₃–K₂CO₃ quaternary system were performed at 1 bar between 555 and 969 °C using the double-bob Archimedean method. Our density data on the end-member alkali carbonate liquids are in excellent agreement with the NIST standards compiled by Janz (1992). The results were fitted to a volume equation that is linear in composition and temperature; this model recovers the measured volumes within experimental error (±0.18% on average, with a maximum residual of ±0.50%). Our results indicate that the density of the CaCO₃ component in natrocarbonate liquids is 2.502 (±0.014) g/cm³ at 800 °C and 1 bar, which is within the range of silicate melts; its coefficient of thermal expansion is 1.8 (±0.5)×10^–4 K^–1 at 800 °C. Although the volumes of carbonate liquids mix linearly with respect to carbonate components, they do not mix linearly with silicate liquids. Our data are used with those in the literature to estimate the value of in alkaline silicate magmas (20 cm³/mol at 1400 °C and 20 kbar), where CO₂ is dissolved as carbonate in close association with Ca. Our volume measurements are combined with sound speed data in the literature to derive the compressibility of the end-member liquids Li₂CO₃, Na₂CO₃, and K₂CO₃. These results are combined with calorimetric data to calculate the fusion curves for Li₂CO₃, Na₂CO₃, and K₂CO₃ to 5 kbar; the calculations are in excellent agreement with experimental determinations of the respective melting reactions.Editorial responsibility: I Carmichael     

CO2 solubility and speciation in intermediate (andesitic) melts: the role of H2O and composition

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.     

Structure and properties of fluorine-bearing aluminosilicate melts: the system Na2O-Al2O3-SiO2-F at 1 atm

The chemical interaction between fluorine and highly polymerized sodium aluminosilicate melts [Al/(Al+Si)= 0.125–0.250 on the join NaAlO₂-SiO₂] has been studied with Raman spectroscopy. Fluorine is dissolved to form F^– ions that are electrically neutralized with Na⁺ or Al³⁺. There is no evidence for association of fluorine with either Si⁴⁺ or Al³⁺ in four-fold coordination and no evidence of fluorine in six-fold coordination with Si⁴⁺ in these melt compositions. Upon solution of fluorine nonbridging oxygens are formed and are a part of structural units with nonbridging oxygen per tetrahedral cations (NBO/T) about 2 and 1. The proportions of these two depolymerized units in the melts increase systematically with increasing F/(F+O) at constant Al/(Al+Si) and with decreasing Al/(Al+Si) at constant F/(F+O). Depolymerization (increasing NBO/T) of silicate melts results from a fraction of aluminum and alkalies (in the present study; Na⁺) reacting to form fluoride complexes. In this process an equivalent amount of Na⁺ (orginally required for Al-³⁺charge-balance) or Al³⁺ (originally required Na⁺ to exist in tetrahedral coordination) become network-modifiers.The structural data have been used to develop a method for calculating the viscosity of fluorine-bearing sodium aluminosilicate melts at 1 atm. Where experimental viscosity data are available, the calculated and measured values are within 5% of each other.A method is also suggested by which the liquidus phase equilibria of fluorine-bearing aluminosilicate melts may be predicted. In accord with published experimental data it is suggested, for example, that — on the basis of the determined solubility mechanism of fluorine in aluminosilicate melts — with increasing fluorine content of feldspar-quartz systems, the liquidus boundaries between aluminosilicate minerals (e.g., feldspars) and quartz shift away from silica.     

CO2 solubility in dacitic melts equilibrated with H2O-CO2 fluids: Implications for modeling the solubility of CO2 in silicic melts

The solubility of CO₂ in dacitic melts equilibrated with H₂O-CO₂ fluids was experimentally investigated at 1250°C and 100 to 500 MPa. CO₂ is dissolved in dacitic glasses as molecular CO₂ and carbonate. The quantification of total CO₂ in the glasses by mid-infrared (MIR) spectroscopy is difficult because the weak carbonate bands at 1430 and 1530 cm⁻¹ can not be reliably separated from background features in the spectra. Furthermore, the ratio of CO_2,mol/carbonate in the quenched glasses strongly decreases with increasing water content. Due to the difficulties in quantifying CO₂ species concentrations from the MIR spectra we have measured total CO₂ contents of dacitic glasses by secondary ion mass spectrometry (SIMS).At all pressures, the dependence of CO₂ solubility in dacitic melts on x^fluid_CO2,total shows a strong positive deviation from linearity with almost constant CO₂ solubility at x_CO2fluid > 0.8 (maximum CO₂ solubility of 795 ± 41, 1376 ± 73 and 2949 ± 166 ppm at 100, 200 and 500 MPa, respectively), indicating that dissolved water strongly enhances the solubility of CO₂. A similar nonlinear variation of CO₂ solubility with x_CO2fluid has been observed for rhyolitic melts in which carbon dioxide is incorporated exclusively as molecular CO₂ (Tamic et al., 2001). We infer that water species in the melt do not only stabilize carbonate groups as has been suggested earlier but also CO₂ molecules.A thermodynamic model describing the dependence of the CO₂ solubility in hydrous rhyolitic and dacitic melts on T, P, f_CO2 and the mol fraction of water in the melt (x_water) has been developed. An exponential variation of the equilibrium constant K₁ with x_water is proposed to account for the nonlinear dependence of x_CO2,totalmelt on x_CO2fluid. The model reproduces the CO₂ solubility data for dacitic melts within ±14% relative and the data for rhyolitic melts within 10% relative in the pressure range 100-500 MPa (except for six outliers at low x_CO2fluid). Data obtained for rhyolitic melts at 75 MPa and 850°C show a stronger deviation from the model, suggesting a change in the solubility behavior of CO₂ at low pressures (a Henrian behavior of the CO₂ solubility is observed at low pressure and low H₂O concentrations in the melt). We recommend to use our model only in the pressure range 100-500 MPa and in the x_CO2fluid range 0.1-0.95. The thermodynamic modeling indicates that the partial molar volume of total CO₂ is much lower in rhyolitic melts (31.7 cm³/mol) than in dacitic melts (46.6 cm³/mol). The dissolution enthalpy for CO₂ in hydrous rhyolitic melts was found to be negligible. This result suggests that temperature is of minor importance for CO₂ solubility in silicic melts.     

Carbon dioxide and argon diffusion in silicate melts: Insights into the CO2 speciation in magmas

To investigate the influence of temperature and composition on the diffusivities of dissolved carbon dioxide and argon in silicate melts, diffusion experiments were performed at magmatic pressure and temperature conditions in (a) albite melts with excess Na₂O (0-8.6 wt%) and a constant Si/Al ratio of 3, and (b) albite₇₀quartz₃₀ to jadeite melts with decreasing SiO₂ content and a constant Na/Al ratio of 1. We obtained diffusion coefficients at 500 MPa and 1323-1673 K. In the fully polymerized system Ab₇₀Qz₃₀ - Jd, the change in composition only has a weak effect on bulk CO₂ diffusivity, but Ar diffusivity increases clearly with decreasing SiO₂ content. In the system Ab + Na₂O, bulk CO₂ and Ar diffusivity increase significantly with gradual depolymerisation. The relatively small change in composition on molar basis in the depolymerized system leads to a significantly larger change in diffusivities compared to the fully polymerized Ab₇₀Qz₃₀-Jd join. Within error, activation energies for bulk CO₂ and Ar diffusion in both systems are identical with decreasing silica content (Ab + Na₂O: 159 ± 25 kJ mol⁻¹ for bulk CO₂ and 130 ± 8 kJ mol⁻¹ for Ar; Ab₇₀Qz₃₀-Jd: 163 ± 16 kJ mol⁻¹ for bulk CO₂ and 148 ± 15 kJ mol⁻¹ for Ar) even though this results in depolymerisation in one system and not the other.Although there is a variation in CO₂ speciation with changing composition as observed in quenched glasses, it has previously established that this is not a true representation of the species present in the melt, with the ratio of molecular CO₂ to carbonate decreasing during quenching. Thus, diffusion coefficients for the individual CO₂ species cannot be directly derived by measuring molecular CO₂ and CO₃^2- concentration-distance profiles in the glasses. To obtain diffusivities of individual CO₂ species, we have made two assumptions that (1) inert Ar can be used as a proxy for molecular CO₂ diffusion characteristics as shown by our previous work and (2) the diffusivity of CO₃²⁻ can be calculated assuming it is identical to network forming components (Si⁴⁺ and Al³⁺). This is derived from viscosity data (Eyring eqn.) and suggests that CO₃²⁻ diffusion would be several orders of magnitude slower than molecular CO₂ diffusion.The systematics of measured bulk CO₂ diffusivity rates and comparison with the Ar proxy all suggest that the faster molecular CO₂ species is much more dominant in melts than measurements on resulting quenched glasses would suggest. This study has confirmed an observation of surprisingly consistent bulk CO₂ diffusivity across a range of natural compositions were Ar diffusivity significantly increases. This is consistent with an actual increase in molecular CO₂ mobility (similar to Ar) that is combined with an increase in the proportion of the slower carbonate in the melt.These results demonstrate that the CO₂ diffusion and speciation model provides an insight into the transport processes in the melt and is promising and an alternative tool to in situ speciation measurements at magmatic conditions, which at the moment are technically extremely difficult. We present the first high pressure high temperature in situ MIR spectra of a CO₂ bearing albitic glass/melt suggesting that molecular CO₂ is a stable species at high temperature, which is qualitatively consistent with the modelled CO₂ speciation data.     

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.     

Carbon isotopic fractionation between CO2 vapour,silicate and carbonate melts: an experimental study to 30 kbar

The carbon isotopic fractionation between CO₂ vapour and sodamelilite (NaCaAlSi₂O₇) melt over a range of pressures and temperatures has been investigated using solid-media piston-cylinder high pressure apparatus. Ag₂C₂O₄ was the source of CO₂ and experimental oxygen fugacity was buffered at hematite-magnetite by the double capsule technique. The abundance and isotopic composition of carbon dissolved in sodamelilite (SM) glass were determined by stepped heating and the ¹³C of coexisting vapour was determined directly by capsule piercing. CO₂ solubility in SM displays a complex behavior with temperature. At pressures up to 10 kbars CO₂ dissolves in SM to form carbonate ion complexes and the solubility data suggest slight negative temperature dependence. Above 20 kbars CO₂ reacts with SM to form immiscible Na-rich silicate and Ca-rich carbonate melts and CO₂ solubility in Na-enriched silicate melt rises with increasing temperature above the liquidus. Measured values for carbon isotopic fractionation between CO₂ vapour and carbonate ions dissoived in sodamelilite melt at 1200°–1400° C and 5–30 kbars average 2.4±0.2, favouring¹³C enrichment in CO₂ vapour. The results are maxima and are independent of pressure and temperature. Similar values of 2 are obtained for the carbon isotopic fractionation between CO₂ vapour and carbonate melts at 1300°–1400° C and 20–30 kbars.     

Phosphorus solubility mechanisms in haplogranitic aluminosilicate glass and melt: effect of temperature and aluminum content

The solubility behavior of phosphorus in glasses and melts in the system Na₂O-Al₂O₃-SiO₂-P₂O₅ has been examined as a function of temperature and Al₂O₃ content with microRaman spectroscopy. The Al₂O₃ was added (2, 4, 5, 6, and 8 mol% Al₂O₃) to melts with 80 mol% SiO₂ and ∼2 mol% P₂O₅. The compositions range from peralkaline, via meta-aluminous to peraluminous. Raman spectra were obtained of both the phosphorus-free and phosphorous-bearing glasses and melts between 25 and 1218 °C. The Raman spectrum of Al-free, P-bearing glass exhibits a characteristic strong band near 940 cm⁻¹ assigned to P=O stretching in orthophosphate complexes together with a weaker band near 1000 cm⁻¹ assigned P₂O₇ complexes. With increasing Al content, the proportion of P₂O₇ initially increases relative to PO₄ and is joined by AlPO₄ complexes which exhibit a characteristic P-O stretch mode slightly above 1100 cm⁻¹. The latter complex appears to dominate in meta-aluminosilicate glass and is the only phosphate complex in peraluminous glasses. When P-bearing peralkaline silicate and aluminosilicate glasses are transformed to supercooled melts, there is a rapid decrease in PO₄/P₂O₇ so that in the molten state, PO₄ units are barely discernible. The P₂O₇/AlPO₄ abundance ratio in peralkaline compositions increases with increasing temperature. This decrease in PO₄/P₂O₇ with increasing temperature results in depolymerization of the silicate melts. Dissolved P₂O₅ in peraluminous glass and melts forms AlPO₄ complexes only. This solution mechanism has no discernible influence on the aluminosilicate melt structure. There is no effect of temperature on this solution mechanism. Received: 7 October 1997 / Accepted: 11 May 1998     

The influence of bulk composition on the speciation of water in silicate glasses

Infrared spectroscopy was used to determine the concentrations of molecular water and hydroxyl groups in hydrous rhyolitic, orthoclasic, jadeitic, and Ca–Al-silicate glasses synthesized by quenching of melts from elevated presure and temperature. The rhyolitic glasses and some of the Ca–Al-silicate glasses were quenched from water-vapor-saturated melts and used to determine the solubility of water in melts of these compositions. For all compositions studied, hydroxyl groups are the dominant hydrous species at low total water contents, whereas molecular water dominates at elevated water contents. Although the trends in species concentrations in all these compositions are similar, the proportions of the two hydrous species are influenced by silicate chemistry: increasing silica content and K relative to Na both favor molecular water over hydroxyl. Results on rhyolitic glass demonstrate that molecular water is also favored by decreasing temperature at T<850°C. For rhyolitic glasses quenched from vapor-saturated melts, the mole fraction of molecular water is proportional to water fugacity for P(H₂O)1500 bars, demonstrating that the behavior of molecular water is approximately Henrian at total water contents up to at least several weight percent. Data on water solubility for albitic, orthoclasic, and Ca–Al-silicate melts to higher pressures can also be fit by assuming Henrian behavior for molecular water and can be used to set constraints on the partial molar volume of water in these melts. The demonstration of Henry's law for molecular water in these liquids provides a link between spectroscopic measurements of microscopic species concentrations and macroscopic thermodynamic properties.     

Oxygen diffusion in three silicate melts along the join diopside-anorthite

The self-diffusion of oxygen has been measured for three silicate melts along the join diopsideanorthite. The experiments were done by isotope exchange between an “infinite” reservoir of oxygen gas and spheres of melt. The oxygen self-diffusion coefficients for the three melts are given as: C-1(diopside): D = 1.64 × 10¹ exp(?(63.2 ± 20)(kcal/mole)/RT) cm²/sec C-2(Di₅₈An₄₂): D = 1.35 × 10^?1 exp(?(46.8 ± 9)(kcal/mole)/RT) cm²/sec C-3(Di₄₀An₆₀): D = 1.29 × 10^?2 exp(?(44.2 ± 6)(kcal/mole)/RT) cm²/secThe self-diffusion coefficients do not agree with the Eyring equation unless mean ionic jump distances (λ) considerably larger than the diameter of oxygen anion are assumed. However, the sense of variation of the actual diffusivities is as the Eyring equation predicts.Consideration of the results of this study and the bulk of previous work shows that oxygen appears to conform to the compensation law for cationic diffusion in silicate melts and glasses. The range of oxygen diffusivities was also found to encompass the field of divalent cation diffusivities in silicate melts.Those results imply that the diffusion of oxygen in silicate melts may involve a contribution from a cation-like diffusion mechanism (discrete O^2? anions) as well as contributions from the diffusion of larger structural units.     

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.     

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.     

Solubility of water and carbon dioxide in an icelandite at 1400 °C and 10 kilobars

The concentrations of water and carbon dissolved in an icelandite glass quenched from 1400 °C and 10 kbar were measured using Fourier transform infra-red spectroscopy and elemental analyses of carbon and hydrogen. Only carbon dioxide and water were observed in the fluid phase as analysed after quenching with a qudrupole mass analyser. The mole fraction of carbon dioxide in the fluid phase ranged from 0.36 to 0.95. Carbon is dissolved as carbonate except at the highest CO₂ fluid fugacity, where a small amount of molecular CO₂ is observed. Dissolved carbon in the glasses, calculated as CO₂, remained constant at approximately 1 wt %, in spite of the different CO₂ fluid fugacities. Water was dissolved as molecular water and as hydroxyl groups, the hydroxyl concentration in the quenched glasses remaining almost constant over the whole interval, whereas the molecular water dissolves in accordance with Henry's law. Molecular water peaks at 5200␣cm⁻¹ and 1630 cm⁻¹, the hydroxyl peak at 4500␣cm⁻¹, and the carbonate peaks at 1400 cm⁻¹–1550 cm⁻¹ have been calibrated using elemental analyses of C and H in the quenched glasses. As molecular water decreases in the melt the higher wavenumber carbonate peak is observed to move towards the molecular water peak at 1630 cm⁻¹ causing a split of the carbonate peaks, ranging from 45 cm⁻¹ to 100 cm⁻¹. Received: 15 November 1995 / Accepted: 21 September 1996     

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.     

Effect of boron on the water speciation in (alumino)silicate melts and glasses

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.     

A Raman spectroscopic study of H/D isotopically substituted hydrous aluminosilicate glasses

We have obtained high quality Raman spectra for two H/D isotopically substituted hydrous aluminosilicate glasses with compositions along the NaAlSi₃O₈-SiO₂ join. Consistent with the results of previous studies, the isotope shift for the band near 900 cm^–1, whose intensity grows with increasing water content, is extremely small: v _h /v _d = 1.004 ± 0.004. The lack of a definite H/D isotope shift for this band does not, however, preclude its association with a vibration of a hydrous species in the glass, because of likely strong coupling between different vibrational modes of hydrated framework species. The 900 cm^–1 band could well be due to a T — OH (T = Si, Al) stretching or bending vibration in the hydrous glass, as required by the presence of a combination band near 4500 cm^–1 in near-infrared spectra.     

Solution mechanisms of silicate in aqueous fluid and H2O in coexisting silicate melts determined in-situ at high pressure and high temperature

The structure of H₂O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na₂O·4SiO₂–H₂O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H₂O at nominal densities, ρ_fluid, (from EOS of pure H₂O) of 0.90 and 0.78 g/cm³. With ρ_fluid = 0.78 g/cm³, water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 °C) and pressure (677 MPa), whereas with ρ_fluid = 0.90 g/cm³, a homogeneous single-phase liquid phase exists through the temperature and pressure range (25–800 °C, 0.1–1033 MPa). Less than 5 vol% quartz precipitates near 650 °C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na₂O·4SiO₂ starting material.Molecular H₂O (H₂O°) and structurally bonded OH groups were observed in coexisting melts and fluids as well as in supercritical liquids. Their OH/(H₂O)-ratio is positively correlated with temperature. The OH/(H₂O)° in melts is greater than in coexisting fluids. Structural units of Q³, Q², Q¹, and Q⁰ type are observed in all phases under all conditions. An expression of the form, 12Q³ + 13H₂O2Q² + 6Q¹ + 4Q⁰, describes the equilibrium among those structural units. This equilibrium shifts to the right with increasing pressure and temperature with a ΔH of the reaction near 425 kJ/mol.