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1.
The heat capacity and vibrational entropy of a calcium aluminate and three peraluminous calcium aluminosilicate glasses have been determined from 2 to 300 K by heat-pulse relaxation calorimetry. Together with previous adiabatic data for six other glasses in the system CaO-Al2O3-SiO2, these results have been used to determine partial molar heat capacities and entropies for five species namely, SiO2, CaO and three different sorts of Al2O3 in which Al is 4-, 5- and 6-fold coordinated by oxygen. Given the determining role of oxygen coordination on low-temperature heat capacity, the composition independent entropies found for SiO2 and CaO indicate that short-range order around Si and Ca is not sensitive to aluminum speciation up to the highest fraction of 25% observed for VAl by NMR spectroscopy. Because of the higher room-temperature vibrational entropy of IVAl2O3 (72.8 J/mol K) compared to VAl2O3 (48.5 J/mol K), temperature-induced changes from IVAl to VAl give rise to a small negative contribution of the order of 1 J/mol K to the partial molar configurational heat capacity of Al2O3 in melts. Near 0 K, pure SiO2 glass distinguishes itself by the importance of the calorimetric boson peak. On a g atom basis, the maximum of this peak varies with the composition of calcium aluminosilicate glasses by a factor of about 2. It does not show smooth variations, however, either as a function of SiO2 content, at constant CaO/Al2O3 ratio, or as a function of Al2O3 content, at constant SiO2 content.  相似文献   

2.
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   

3.
The sodium solubility in silicate melts in the CaO-MgO-SiO2 (CMS) system at 1400 °C has been measured by using a closed thermochemical reactor designed to control alkali metal activity. In this reactor, Na(g) evaporation from a Na2O-xSiO2 melt imposes an alkali metal vapor pressure in equilibrium with the molten silicate samples. Because of equilibrium conditions in the reactor, the activity of sodium-metal oxide in the molten samples is the same as that of the source, i.e., aNa2O(sample) = aNa2O(source). This design also allows to determine the sodium oxide activity coefficient in the samples. Thirty-three different CMS compositions were studied. The results show that the amount of sodium entering from the gas phase (i.e., Na2O solubility) is strongly sensitive to silica content of the melt and, to a lesser extent, the relative amounts of CaO and MgO. Despite the large range of tested melt compositions (0 < CaO and MgO < 40; 40 < SiO2 < 100; in wt%), we found that Na2O solubility is conveniently modeled as a linear function of the optical basicity (Λ) calculated on a Na-free basis melt composition. In our experiments, γNa2O(sample) ranges from 7 × 10−7 to 5 × 10−6, indicating a strongly non-ideal behavior of Na2O solubility in the studied CMS melts (γNa2O(sample) ? 1). In addition to showing the effect of sodium on phase relationships in the CMS system, this Na2O solubility study brings valuable new constraints on how melt structure controls the solubility of Na in the CMS silicate melts. Our results suggest that Na2O addition causes depolymerization of the melt by preferential breaking of Si-O-Si bonds of the most polymerized tetrahedral sites, mainly Q4.  相似文献   

4.
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  相似文献   

5.
The heat capacities of 29 glasses and supercooled liquids in the Na2O-SiO2, Na2O-Al2O3-SiO2, Na2O-(FeO)-Fe2O3-SiO2, and Na2O-TiO2-SiO2 systems were measured in air from 328 to 998 K with a differential scanning calorimeter. The reproducibility of the data determined from multiple heat capacity runs on a single crystal MgO standard is within ± 1% of the accepted values at temperatures ≤ 800 K and within ± 1.5% between 800 and 1000 K. Within the resolution of the data, the heat capacities of sodium silicate and sodium aluminosilicate liquids are temperature independent. Heat capacity data in the supercooled liquid region for the sodium silicates and sodium aluminosilicates were combined and modelled assuming a linear compositional dependence. The derived values for the partial molar heat capacities of Na2O, Al2O3, and SiO2 are 112.35 ± 0.42, 153.16 ± 0.82, and 76.38 ± 0.20 J/gfw · K respectively. The partial molar heat capacities of Fe2O3 and TiO2 could not be determined in the same manner because the heat capacities of the Fe2O3- and TiO2-bearing sodium silicate melts showed varying degrees of negative temperature dependence. The negative temperature dependence to the configurational C P may be related to the occurrence of sub-microscopic domains (relatively polymerized and depolymerized) that break down to a more homogeneous melt structure with increasing temperature. Such an interpretation is consistent with data from in situ Raman, Mössbauer, and X-ray absorption fine structure (XAFS) spectroscopic studies on similar melts.  相似文献   

6.
The influence of ferrous and ferric iron on the low-temperature heat capacity and vibrational entropy of silicate glasses has been determined by adiabatic calorimetry. Two pairs of samples based on sodium disilicate and calcium Tschermak molecule compositions have been studied. Along with previous data for another Fe-bearing glass, these results have been used to complement the available set of composition independent partial molar relative entropies of oxides in silicate glasses with S298 − S0 values of 56.7 and 116 J/mol for FeO and Fe2O3, respectively. The calorimetric data indicate that the fraction of fivefold coordinated Al is significant in the CaO-“FeO”-Al2O3-SiO2 system and that association of Ca2+ and Na+ with Fe3+ in tetrahedral coordination for charge compensation does not entail significant changes in coordination for these two cations. At very low temperatures, however, the heat capacity is no longer an additive function of composition because of unexpectedly high positive deviations from Debye laws. These anomalies are stronger for the reduced than the oxidized glasses and considerably larger than for iron-free glasses, but their origin cannot be established from the present measurements.  相似文献   

7.
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   

8.
A revised model for the volume and thermal expansivity of K2O-Na2O-CaO-MgO-Al2O3-SiO2 liquids, which can be applied at crustal magmatic temperatures, has been derived from new low temperature (701–1092 K) density measurements on sixteen supercooled liquids, for which high temperature (1421–1896 K) liquid density data are available. These data were combined with similar measurements previously performed by the present author on eight sodium aluminosilicate samples, for which high temperature density measurements are also available. Compositions (in mol%) range from 37 to 75% SiO2, 0 to 27% Al2O3, 0 to 38% MgO, 0 to 43% CaO, 0 to 33% Na2O and 0 to 29% K2O. The strategy employed for the low temperature density measurements is based on the assumption that the volume of a glass is equal to that of the liquid at the limiting fictive temperature, T f ′. The volume of the glass and liquid at T f ′ was obtained from the glass density at 298 K and the glass thermal expansion coefficient from 298 K to T f ′. The low temperature volume data were combined with the existing high temperature measurements to derive a constant thermal expansivity of each liquid over a wide temperature interval (767–1127 degrees) with a fitted 1 error of 0.5 to 5.7%. Calibration of a linear model equation leads to fitted values of Vˉ i ±1 (cc/mol) at 1373 K for SiO2 (26.86 ± 0.03), Al2O3 (37.42±0.09), MgO (10.71±0.08), CaO (15.41±0.06), Na2O (26.57±0.06), K2O (42.45 ± 0.09), and fitted values of dVˉ i /dT (10−3 cc/mol-K) for MgO (3.27±0.17), CaO (3.74±0.12), Na2O (7.68±0.10) and K2O (12.08±0.20). The results indicate that neither SiO2 nor Al2O3 contribute to the thermal expansivity of the liquids, and that dV/dT liq is independent of temperature between 701 and 1896 K over a wide range of composition. Between 59 and 78% of the thermal expansivity of the experimental liquids is derived from configurational (vs vibrational) contributions. Measured volumes and thermal expansivities can be recovered with this model with a standard deviation of 0.25% and 5.7%, respectively. Received: 2 August 1996 / Accepted: 12 June 1997  相似文献   

9.
The solubility behavior of K2O, Na2O, Al2O3, and SiO2 in silicate-saturated aqueous fluid and coexisting H2O-saturated silicate melts in the systems K2O-Al2O3-SiO2-H2O and Na2O-Al2O3-SiO2-H2O has been examined in the 1- to 2-GPa pressure range at 1100°C. Glasses of Na- and K-tetrasilicate compositions with 0, 3, and 6 mol% Al2O3 were used as starting materials. In both systems, the oxides dissolve incongruently in aqueous fluid and silicate melt. When recalculated to an anhydrous basis, the aqueous fluids are enriched in alkalis and depleted in silica and alumina relative to their proportions in the starting materials. The extent of incongruency is more pronounced in the Na2O-Al2O3-SiO2-H2O system than in the K2O-Al2O3-SiO2-H2O system.The partition coefficients of the oxides, Doxidefluid/melt, are linear and positive functions of the oxide concentration in the fluid for each composition. There is a slight dependence of the partition coefficients on bulk composition. No effect of pressure could be discerned. For alkali metals, the fluid/melt partition coefficients range from 0.06 to 0.8. For Al2O3 this range is 0.01 to 0.2, and for SiO2, it is 0.01 to 0.32. For all compositions, DK2Ofluid/melt∼DNa2Ofluid/melt>DSiO2fluid/melt>DAl2O3fluid/melt for the same oxide concentration in the fluid. DK2Ofluid/melt, DNa2Ofluid/melt, and DSiO2fluid/melt correlate negatively with the Al2O3 content of the systems. This correlation is consistent with a solubility model of alkalis that involve associated KOH°, NaOH°, silicate, and aluminate complexes.  相似文献   

10.
The effect of fluorine and fluorine + chlorine on melt viscosities in the system Na2O-Fe2O3-Al2O3-SiO2 has been investigated. Shear viscosities of melts ranging in composition from peraluminous [(Na2O + FeO) < (Al2O3 + Fe2O3)] to peralkaline [(Na2O + FeO) > (Al2O3 + Fe2O3)] were determined over a temperature range 560-890 °C at room pressure in a nitrogen atmosphere. Viscosities were determined using the micropenetration technique in the range of 108.8 to 1012.0 Pa s. The compositions are based on addition of FeF3 and FeCl3 to aluminosilicate melts with a fixed amount of SiO2 (67 mol%). Although there was a significant loss of F and Cl during glass syntheses, none occurred during the viscometry experiments. The presence of fluorine causes a decrease in the viscosity of all melts investigated. This is in agreement with the structural model that two fluorines replace one oxygen; resulting in a depolymerisation of the melt and thus a decrease in viscosity. The presence of both chlorine and fluorine results in a slight increase in the viscosity of peraluminous melts and a decrease in viscosity of peralkaline melts. The variation in viscosity produced by the addition of both fluorine and chlorine is the opposite to that observed in the same composition melts, with the addition of chlorine alone (Zimova M. and Webb S.L. (2006) The effect of chlorine on the viscosity of Na2O-Fe2O3-Al2O3-SiO2 melts. Am. Mineral.91, 344-352). This suggests that the structural interaction of chlorine and fluorine is not linear and the rheology of magmas containing both volatiles is more complex than previously assumed.  相似文献   

11.
Short and medium range order of silica and sodium silicate glasses have been investigated from a quantitative analysis of 29Si MAS NMR and 23Na, 17O MQMAS NMR spectra. The method described enables the extraction of the underlying 17O NMR parameter distributions of bridging oxygens (BOs) and non-bridging oxygens (NBOs), and yields site populations which are confirmed by 29Si NMR data. The extracted NMR parameter distributions and their variations with respect to the glass chemical composition can then be analyzed in terms of local structural features (bond angles and bond lengths, coordination numbers) with the help of molecular dynamics simulations combined with first-principles calculations of NMR parameters. Correlations of relevant structural parameters with 23Na, 29Si and 17O NMR interactions (isotropic chemical shift δiso, quadrupolar coupling constant CQ and quadrupolar asymmetry parameter ηQ) are re-examined and their applicability is discussed. These data offer better insights into the structural organization of the glass network, including both chemical and topological disorder. Adding sodium to pure silica significantly diminishes the Si-O-Si bond angles and leads to a longer mean Si-O bond length with a slight decrease of the mean Na-O bond length. Moreover, the present data are in favor of a homogeneous distribution of Na around both oxygen species in the silicate network. Finally, our approach was found to be sensitive enough to investigate the effect of addition of a small quantity of molybdenum oxide (about 1 mol%) on the 17O MAS spectrum, opening new possibilities for investigating the Mo environment in silicate glasses.  相似文献   

12.
Configurational changes with temperature are important for the thermodynamic and transport properties of most aluminosilicate melts, but in general are not well understood. Here, we present high-resolution 27Al and 17O NMR data on several calcium aluminosilicate glasses prepared with varying quench rates and thus with fictive temperatures that span ranges up to about 200 K. In all compositions the content of five-coordinated aluminum increases with fictive temperature, in agreement with recent high temperature NMR data on melts. In a glass of CaAl2Si2O8 (“anorthite”) composition, the content of non-bridging oxygens also increases with temperature; however this effect was not observed in a sample with a much higher CaO/Al2O3 ratio. We present a consistent notation for reactions among structural species in these systems that clarify why in some cases, high-coordinated network cations may appear on the same side of the reaction, while in others they occur on the opposite sides: the key difference is in accounting for all coordination changes for oxygens. Mixing of non-bridging oxygens and of high-coordinated aluminum make significant contributions to the overall configurational entropy and heat capacity of the melts, as does the mixing of various bridging oxygens and of tetrahedral network cations. Other, less well known, types of increase in disorder with temperature may be important as well.  相似文献   

13.
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.  相似文献   

14.
Solubility and solution mechanisms of H2O in depolymerized melts in the system Na2O-Al2O3-SiO2 were deduced from spectroscopic data of glasses quenched from melts at 1100 °C at 0.8-2.0 GPa. Data were obtained along a join with fixed nominal NBO/T = 0.5 of the anhydrous materials [Na2Si4O9-Na2(NaAl)4O9] with Al/(Al+Si) = 0.00-0.25. The H2O solubility was fitted to the expression, XH2O=0.20+0.0020fH2O-0.7XAl+0.9(XAl)2, where XH2O is the mole fraction of H2O (calculated with O = 1), fH2O the fugacity of H2O, and XAl = Al/(Al+Si). Partial molar volume of H2O in the melts, , calculated from the H2O-solulbility data assuming ideal mixing of melt-H2O solutions, is 12.5 cm3/mol for Al-free melts and decreases linearly to 8.9 cm3/mol for melts with Al/(Al+Si) ∼ 0.25. However, if recent suggestion that is composition-independent is applied to constrain activity-composition relations of the hydrous melts, the activity coefficient of H2O, , increases with Al/(Al+Si).Solution mechanisms of H2O were obtained by combining Raman and 29Si NMR spectroscopic data. Degree of melt depolymerization, NBO/T, increases with H2O content. The rate of NBO/T-change with H2O is negatively correlated with H2O and positively correlated with Al/(Al+Si). The main depolymerization reaction involves breakage of oxygen bridges in Q4-species to form Q2 species. Steric hindrance appears to restrict bonding of H+ with nonbridging oxygen in Q3 species. The presence of Al3+ does not affect the water solution mechanisms significantly.  相似文献   

15.
In 29Si-NMR, it has so far been accepted that the chemical shifts of Qn species (SiO4 units containing n bridging oxygens) were equivalent between alkali borosilicate and boron-free alkali silicate glasses. In the sodium borosilicate glasses with low sodium content, however, a contradiction was confirmed in the estimation of alkali distribution; 11B NMR suggested that Na ions were entirely distributed to borate groups to form BO4 units, whereas a −90 ppm component was also observed in 29Si-NMR spectra, which has been attributed to Q3 species associated with a nonbridging oxygen (NBO). Then, cluster molecular orbital calculations were performed to interpret the −90 ppm component in the borosilicate glasses. It was found that a silicon atom which had two tetrahedral borons (B4) as its second nearest neighbors was similar in atomic charge and Si2p energy to the Q3 species in boron-free alkali silicates. Unequal distribution of electrons in Si-O-B4 bridging bonds was also found, where much electrons were localized on the Si-O bonds. It was finally concluded that the Si-O-B4 bridges with narrow bond angle were responsible for the −90 ppm 29Si component in the borosilicate glasses. There still remained another interpretation; the Q3 species were actually present in the glasses, and NBOs in the Q3 species were derived from the tricluster groups, such as (O3Si)O(BO3)2. In the glasses with low sodium content, however, it was concluded that the tricluster groups were not so abundant to contribute to the −90 ppm component.  相似文献   

16.
We have measured the water solubility between 1 atmosphere and 5 kilobars for a calcium aluminosilicate melt of molar composition CaO 0.28, Al2O3 0.06, SiO2 0.66 (An9Wo38Qz53). The water contents were measured via thermogravimetric analysis of isobarically quenched glasses, and range from 0.121 wt% H2O near 1 aim to 9.25 wt% H2O at 5 kilobars. The molar water solubility lies between those of SiO2 and albite melts below around three kilobars, and crosses the albite solubility curve above this pressure. The present results are compared with data in the literature on related calcium aluminosilicate melts. There seems to be little variation of water solubility with composition for calcium aluminosilicate melts, unlike analogous alkali aluminosilicate compositions. Examination of the data suggests that there may be a maximum in molar water solubility along the albite-anorthite join.  相似文献   

17.
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   

18.
Incremental amounts of Na2O and K2O added to immiscible melts in the MgO-CaO-TiO2-Al2O3 SiO2 system cause a decrease in critical temperature, phase separation and change in the pattern of Al2O3 partitioning. Al2O3, which is concentrated in the low SiO2 immiscible melts in the alkali-free system, is increasingly partitioned into the high-SiO2 immiscible melt as the alkali/aluminium ratio is increased. However, K2O is more effective than Na2O in stabilizing Al2O2 in the SiO2-rich melt. The coordination changes occurring in the aluminosilicate melts upon the addition of the alkali oxides are described by CaAl2O4+2SiOK=2KAlO2+SiOCaOSi where K (or Na) displaces Ca as the charge-balancing cation for the networkforming AlO4 tetrahedra. The increased stability of the AlO4 species in the highly polymerized SiO2-rich melt and the consequent shrinkage of the miscibility gap is ascribed to positive configurational entropy and negative enthalpy changes associated with the formation of K, Na-AlO4 species. Element partition systematics indicate that (Na, K)AlO2 species favor the more polymerized, CaAl2O4 and TiO2 species, the less polymerized silicate structure in the melt.  相似文献   

19.
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.  相似文献   

20.
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 (XSiO2=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.  相似文献   

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