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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
1 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, U.S.A.
2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
Correspondence: * E-mail: kkelsey{at}stanford.edu
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ABSTRACT |
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 Top Abstract IntroductionMaterials and methodsResultsDiscussionAcknowledgmentsReferences cited |
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Key Words: NMR • aluminosilicate • glass • high pressure • coordination • aluminum • silicon • sodium
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INTRODUCTION |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionAcknowledgmentsReferences cited |
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As a melt is compressed, network-forming cations (including Si and Al) commonly respond with an increase in coordination number. High-coordinated Si has been previously reported in high-pressure silicate glasses (Xue et al. 1989, 1991; Stebbins and Poe 1999; Allwardt et al. 2004), and high-coordinated Al has been observed using various techniques in aluminosilicate glasses (Wolf and McMillan 1995; Yarger et al. 1995; Li et al. 1995; Poe et al. 2001; Lee et al. 2006; Allwardt et al. 2005a, 2005b; Kelsey et al. 2009). However, in aluminosilicate glasses the Al apparently changes coordination more readily than does the Si and, to date, high-coordinated Si has not been directly observed in high-pressure aluminosilicate glasses (Yarger et al. 1995; Allwardt et al. 2007; Kelsey et al. 2009), although it has been inferred from 17O 3QMAS NMR (Lee 2004). It has been hypothesized that if both Al and Si are present in a glass, the Al will change coordination number instead of the Si because Al is better charge balanced with 5 or 6 oxygen ions than is Si and has a slightly larger cation radius. In aluminosilicate glasses, it appears that the dominant, recovered structural change around the Si cations involves the decrease in Si-O-(Si,Al) bond angles instead of an increase in coordination. Composition strongly influences the amount of high-coordinated Al and Si generated: variations in both NBO/T ratios and in the field strength of the modifier cations are known to be especially important (Xue et al. 1991; Allwardt et al. 2005b; Kelsey et al. 2009).
Structural changes in network modifier environments with pressure are also known, and must be related to changes in the network. Sodium aluminosilicate glasses are especially useful experimental systems for studying these changes, as high-resolution NMR can characterize the short-range structure around all of the cations (as well as the oxygen anions), thus providing a more complete understanding of the interactions and trade-offs among different pressure effects. Large, low-charged network-modifying/charge-balancing cations such as Na+ commonly respond to pressure increases through a decrease in average bond lengths as the cation site is compressed (Allwardt et al. 2005b, 2005c; Lee et al. 2006; Kelsey et al. 2009).
Much is known about compositional effects on the structure of sodium aluminosilicate glasses that has bearing on the compositions studied here. For example, previous studies of ambient-pressure sodium aluminosilicate glasses have suggested that Al-Si substitution can take place in two structural units with different inter-tetrahedral angles, whose concentrations depend on Al content (Neuville and Mysen 1996); that Al-Si ordering increases with increasing Al content (Toplis et al. 1997); and that aluminum avoidance (i.e., the absence of Al-O-Al clusters) is nearly complete but does vary with Al content (Lee and Stebbins 1999). Mysen et al. (2003), in a study of glasses along the Na2Si3O7–Na2(NaAl)3O7 join, observed an increase in the frequency of the 29Si NMR peak maximum with increasing Al content and determined that the Al is dominantly located in Q4 structural units. Lee and Stebbins (2003) used 23Na NMR to conclude that substituting charge-balanced (NaAlO2) for SiO2 causes a decrease in average Na-O bond distances on the SiO2–NaAlO2 join.
Several studies of alkali silicate and aluminosilicate glasses have also described structural changes in densified glasses quenched from melts at high pressure and returned to ambient conditions, the approach followed here. As sodium silicate glasses densify, V,VISi forms and is most abundant in the Na2Si4O9 composition (Xue et al. 1989, 1991). It appears that these high-coordinated species are initially generated at the expense of non-bridging oxygen (NBO), but may involve bridging O atoms (BO) as well at higher pressures (Wolf et al. 1990), as has also been reported for high-pressure potassium silicate glasses (Allwardt et al. 2004). Furthermore, in high-pressure sodium silicates, changes in the IVSi NMR line shape have been attributed to increased Q species disproportion as Q3 are converted to Q2 and Q4 species, along with a decrease in the mean Si-O-Si angle and the development of Si-O-V,VISi (Xue et al. 1989, 1991). In Na2Si4O9 glasses, 23Na NMR suggested a very slight increase in Na-O distances with increasing pressure (Lee et al. 2006). In sodium aluminosilicate glasses, significant amounts of high-coordinated Al have been observed. However, this appears to be at the expense of the highly coordinated Si, as no V,VISi has been directly detected, although it has been suspected from 17O NMR (Lee 2004). As for the Al-free compositions, increasing pressure also causes a decrease in NBO, indicating that this may be involved in the generation of high-coordinated aluminum (Lee 2004; Lee et al. 2004). This effect has been reported for potassium and calcium aluminosilicate glasses (Allwardt et al. 2005c). There also appears to be a decrease in Na-O distances in densified sodium aluminosilicate glasses (Yarger et al. 1995; Allwardt et al. 2005b; Lee et al. 2006).
In this study, we use 29Si, 27Al, and 23Na NMR to examine sodium aluminosilicate glasses quenched from melts at ambient pressure and at 6 GPa, with compositions (Fig. 1
) based on adding Na2Al2O4 to Na2Si3O7 (NS3 series) and to Na2Si4O9 (NS4 series). Given the likelihood that most NBO are associated with Si, not Al, the nominal (low pressure) ratio of NBO/Si should remain roughly constant along each of these joins, simplifying interpretations of spectra. The range of Al contents was chosen such that, at its low end, even the extreme case of transformation of all Al to VIAl would allow some increase in Si coordination if the mean Si+Al coordination were similar to that reported in Al-free glasses. At the high end of the range, this same mean coordination change could be accommodated entirely by Al.
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MATERIALS AND METHODS |
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) were formed from mixtures of Na2CO3, Al2O3, and SiO2 (95% 29Si for some samples), with 0.15 wt% Co3O4 added to speed spin-lattice relaxation rates. These mixtures were melted in air for 45 min in crimped platinum tubes (to minimize Na loss) in batches of 0.5–2 g at ambient pressure and 1300 °C and quenched by dipping the tubes in water. To ensure compositional homogeneity and full dissolution of Al2O3, some of the glasses were reground and re-melted for 30 min. Starting glasses were then melted at 1620–1720 °C and 6 GPa in a Walker-style multi-anvil device (Table 1). Glass powders were placed in welded Pt capsules with outer diameters of 4 mm and lengths of 3.0–4.0 mm and run in 25/15 type injection-molded MgO + spinel assemblies with LaCrO3 heaters. The press calibration was performed at 1200 °C and extrapolated up to 1600 °C. We estimate the pressure uncertainty to be ±0.5 GPa. For all runs, the samples were first compressed to the desired pressure, and then heated at 300–500° per min to the desired temperature, which was maintained for about 5 min before the samples were quenched by turning off the furnace. Given what is known about melt viscosity, structural relaxation times in all of these compositions at temperatures above their liquidi should be much shorter than the run times (Webb and Dingwell 1995). The temperature was controlled using a W5Re-W26Re thermocouple separated from the capsule by a 0.35 mm thick alumina disk. Once quenched, the samples were decompressed over a period of 18–24 h. Electron microprobe analyses were obtained on 5–15 random spots on at least three separate fragments of each sample, at 15 kV with a sample current of 17 nA and a 10 µm spot size. At each point, Na was analyzed first, with a mean atomic number background correction procedure that eliminated off-peak background data collection (Donovan and Tingle 1996), and a 20 s counting time, all of which minimized Na migration away from the electron beam. All samples had the desired composition within uncertainties (standard deviation for each oxide component was less than 1% absolute), with no evidence of heterogeneity. No crystals were detected using optical microscopy, 29Si NMR, or 27Al NMR in any of the glass samples except for the NS4 glass with Al/(Al+Si) = 0.20, which contained a small amount (~3%) of crystalline jadeite detectable in both the 29Si NMR and 27Al NMR spectra. Microprobe analysis indicated that the glassy portion of this sample maintained the nominal contents of Al2O3, SiO2, and Na2O.
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Density measurements
Glass densities were measured using the sink/float method, in which all samples of a given series were placed in a small beaker containing a known amount of diiodomethane. Acetone was added in excess until all samples were denser than the solution. The acetone was then gradually evaporated until each sample started to float, at which point the weight of the solution was measured and used to calculate the weight of acetone and density of the solution. Once all samples were floating, additional acetone was added and the measurement was reproduced at least once. A CaMgSi2O6 glass standard with a density of 2.8 g/cm3 was analyzed both before and after each series of glasses. For each sample, the largest portion of the glass was chosen for analysis. However, for one glass sample in each series, three separate glass shards were chosen for analysis to test for uniformity. The densities of the 1 atm samples are not reported because air bubbles caused inconsistent measurements among different fragments of the same glass. However, several literature reports of the densities of sodium silicate glasses provide useful comparisons to those of the high-pressure samples (see below).
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RESULTS |
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and the coordination numbers obtained by fitting are given in Table 2. The spectra consist of signals from up to three different Si environments, IVSi, VSi, and VISi, which have maxima located at about –90, –150, and –200 ppm, respectively. The NS4 glass with Al/(Al+Si) = 0.20 contains about 3% crystalline jadeite, as labeled. Based on data for pure synthetic jadeite (Kelsey et al. 2007), this signal was subtracted before the proportions of the various coordination numbers and the centers of gravity were determined. The NS3 series has lower Si coordination numbers relative to the NS4 series at similar Al contents, which is consistent with previous results suggesting that the Na2Si4O9 composition may be near a maximum with respect to Si coordination increases (Xue et al. 1991). In the NS3 series, the samples with and without Al have nearly identical amounts of VSi and VISi, indicating that the presence of very small amounts of Al does not significantly affect the generation of high-coordinated Si. Similarly, the glass with Al/(Al+Si) of 0.006 in the NS4 series yields results quite similar to those of previous work on an Al-free Na2Si4O9 glass synthesized at 6 GPa (Xue et al. 1991), which contained 3.0% VSi and 2.1% VISi. When larger amounts of Al are added (as NaAlO2), there is a gradual decrease in the Si coordination, and there is only 0.3% VSi and less than 0.2% VISi when Al/(Al+Si) reaches 0.20. V,VISi was not detected at the 0.2% level in any of the 1 atm glasses, but it is likely that tiny amounts of VSi are present, as 0.045% of this species was observed in a more detailed study of 1 atm Na2Si4O9 glasses (Stebbins and McMillan 1993). As the different Si resonances are well resolved, the error in each of the Si coordination values reported in Table 2 is ±0.2, resulting in an uncertainty in the mean Si coordination number of ±0.005.
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and 4 show the IVSi regions of the 29Si MAS NMR spectra in more detail. The center of gravity for each IVSi peak was also measured (Table 2
). The IVSi resonance in the 1 atm NS4 glass with the lowest Al content clearly shows two overlapping components, centered at around –93 and –106 ppm. These are well known to be associated with the Q3 and Q4 species (Kirkpatrick 1988; MacKenzie and Smith 2002). As pressure increases, both of these peaks shift toward higher frequency, indicating a decrease in the average Si-O-Si angle, and merge together, consistent with earlier work (Xue et al. 1991). Previous observations suggested that increasing pressure favors the right-hand side of the reaction 2Q3 = Q2 + Q4 (Xue et al. 1989, 1991; Dickenson et al. 1990). However, in the compositions studied here the Q2 content is quite low and we expect this to have at most a minor effect. As the Al content increases, the IVSi resonance shifts toward higher frequency as the Si atoms gain more Al neighbors. The IVSi resonance in the 1 atm NS3 glass (Fig. 4) also has partially resolved Q3 and Q4 peaks, but the latter is lower due to the reduced silica content and higher NBO/Si. As pressure increases, these two resonances again both shift toward higher frequency and merge together. However, as the Al/(Al+Si) increases from 0 to 0.013 in the NS3 series, there is a slight decrease in the center of gravity for the IVSi resonance, opposite to the trend observed in the NS4 series of glasses. Although it is possible that there is a slight increase in the NBO content with increasing Al/(Al+Si), there may also be a small, undetected difference in Na content. In any case, this trend would not likely continue at higher Al contents, as Mysen et al. (2003) observed an increase in the peak maximum of the IVSi resonance with increasing Al content in glasses similar to the NS3 series but with higher Al contents. The latter is also consistent with the trend observed here in the NS4 series. Because each IVSi peak is made up of contributions from Q2, Q3, and Q4 species with varying numbers of Al neighbors, we did not attempt to fit the data to quantify the speciation. There is an uncertainty of ±0.2 ppm in the centers of gravity as the IVSi resonances are all fully resolved from the VSi and VISi.
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and 6 and the coordination numbers obtained by fitting these spectra are given in Table 3. There are three distinct resonances associated with IVAl, VAl, and VIAl located around 57, 22, and –7 ppm, respectively. In all of the 1 atm glasses, VAl is present and resolved well enough to quantify (Table 3), as is the case for both VAl and VIAl in the 6 GPa glasses. However, no VIAl is detected in the 1 atm glasses (0.1% detection limit). Because of distributions of quadrupolar coupling constants, each resonance is slightly skewed, with a lower frequency "tail," and was thus fit with two Gaussian lineshapes, as discussed previously (Kelsey et al. 2008), to be consistent with the extensive multifield NMR study by Neuville et al. (2006). In the NS4 sample with Al/(Al+Si) = 0.20, a small sharp peak at –2 ppm (labeled as "J") is consistent with the presence of about 3% crystalline jadeite, as also detected in the 29Si spectrum. Again based on data for pure jadeite (Kelsey et al. 2007), this signal was subtracted from the spectrum before the coordination numbers and center of gravity were determined. There is also a very small inflection in some of the spectra near –10 ppm, which is an artifact of the rotor background subtraction. In the NS4 series, at both 1 atm and 6 GPa, as the Al content increases, there is a decrease in mean Al coordination, similar to the decrease for Si discussed above. The NS3 glass with Al/(Al+Si) = 0.013 has an average Al coordination number of 5.49, which is apparently higher than has been previously reported for any alumina-bearing oxide glass synthesized up to 10 GPa (possibly excepting glasses very rich in phosphorus, Brow et al. 1993). This extreme value is clearly related to the low Al content, although the mean Al coordination of the NS4 glass with even lower Al content is somewhat less, indicating a complex relationship between composition and this aspect of the network structure. Because the resonances are well resolved and the spectra were fit using consistent approximations, the uncertainties in mean Al coordination values are about ±0.01 relative to data in this paper; variations in absolute estimates made with other fitting assumptions are likely to be somewhat larger, probably about ±0.02.
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. There is a slight increase in the frequency of the IVAl resonance and a decrease in that for VAl with increasing pressure as well as an increase in the frequencies for all of the peaks with increasing Al content at both 1 atm and 6 GPa. This shift may be due to changes in the Al-O-Si bond angles, a change in the coordination of the surrounding Si atoms with pressure or Al content, or a change in the number of second neighbor Al atoms. It is also possible that as the Al content increases there are additional Al-O-Al linkages, although none of the compositions are expected to have large amounts of this species (Lee and Stebbins 1999). The asymmetry of each resonance causes some peak overlap and uncertainty in calculating the center of gravity, which we estimate as ±0.3 ppm.
23Na NMR
23Na MAS NMR spectra for the NS3 and NS4 glasses were collected at 18.8 and 14.1 T; the latter are shown in Figure 7. For each sample, the single resonance is slightly asymmetrical, with a minor "tail" toward lower frequency, and shifts toward higher frequency with increasing pressure. As 23Na peak positions and shapes in similar samples are controlled primarily by chemical shifts and not second-order quadrupolar effects at these high fields (Lee and Stebbins 2003), this change indicates a decrease in the average Na-O bond distance and/or coordination number with increasing pressure (Xue and Stebbins 1993). The centers of gravity were determined at both fields and were used to calculate the mean isotropic chemical shifts (Schmidt et al. 2000) (Table 4). In the NS3 series, the addition of small amounts of Al does not have a significant effect on the observed Na environment. However, there is a shift of just under 1 ppm toward higher frequency with increasing pressure, indicating a decrease in Na-O bond distances of about 0.002(1) nm. In the NS4 series, there is a similar increase in mean isotropic chemical shift with increasing pressure. However, this shift increases from 0.6 to 2.1 as the Al/(Al+Si) increases from 0.006 to 0.20, corresponding to decreases in Na-O bond distances of about 0.001 to 0.004 nm. The ambient pressure glasses show only slight changes in chemical shifts with increasing Al content, although there seems to be a greater effect of composition in the 6 GPa glasses. Lee et al. (2006) reported a slight decrease in the frequency of the 23Na peak at 14.1 T with pressure in a Na2Si4O9 glass, which seems inconsistent with our results, but did describe increases in peak positions with pressure in Al-containing glasses. Our results are also consistent with those of Allwardt et al. (2005b) and Kelsey et al. (2009), who observed increases in 23Na isotropic chemical shifts with increasing pressure of similar magnitude to those described here. We estimate absolute uncertainties of about ±0.5 ppm in the mean chemical shifts. However, relative values when comparing two spectra collected on similar materials under identical conditions are again probably more precise, about ±0.3 ppm.
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. Tiny air bubbles in the 1 atm glasses prevented accurate measurements of their densities. In the table we thus include typical values from numerous literature reports of ambient pressure densities, which vary from study to study by about ±0.05 g/cm3. (See also several recent compilations by Doweidar 1996, 2001.) All of the 6 GPa glasses show density increases of 10–15% relative to 1 atm glasses. In the NS3 series, there is a slight increase in glass density with increasing Al content, which agrees with the published 1 atm glass densities. However, in the NS4 series, there may be a slightly more complicated trend where there is an increase in density and then a decrease. This trend is likely to be real because all samples of that series were analyzed together and we could directly observe the density of each sample relative to the others by observing the order in which they floated. Although the absolute uncertainty for the glass densities is about ±0.02 g/cm3, smaller differences of only about ±0.01 g/cm3 could be detected for glasses analyzed together.
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DISCUSSION |
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shows the mean Al, Si, and combined (Al+Si) coordination numbers as well as the densities of the high-pressure NS3 and NS4 glasses. The combined (Al+Si) coordination number is calculated by taking the weighted average of both the Al and Si coordination numbers, based on the glass composition.
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In both the NS3 and NS4 series, the Al and Si coordination numbers decrease with increasing Al content. However, the (Al+Si) mean coordination number increases slightly with increasing Al/(Al+Si) (Table 3; Fig. 8); the density may increase slightly as well. When the two series are compared, the NS4 has higher Si coordination numbers at similar Al contents, supporting the previous results of Xue et al. (1991) suggesting that the NBO concentration in Na2Si4O9 may be optimal for high-Si coordination.
As a glass or melt is compressed, the environment around a given cation may respond in different ways, including coordination number increases, decreases in bond distances and angles, or changes in the types and amounts of linkages present (e.g., Si-O-Al). In the case of Al and Si, perhaps the most obvious structural change is an increase in coordination number. However, with increasing pressure, 29Si NMR spectra also show a shift toward higher frequency and a narrowing of the IVSi resonance (Table 2; Figs. 3 and 4), probably indicating a decrease in the Si-O-Si and Si-O-Al bond angles. In general, this finding agrees with the conclusions from previous extensive studies of high-pressure aluminosilicate glasses by Raman spectroscopy (Mysen and Richet 2005; Poe et al. 2001). High-resolution 17O NMR on high-pressure alkali silicate and alkali aluminosilicate glasses may also reveal changes in network bond angles and their distributions (Lee et al. 2003). There is as well an increase in similarity between the NMR resonances of the different Q species as Na-NBO distances decrease. There must also be increases in Si-O-V,VISi and Si-O-V,VIAl linkages with increasing pressure. For both series of glasses, the shift in frequency of the IVSi peak with pressure is larger at lower Al contents, implying that the Si tetrahedral environments experience larger changes with pressure at lower Al contents, along with their faster rate of conversion to high-coordinated species.
In the 27Al NMR spectra, there also appear to be slight increases in the frequency of the IVAl resonance in the high-pressure glasses (Table 3). These may correspond to chemical shift changes in the same direction as that observed for IVSi sites, and may thus be related to the same types of changes in bond angles and polyhedral linkages. The VAl peaks seem to shift in the opposite direction, perhaps due to the development of links between high-coordinated Al sites. The 23Na NMR peaks (and calculated mean isotropic chemical shifts) for all of the glasses shift to higher frequency with increasing pressure (Fig. 7), as also seen previously in sodium aluminosilicate glasses (Yarger et al. 1995; Lee et al. 2006; Kelsey et al. 2009), probably indicating a decrease in the average Na-O bond distances of about 0.001 to 0.004 nm, with larger changes at higher Al contents. Narrowing of network bond angles may be expected as Na+ sites are compressed. All of these structural effects must eventually be linked together to try to form a more complete picture of compression mechanisms.
Thermodynamic implications
We reiterate that quenched and decompressed glasses are unlikely to provide perfectly accurate snapshots of equilibrium melt structure: some effects on glass structure have been shown to reverse on decompression (Wolf and McMillan 1995; Farber and Williams 1996), and to be better retained at decompression rates higher than those generally considered safe for the type of apparatus used here (Allwardt et al. 2005c). However, if the observed glass structures at least approximate those of the high-pressure melts at their glass transition temperatures, some thermodynamic analysis can provide a useful way of comparing data sets and determining whether observed changes with composition are sensible. For example, a rough initial approximation of the distribution of VSi and VAl coordinations can be made by considering the reaction:
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for which an ideal, apparent equilibrium constant can be written in terms of observed mole fractions:
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Other reactions involving VISi and VIAl could of course be written and would have apparent equilibrium constants such as:
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"Exchange" reactions written in this form may be somewhat misleading or incomplete, as they do not explicitly account for changes in the oxygen species and distribution of charge-balancing cations that must accompany network cation coordination shifts (Stebbins et al. 2008). The resulting K values are not intended as quantitative predictors of speciation in other experimental systems. However, they may serve to check for consistency among data in a compositional series. Values of log10(KSA) calculated from the data in Tables 2 and 3 are given in Table 6. All log10(KSA–45) are within the range of –2 ±0.2, and the values for log10(KSA–46) may be a bit lower. For the sample with Al/(Al+Si) = 0.20, the tabulated value for log10(KSA–46) is based on varying the (unobserved) VISi concentration from 0.05 to 0.20%, which is a reasonable guess based on detection limits and trends for the other samples. For both sets of KSA, there may be some systematic effect of composition, as there appears to be a slight decrease in log10(KSA) with increasing Al content in the NS4 series, but this is small with respect to uncertainties. In any case, the low values are expected from the relative ease of formation of VAl compared to VSi.
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with
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A reaction can also be written relating the distribution of Si coordinations and would have:
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As shown in Table 6, log10(KAA) and log10(KSS) values are all similar, in the range of 0.1 ±0.1 and – 1.6 ±0.1, respectively. As with the log10(KSA–45), the value for log10(KSS) for the sample with Al/(Al+Si) = 0.20 is based on VISi ranging from 0.05 to 0.20.
In a previous study, an analysis of the density increases observed for high-pressure Ca aluminosilicate glasses in terms of estimated partial molar volumes of IVAl and VAl indicated that although network cation coordination may be the most visible structural result of densification, other changes must occur that have similar or even greater consequences for the bulk properties (Allwardt et al. 2005b). We apply that approach here by calculating molar volumes near Tg and ambient pressure from partial molar volumes of oxide components tabulated at 1023 K (Lange 1997). As a crude estimate of the effects on volume of observed Si and Al coordination changes, we then approximate the partial molar volumes of V,VISiO2 and V,VIAl2O3 as fixed fractions, F, of the 1 bar values (Lange 1997). For simplicity, we consider F constant for all four high-coordinated species; extreme minimum values of F might be taken as about 0.52 (the molar volume of stishovite divided by that of the IVSiO2 melt component) or as 0.68 (the molar volume of corundum divided by that of the IVAl2O3 melt component). Using observed contents of high-coordinated Al and Si, the predicted contributions to the overall volume decreases range from about 1% (F = 0.8) to 2% (F = 0.6). These are far less than the observed density increases of 10 to 15% (Table 5). This reinforces the conclusion that much of the densification (in these compositions and pressure range) is accommodated by compression of alkali cation sites and narrowing of network bond angles, as qualitatively indicated by observed changes in the 29Si, 27Al, and 23Na NMR spectra.
Another intriguing outcome of this approach can help to rationalize the seemingly dramatic effects of Al content on Al and Si coordination changes with pressure. Figure 9 shows, for F = 0.6, the percent volume decreases predicted separately from the observed Si and Al coordination changes, together with their sum; plots for larger values of F are similar, with all volume changes scaled down in proportion. For the three glasses in the NS4 series, the total predicted volume change is nearly constant, with a greater proportion resulting from Al as Al/(Al+Si) increases. Rather remarkably, these glasses behave as if there is a constant, relatively small proportion (10–20%) of the overall densification that is taken up by network cation coordination increases, regardless of composition. Because of the relative ease of formation of high-coordinated Al relative to Si, the coordination number of Al is very high at low Al contents, but this contributes only a small part of the volume decrease, which in turn is taken up mostly by coordination increase of Si; at higher Al contents, the same volume decrease can occur with little or no Si coordination change. The one available set of data for the NS3 series gives similar predicted volume changes, but probably somewhat reduced by the higher NBO content. Whether or not this trade-off among volume changes attributable to Si and Al coordination changes prevails over a wider compositional range, and how it relates to effects of other structural changes, will be important questions for future studies.
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ACKNOWLEDGMENTS |
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Footnotes |
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MANUSCRIPT RECEIVED December 19, 2008; MANUSCRIPT ACCEPTED April 24, 2009
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REFERENCES CITED |
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