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Effect of water on the high-pressure structural behavior of anorthite-diopside eutectic glass
Journal of Non-Crystalline Solids 452 (2016) 312–319
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Effect of water on the high-pressure structural behavior of anorthite-diopside eutectic glass Wesley Helwig a, Emmanuel Soignard b, James A. Tyburczy c,⁎ a b c
Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, United States Leroy Eyring Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704, United States School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, United States
a r t i c l e
i n f o
Article history: Received 12 May 2016 Received in revised form 19 August 2016 Accepted 22 August 2016 Available online xxxx Keywords: Glass structure Raman spectroscopy High pressure Anorthite-diopside glass Aluminum coordination Triclusters Polymerization
a b s t r a c t We employed in situ high pressure Raman spectroscopy to examine structural variations in hydrous versus anhydrous aluminosilicate glasses as a function of pressure. Raman spectra of anhydrous and water-saturated (6.7 wt% H2O) eutectic anorthite – diopside (An-Di) glasses were collected at pressures up to 10 GPa in a diamond anvil cell (DAC). Both glasses exhibited a distinct change in compression mechanism at about 2.5 GPa. From 0 to 2.5 GPa each glass depolymerizes. Above 2.5 GPa the anhydrous glass becomes less polymerized, whereas the hydrous glass becomes more polymerized as pressure is increased up to 10 GPa. These differences are explained in terms of Al-coordination and formation of triclusters. For the dry glass, at pressures below 2.5 GPa, depolymerization occurs by means of tricluster (OT3) formation in which bridging oxygens (BO) become triply coordinated to a third network forming cation, preferentially IVAl, thereby increasing the coordination to VAl. At pressures N 2.5 GPa compression induced coordination to non-bridging oxygens (NBO) causes tetrahedral IVAl to become highly coordinated V,VIAl. Network modifying cations (Ca and Mg) coordinated to NBO at ambient conditions become charge-balancing cations for V,VIAl at elevated pressure, resulting in decreased polymerization. For the wet glass, compression up to 2.5 GPa causes protons in H2O to depolymerize Al tetrahedra into Al-OH. At pressures N 2.5 GPa most of the highly coordinated Al is present as VIAl and network polymerization increases with the formation of M-OH (M = Ca, Mg) groups that enable Si-O-Si bonds (BO). Bulk modulus measurements support increased polymerization with the wet glass (K0 = 16 ± 2GPa) shown to be more compressible than the dry glass (K0 = 52 ± 5 GPa). © 2016 Elsevier B.V. All rights reserved.
1. Introduction Understanding the mechanics of the deep Earth requires understanding properties of silicate melts under deep Earth conditions. In this investigation, in-situ high-pressure Raman spectroscopy and relative volume measurements were collected on two glass compositions within a DAC at room temperature from 0 to 10.5 GPa. The compositions were a dry and water-saturated glass near the An-Di eutectic. These measurements were made to examine the effects of pressurequenching and high water content on glass structure. This glass composition is of interest because aluminosilicates in the anorthite-diopsideforsterite (An-Di-Fo) ternary have been proposed as analogues to mid ocean ridge basaltic melts (MORB) [1–6]. While structures of glasses have been studied at ambient conditions as well as quenched high pressure and/or temperature [7–13], there are few studies of the An-Di eutectic using in-situ high pressure studies via Raman spectroscopy. Structural studies of melts are important because those properties have been correlated with melt viscosity, melt density, elastic ⁎ Corresponding author. E-mail address: [email protected] (J.A. Tyburczy).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.08.030 0022-3093/© 2016 Elsevier B.V. All rights reserved.
properties, and thermodynamic properties [3,14–17]. As well, the effects of H2O on melts and the significance for Earth's interior have also been examined [2,4,18–25]. Those studies indicate that water has significant effects on melt properties and can thus be correlated to structural features. Poe et al. [12] examined an anhydrous calcium-aluminosilicate glass quenched from pressures to 12 GPa using Raman spectroscopy and XANES. They proposed that for pressures from 0 to 6 GPa, the formation of triply-coordinated oxygen (‘triclusters’) occurs without affecting the coordination of aluminum. At pressures above 6 GPa a mechanism change occurred and the quenched samples exhibited spectra consistent with either an increase in aluminum coordination or a reduction in Al-O-Al bond angle. Xue and Kanzaki [13]examined quenched undersaturated water bearing glasses in the An-Di system using 1H and 27Al NMR at ambient pressure. They concluded that dissolution of water in these glasses involves competition between formation of SiOH and AlOH species that depolymerize the glasses and formation of free OH that serves to polymerize the melts. Stronger field strength cations (Ca, Mg) enhance the tendency to polymerize whereas alkali cations (Na, K) act to depolymerize the network. Aluminum is predominantly four-coordinated to oxygen (IVAl) but a small amount of five
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coordinated aluminum (VAl) occurs in all glasses and some six-coordinated Al (VIAl) occurs in all hydrous glasses. Mysen et al. [26] studied H2O-saturated melts in the Na-Al-Si-O system in situ at pressure to 0.8 GPa and temperatures to 800 °C, observing that increasing H2O depolymerizes the melts. In this investigation, we perform in-situ high pressure (0–10 GPa) room temperature Raman spectroscopy on a dry and water-saturated glass with composition near the An-Di eutectic (40CaAl2Si2O860CaMgSi2O6 wt%; the eutectic is An 42 wt% Di 58 wt%). This composition was chosen for its relatively low melting temperature at 1 bar pressure of approximately 1270 °C [3]. In-situ Raman spectra and relative volume measurements were collected on both glasses at room temperature in a DAC up to pressures of 10.5 GPa. 1.1. Raman spectra The 500–600 cm−1 Raman spectral region of aluminosilicate glasses has been associated with T-O-T (T = Si, Al) bending/stretching motions between variably polymerized network forming cations [27] and we assign these modes to the 500–600 cm−1 spectral region of anhydrous and hydrous glass studies here. The degree of polymerization can be denoted as Qn species (Q = Si, Al and n = 4, 3, 2, 1 BO). Overlapping of Raman bands in this region prevents assignment to specific Qn [8,24, 27,28]. The 600–800 cm−1 bands correspond to stretching/bending vibrations associated with V,VIAl upon compression [12,28–39]. The 650– 670 and 720–750 cm−1 bands are respectively correlated to the occurrence of VAl and VIAl in each glass [34,39–41]. The 800–1200 cm−1 region is assigned to T-NBO stretching, which NBO are coordinated to network modifying cations Ca and Mg. Prior investigations have assigned individual bands to specific Qn (n = 3,2,1,0 BOs), in which higher frequency bands in the region are representative of higher Qn species [26,42–47]. However, the positions of these bands shift as a function of polymerization, modifier cation species (Ca, Mg, Na, K), and with the ratio of Al/(Si + Al) as shown in studies of calcium-magnesium aluminosilicate (CMAS) glasses [48]. Although the assignment of specific peaks to specific Qn species may be difficult, it will be assumed the 1020–1040 cm−1 band can be assigned to Q3, given the likelihood of its presence based on the glass degree of polymerization (NBO/T = 0.86) and associated Qn speciation [14,43,48]. Both the 910–930 cm− 1 and 960–980 cm− 1 bands are assigned to Qn b 3. 2. Experimental details The anhydrous glass (herein referred to as the ‘dry’ glass) was prepared using a mixture of high purity SiO2 (Alfa Aesar 99.999%), Al2O3 (Sigma Aldrich 99.998%), CaCO3 (Alfa Aesar 99.997%), and MgCO3 (99.998%). The fine powder mixture was decarbonated in a platinum crucible at 900 °C for 1 h in air. The sample was then melted in a MoSi2 vertical tube furnace in air at 1500 °C for 10 min. The molten sample was drop-quenched in water by burning through the support wire resulting in a clear colorless glass. The composition was determined by electron probe microanalysis (Table 1). No elements other than Si, Al, Mg, Ca and O were present in measurable quantity. Table 1 Average dry glass oxide weight per cent determined by electron probe micro-analysis, and theoretical weight % from the eutectic composition. Errors are standard deviations over five repeat measurements. Oxide
Weight (%) measured
Weight (%) (theoretical)
SiO2 Al2O3 MgO CaO Total weight percent
51.1 ± 0.3 17.0 ± 0.1 9.4 ± 0.2 23.7 ± 0.4 101.4 ± 0.5
49.8 16.9 10.0 23.2 100
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To prepare the hydrous glass (herein referred to as the ‘wet’ glass), 25 mg of the dry glass were loaded into a 5 mm outside diameter, 0.127 mm wall thickness platinum capsule with an excess of 18 MΩ (Millipore ultrapure) water. The capsule was welded shut while wrapped in a wet paper towel to minimize water loss. The sample was melted at 0.4 GPa at 1300 °C in a 1/2-inch diameter piston cylinder apparatus for 1 day. Upon opening the capsule liquid water emerged, indicating that the integrity of the capsule was maintained during the high pressure treatment and that the sample (a clear colorless glass) was water saturated. The pressurized sample's water content was 6.7 ± 0.2 wt% determined by secondary ion mass spectroscopy (SIMS) at the ASU National SIMS facility using basaltic glass standards [49]. This water content is consistent with water solubility for similar composition determined by Benne and Behrens [50]. In situ Raman spectra were measured in a cylindrical diamond anvil cell fitted with low-fluorescence diamonds with 300 μm culets. The sample was loaded in a 130 μm diameter hole drilled into a T301 steel gasket. No pressure-transmitting medium was used and a few ruby chips were added to determine pressure by the ruby fluorescence method [51]. Raman spectra were collected using a custom built Raman system composed of a 532 nm Coherent® Compass Laser with a maximum power of 100 mW. The laser was focused onto the sample using a 50× ultra long working distance Mitutoyo objective with a 0.42 N.A. To minimize the Raman signal and fluorescence from the diamond, a spatial filter was used. The signal was discriminated from the laser light using a Kaiser® laser bandpass filter and a Semrock® edge filter followed by a quarter wave plate to reduce the preferred polarization response of the spectrometer. The signal was collected using a 300i Acton spectrometer and a Princeton Instrument liquid nitrogen cooled back thinned CCD detector. Polarized (VV) and depolarized (VH) Raman spectra were collected at ambient pressure by inserting a 100:1 polarizing beam cube and a half-wave plate. It should be noted that the depolarized spectra were collected for twice as long as the polarized spectra to obtain similar signal to noise ratios (360 s versus 180 s). Unpolarized in-situ Raman spectra were collected in the DAC to pressures just above 10 GPa. Spectra were collected for 5 min with 6 accumulations and calibrated by a quartz spectrum. Raman spectra were fit to a series of Gaussian peaks using least squares optimization in a custom MATLAB® fitting interface to fit peak position, width, and amplitude. We chose the minimum number of peaks needed to yield a featureless difference curve. For each glass, the peak number, peak position and peak width were linked during the fitting of the ambient pressure polarized, unpolarized and depolarized spectra of each glass. The fitting resulted in seven peaks, at approximately 530–540, 580–600, 650–670, 720–750, 910–930, 960–980, and 1020–1040 cm−1 (Fig. 1, Table 2). These peak positions and peak half widths were used as starting parameters to fit the high pressure in-situ unpolarized Raman spectra. To minimize the number of free parameters, the peak width was fixed for each of the bands at ambient-pressure values and only the peak intensity and position were refined. The errors were determined using the minimization function at the 1-sigma level in the ‘compint’ function in MATLAB®. To facilitate comparisons between spectra taken at different pressures, spectra at each pressure were scaled to the peak maximum in the 800 to 1200 cm−1 range. A non-uniform Raman system response interfered below 350 cm− 1 and increased linearly from 0 to 350 cm− 1. Three bands below 500 cm−1 were fit in the dry and wet glass Raman spectra to account for this system response (shaded in Fig. 1); Raman bands in this region are not interpreted. Additionally, system response created a baseline across all Raman spectra. A zero slope constant was subtracted from individual Raman spectra to minimize the background signal. The unpolarized in-situ spectra were reversible upon decompression. Polarized spectra were not measured on samples recovered from the DAC after compression. Compression spectra of the anhydrous and hydrous glasses are shown in Figs. 2 and 3, respectively.
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Fig. 1. Ambient pressure polarized Raman spectra of (a) anhydrous glass (VV), (b) anhydrous glass (VH), (c) hydrous glass (VV) and (d) hydrous glass (VH). Peak fits (dashed lines) are shown; shaded peaks are due to system response. Peak locations and half-widths are virtually the same between polarizations. VH spectra were collected for twice as long as VV spectra to obtain similar signal-to-noise ratios. (360 s versus 180 s).
Relative volume change during compression was measured in the DAC for both anhydrous and hydrous glasses loaded in 16:4:1 methanol:ethanol:water pressure medium using the method of Amin et al. [52]. Lateral dimensions of a thin piece of glass were determined from digital microscope images using imaging software. The relatively large error bars on the measurements are the result of the poor visual contrast between the pressure medium and the sample, both of which are colorless. The relative volume data was fitted to a second order Birch-Murnaghan equation of state to determine the bulk modulus (K0). Relative volume curves and fits are shown in Fig. 4. Zero-pressure density is about 2619 g/l using the relation of Benne and Behrens [50].
Fig. 2. In situ unpolarized Raman spectra of compressed anhydrous glass from ambient pressure to approximately 10 GPa. Notable features are the growth of the 650– 670 cm−1 and the decrease in 500–600 cm−1 band intensities at pressures up to 2.5 GPa, followed by the growth of the 650–670 cm−1 and the 720–750 cm−1 bands to pressures up to 10 GPa. Additionally at pressures N2.5 GPa, 500–600 cm−1 intensities remain minimal. These observations support pressure-induced coordination of bridging oxygens to IVAl to form VAl. Loss of bridging oxygens depolymerizes network forming tetrahedra. As pressure increases from 2.5 GPa to 10 GPa, both VAl and VIAl become present, acting as network-modifying cations while not contributing to networkforming polymerization.
3. Results and discussion 3.1. Results Figs. 2 and 3 display the compression spectra of the dry and wet glass, respectively. The most notable features are the differences between 500-
Table 2 Assignment of Raman bands. Column 4 summarizes structural differences between hydrous glass and dry glass based on relative band intensities. Bands (cm−1)
Assignment
References
Hydrous glass relative to dry glass
530–540 580–600 650–670 720–750 910–930 960–980 1020–1040
T-O-T stretch of Qn, n = 1,2,3,4 BOs T-O-T stretch of Qn n = 1,2,3,4 BOs V Al-O-T stretch/bend VI Al-O-T stretch/bend T-NBO stretch of Qn, n b 3 BOs T-NBO stretch of Qn, n b 3 BOs T-NBO stretch likely Q3
Merzbacher and White [27] Merzbacher and White [27] Neuville et al. [16] Neuville et al. [16] McMillan [43] McMillan [43] McMillan [43]
More polymerized More Polymerized V Al somewhat abundant VI Al somewhat abundant Qnb3 not as abundant nb3 Q not as abundant Q3 very abundant, indicative of more polymerization
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315
1
hydrated sample dry sample
relative volume (V / Vo)
0.95
0.9
0.85
0.8
0.75
0.7 0
2
4
6
8
10
Pressure (GPa) Fig. 4. Relative volume versus pressure for the dry (filled circles) and hydrous (open squares) glass along with 3rd order Birch-Murnaghan fit to the data using pressure derivative K0′ = 4. The respective K0 are 52 (5) GPa and 16 (2) GPa. Greater compressibility in the wet glass is consistent with prior compressibility measurements but the wet-dry difference is greater than for basaltic compositions [53].
Fig. 3. In situ unpolarized Raman spectra of compressed hydrous glass (6.7 wt%, 18.2 mol% H2O). Notable features are the drop in intensity of 500–600 cm−1 region and growth of the 960–980 cm−1 band at pressures up to 2.5 GPa, followed by the growth of 500–600 cm−1 intensities and the 720–750 cm−1 band at pressures above 2.5 GPa. These observations indicate depolymerization of the network at pressures up to 2.5 GPa, followed by polymerization and the increasing presence of VIAl at higher pressures.
600 cm−1 and 600–800 cm−1 band intensities between the two glasses. The structural implications will be discussed in the following sections. Fig. 4 shows the compressibility measurements of both glasses up to about 10 GPa. The fitted bulk moduli show the hydrous glass (K 0 = 16 ± 2 GPa) to be more compressible than the dry glass (K0 = 52 ± 5 GPa) (Fig. 4). This observation is consistent with that of Whittington et al. [53] who showed higher compressibility of hydrated silicate (basaltic) glasses compared to dry compositions although the anhydrous-to-hydrous differences in compressibility are much greater in our work here, suggesting a strong compositional dependence. Raman band positions shift systematically upward in frequency with pressure for both glasses (Fig. 5). This shift is generally due to increased vibrational frequencies of bending/stretching modes as pressure decreases bond lengths and angles [54,55]. Consistency of peak positions between the glasses at ambient (Fig. 1) and elevated pressures (Figs. 2, 3) indicates the same structural elements can be assigned to each peak (Table 2). The slope in peak position versus pressure is greatest up to about 2.5 GPa, which correlates with notable changes in peak
Fig. 5. Raman peak positions versus pressure in dry (filled circles) and hydrated (open squares) glasses. The increase of frequency is consistent with pressure shortened bond lengths and smaller bending angles of vibrational modes [54,55].
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Fig. 6. Peak areas relative to the 800–1200 cm−1 band region versus pressure. (a) 650–670 cm−1 Raman band associated with VAl. (b) 720–750 cm−1 Raman band associated with VIAl. Filled circles represent the anhydrous glass; open squares represent the hydrous glass. The 720–750 cm−1 band associated with VIAl generally shows intensity growth above 2.5 GPa in both glasses. Only the anhydrous glass shows intensity growth of the 650–670 cm−1 band associated with VAl above ambient pressures.
intensity (Figs. 2, 3, 5). Peak intensity variations with pressure indicate differing proportions of structural elements (Fig. 6). 3.2. Anhydrous glass compression Compression of the dry glass above ambient pressures induces the formation of triclusters (OT3). Prior viscosity and NMR studies support the notion of tricluster formation in aluminosilicate melts as viscosity decreases with increased pressure for pressures up to about 3 GPa [11, 56–60]. Tricluster formation requires that T-O-T bridging oxygens coordinate to a third network formation cation, preferentially IVAl, to become triply coordinated O. This is evidenced by decreased 500– 600 cm−1 band intensities relative to 800–1200 cm−1 bands suggesting loss of T-O-T bending/stretching modes (Fig. 2) [12,61]. Also observed is the increased intensity of the 650–670 cm−1 band associated with VAl, where prior investigations have suggested VAl as the preferential third network forming cation in OT3 (Fig. 2, 6a, 7) [12,39,59]. At pressures above 2.5 GPa, compression induced coordination to NBO causes IVAl to become highly coordinate V,VIAl, as evidenced by intensity growth of 650–670 cm−1 and 720–750 cm−1 bands (Fig. 2, 6a,b, 8). This process of highly coordinated aluminum formation upon increasing pressure was proposed in NMR studies of An-Di dry and wet glasses [10,13,31,35,62]. Results shown here suggest that highly
coordinate Al acts as a network modifying cation as 500–600 cm− 1 band intensities, associated with T-O-T modes of the polymerized network remain minimal above ambient pressures up to 10 GPa. Growth of the 650–670 cm−1 and 720–750 cm−1 bands, associated with NBO coordination to V,VIAl, make it difficult to determine whether triclusters are formed above 2.5 GPa. 3.3. Hydrous glass compression The following structural interpretations of hydrated glass Raman spectra are based on structural properties proposed by Xue and Kanzaki [13,62] in NMR studies of An-Di glasses. At ambient pressure, hydration appears to increase the relative proportion of polymerized networkforming tetrahedra, where the presence of M-OH (M = Ca, Mg) enables further Si-O-Si bonding which elevates the degree of polymerization of the glass. This is evidenced by the relatively more intense 500– 600 cm−1 peaks in the wet glass (Fig. 1). At pressures up to 2.5 GPa the hydrous glass appears to depolymerize with the formation of AlOH groups, where protons preferentially depolymerize Al networkforming tetrahedra to Qn b 3 species (Fig. 9). This is evidenced by 500– 600 cm−1 band intensity decrease and 960–980 cm−1 band intensity increase, which is associated with more Qnb3 (Fig. 3). At pressures above 2.5 GPa, polymerization increases along with the formation of
Fig. 7. Upon compression of the anhydrous glass up to 2.5 GPa, bridging oxygens coordinate to a third network forming cation to become triply coordinated. The third network forming cation is preferentially IVAl, which becomes VAl (denoted as the pentahedral P ion in the figure) [12]. This is evidenced by decreased intensities in the 500–600 cm−1 region, interpreted as loss of T-O-T modes, as well the growth of the 650–670 cm−1 band assigned to VAl (Fig. 2, 6b).
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Fig. 8. At pressures above 2.5 GPa in the dry glass, NBO previously coordinated to network modifying (M) cations undergo pressure induced coordination to IVAl, which form high coordinate V,VIAl [13]. This is supported by growth of the 650–670 cm−1 band associated with VAl and growth of the 720–750 cm−1 band associated with VIAl (Fig. 1, 6a,b).
network-modifying VIAl. This is evidenced by increased 500–600 cm−1 regional intensity as well as growth of the 720–750 cm−1 band (Fig. 3, 6b). Increased polymerization occurs as hydroxyl groups coordinate to network modifiers Ca and Mg (forming M-OH structures), enabling the formation of Si-O-Si bridging oxygens (Fig. 9). Further supporting polymerization is that the wet glass is more compressible than its dry counterpart (Fig. 4). Prior studies have shown direct correlation between compressibility and polymerization in anhydrous silicate and aluminosilicate glass compositions [53,63,64]. Differing from the anhydrous glass is the lack of evidence for VAl (where VAl was due to the presence of tricluster formation). This is evidenced by there being no display of 650–670 cm−1 band growth above ambient pressures in the hydrous glass, nor is there any strong visual indication of the band in compression spectra (Fig. 3, 6a). It is likely that V Al exists in very low amounts in the hydrous glass, and that water enables VIAl formation at high pressures. The latter conclusion could be because VIAl may offer more sites to store the additional oxygen introduced by water.
become highly coordinate V,VIAl as proposed by Xue and Kanzaki [13] (Fig. 8). Raman spectral evidence and compressibility measurements support water dissolution and structural models of An-Di glass in which water serves to polymerize the melt through formation of highly coordinated Al and M-OH that does not form part of the polymerized network [13] (Fig. 9). Xue and Kanzaki [13] discussed compositionally dependent structural components, whereas this work presents pressure-dependent variations in these structural components. Raman spectral differences between the two glasses upon compression, especially intensity variations of the 500–600 cm− 1 and 600– 800 cm−1 regions, point to significantly different structural responses that are enabled with the presence of water. Particularly important is the tendency for the wet glass to polymerize when compressed up to 10 GPa whereas the dry glass depolymerizes. These observations, also supported by compressibility measurements, indicate the importance of water in influencing melt structure and other properties key to understanding volcanism and the deep Earth. Acknowledgements
4. Conclusions At pressures near 2.5 GPa in the dry glass, Raman spectral evidence supports the model of tricluster formation where BO in T-O-T are lost as they coordinate with IVAl to become high coordinate VAl (Fig. 7) [12,13,62]. At more elevated pressures, NBO coordinate to IVAl to
This research was supported by the NSF grant EAR 0739050 to JAT. This work was partially supported through an Undergraduate Research Internship award from the Arizona State University/NASA Space Grant program (NASA Grant NNX10AI41H). We gratefully acknowledge assistance from the ASU SIMS Facility supported by NSF EAR 1352996.
Fig. 9. Implications for hydrated glass structure using the model of Xue and Kanzaki [13]. Upon hydration, Raman spectra suggest an increase of polymerized network-forming tetrahedra as 500–600 cm−1 band intensities are greater in the hydrous glass (at ambient pressures). Compression up to 2.5 GPa influences a compression mechanism in which water depolymerizes network-forming tetrahedra, likely into Al-OH and M-OH groups. This is evidenced by the relative decrease of 500–600 cm−1 intensities and the increase of the 960–980 cm−1 band intensity in this pressure regime (Figs. 3, 5). Upon compression up to 10 GPa, polymerization increases in addition to highly coordinate Al, preferentially as VIAl-OH. This is evidenced by the increase of 500–600 cm−1 intensities and the growth of the 720–750 cm−1 band associated with VIAl.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Effect of water on the high-pressure structural behavior of anorthite-diopside eutectic glass Wesley Helwig a, Emmanuel Soignard b, James A. Tyburczy c,⁎ a b c
Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, United States Leroy Eyring Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704, United States School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, United States
a r t i c l e
i n f o
Article history: Received 12 May 2016 Received in revised form 19 August 2016 Accepted 22 August 2016 Available online xxxx Keywords: Glass structure Raman spectroscopy High pressure Anorthite-diopside glass Aluminum coordination Triclusters Polymerization
a b s t r a c t We employed in situ high pressure Raman spectroscopy to examine structural variations in hydrous versus anhydrous aluminosilicate glasses as a function of pressure. Raman spectra of anhydrous and water-saturated (6.7 wt% H2O) eutectic anorthite – diopside (An-Di) glasses were collected at pressures up to 10 GPa in a diamond anvil cell (DAC). Both glasses exhibited a distinct change in compression mechanism at about 2.5 GPa. From 0 to 2.5 GPa each glass depolymerizes. Above 2.5 GPa the anhydrous glass becomes less polymerized, whereas the hydrous glass becomes more polymerized as pressure is increased up to 10 GPa. These differences are explained in terms of Al-coordination and formation of triclusters. For the dry glass, at pressures below 2.5 GPa, depolymerization occurs by means of tricluster (OT3) formation in which bridging oxygens (BO) become triply coordinated to a third network forming cation, preferentially IVAl, thereby increasing the coordination to VAl. At pressures N 2.5 GPa compression induced coordination to non-bridging oxygens (NBO) causes tetrahedral IVAl to become highly coordinated V,VIAl. Network modifying cations (Ca and Mg) coordinated to NBO at ambient conditions become charge-balancing cations for V,VIAl at elevated pressure, resulting in decreased polymerization. For the wet glass, compression up to 2.5 GPa causes protons in H2O to depolymerize Al tetrahedra into Al-OH. At pressures N 2.5 GPa most of the highly coordinated Al is present as VIAl and network polymerization increases with the formation of M-OH (M = Ca, Mg) groups that enable Si-O-Si bonds (BO). Bulk modulus measurements support increased polymerization with the wet glass (K0 = 16 ± 2GPa) shown to be more compressible than the dry glass (K0 = 52 ± 5 GPa). © 2016 Elsevier B.V. All rights reserved.
1. Introduction Understanding the mechanics of the deep Earth requires understanding properties of silicate melts under deep Earth conditions. In this investigation, in-situ high-pressure Raman spectroscopy and relative volume measurements were collected on two glass compositions within a DAC at room temperature from 0 to 10.5 GPa. The compositions were a dry and water-saturated glass near the An-Di eutectic. These measurements were made to examine the effects of pressurequenching and high water content on glass structure. This glass composition is of interest because aluminosilicates in the anorthite-diopsideforsterite (An-Di-Fo) ternary have been proposed as analogues to mid ocean ridge basaltic melts (MORB) [1–6]. While structures of glasses have been studied at ambient conditions as well as quenched high pressure and/or temperature [7–13], there are few studies of the An-Di eutectic using in-situ high pressure studies via Raman spectroscopy. Structural studies of melts are important because those properties have been correlated with melt viscosity, melt density, elastic ⁎ Corresponding author. E-mail address: [email protected] (J.A. Tyburczy).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.08.030 0022-3093/© 2016 Elsevier B.V. All rights reserved.
properties, and thermodynamic properties [3,14–17]. As well, the effects of H2O on melts and the significance for Earth's interior have also been examined [2,4,18–25]. Those studies indicate that water has significant effects on melt properties and can thus be correlated to structural features. Poe et al. [12] examined an anhydrous calcium-aluminosilicate glass quenched from pressures to 12 GPa using Raman spectroscopy and XANES. They proposed that for pressures from 0 to 6 GPa, the formation of triply-coordinated oxygen (‘triclusters’) occurs without affecting the coordination of aluminum. At pressures above 6 GPa a mechanism change occurred and the quenched samples exhibited spectra consistent with either an increase in aluminum coordination or a reduction in Al-O-Al bond angle. Xue and Kanzaki [13]examined quenched undersaturated water bearing glasses in the An-Di system using 1H and 27Al NMR at ambient pressure. They concluded that dissolution of water in these glasses involves competition between formation of SiOH and AlOH species that depolymerize the glasses and formation of free OH that serves to polymerize the melts. Stronger field strength cations (Ca, Mg) enhance the tendency to polymerize whereas alkali cations (Na, K) act to depolymerize the network. Aluminum is predominantly four-coordinated to oxygen (IVAl) but a small amount of five
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coordinated aluminum (VAl) occurs in all glasses and some six-coordinated Al (VIAl) occurs in all hydrous glasses. Mysen et al. [26] studied H2O-saturated melts in the Na-Al-Si-O system in situ at pressure to 0.8 GPa and temperatures to 800 °C, observing that increasing H2O depolymerizes the melts. In this investigation, we perform in-situ high pressure (0–10 GPa) room temperature Raman spectroscopy on a dry and water-saturated glass with composition near the An-Di eutectic (40CaAl2Si2O860CaMgSi2O6 wt%; the eutectic is An 42 wt% Di 58 wt%). This composition was chosen for its relatively low melting temperature at 1 bar pressure of approximately 1270 °C [3]. In-situ Raman spectra and relative volume measurements were collected on both glasses at room temperature in a DAC up to pressures of 10.5 GPa. 1.1. Raman spectra The 500–600 cm−1 Raman spectral region of aluminosilicate glasses has been associated with T-O-T (T = Si, Al) bending/stretching motions between variably polymerized network forming cations [27] and we assign these modes to the 500–600 cm−1 spectral region of anhydrous and hydrous glass studies here. The degree of polymerization can be denoted as Qn species (Q = Si, Al and n = 4, 3, 2, 1 BO). Overlapping of Raman bands in this region prevents assignment to specific Qn [8,24, 27,28]. The 600–800 cm−1 bands correspond to stretching/bending vibrations associated with V,VIAl upon compression [12,28–39]. The 650– 670 and 720–750 cm−1 bands are respectively correlated to the occurrence of VAl and VIAl in each glass [34,39–41]. The 800–1200 cm−1 region is assigned to T-NBO stretching, which NBO are coordinated to network modifying cations Ca and Mg. Prior investigations have assigned individual bands to specific Qn (n = 3,2,1,0 BOs), in which higher frequency bands in the region are representative of higher Qn species [26,42–47]. However, the positions of these bands shift as a function of polymerization, modifier cation species (Ca, Mg, Na, K), and with the ratio of Al/(Si + Al) as shown in studies of calcium-magnesium aluminosilicate (CMAS) glasses [48]. Although the assignment of specific peaks to specific Qn species may be difficult, it will be assumed the 1020–1040 cm−1 band can be assigned to Q3, given the likelihood of its presence based on the glass degree of polymerization (NBO/T = 0.86) and associated Qn speciation [14,43,48]. Both the 910–930 cm− 1 and 960–980 cm− 1 bands are assigned to Qn b 3. 2. Experimental details The anhydrous glass (herein referred to as the ‘dry’ glass) was prepared using a mixture of high purity SiO2 (Alfa Aesar 99.999%), Al2O3 (Sigma Aldrich 99.998%), CaCO3 (Alfa Aesar 99.997%), and MgCO3 (99.998%). The fine powder mixture was decarbonated in a platinum crucible at 900 °C for 1 h in air. The sample was then melted in a MoSi2 vertical tube furnace in air at 1500 °C for 10 min. The molten sample was drop-quenched in water by burning through the support wire resulting in a clear colorless glass. The composition was determined by electron probe microanalysis (Table 1). No elements other than Si, Al, Mg, Ca and O were present in measurable quantity. Table 1 Average dry glass oxide weight per cent determined by electron probe micro-analysis, and theoretical weight % from the eutectic composition. Errors are standard deviations over five repeat measurements. Oxide
Weight (%) measured
Weight (%) (theoretical)
SiO2 Al2O3 MgO CaO Total weight percent
51.1 ± 0.3 17.0 ± 0.1 9.4 ± 0.2 23.7 ± 0.4 101.4 ± 0.5
49.8 16.9 10.0 23.2 100
313
To prepare the hydrous glass (herein referred to as the ‘wet’ glass), 25 mg of the dry glass were loaded into a 5 mm outside diameter, 0.127 mm wall thickness platinum capsule with an excess of 18 MΩ (Millipore ultrapure) water. The capsule was welded shut while wrapped in a wet paper towel to minimize water loss. The sample was melted at 0.4 GPa at 1300 °C in a 1/2-inch diameter piston cylinder apparatus for 1 day. Upon opening the capsule liquid water emerged, indicating that the integrity of the capsule was maintained during the high pressure treatment and that the sample (a clear colorless glass) was water saturated. The pressurized sample's water content was 6.7 ± 0.2 wt% determined by secondary ion mass spectroscopy (SIMS) at the ASU National SIMS facility using basaltic glass standards [49]. This water content is consistent with water solubility for similar composition determined by Benne and Behrens [50]. In situ Raman spectra were measured in a cylindrical diamond anvil cell fitted with low-fluorescence diamonds with 300 μm culets. The sample was loaded in a 130 μm diameter hole drilled into a T301 steel gasket. No pressure-transmitting medium was used and a few ruby chips were added to determine pressure by the ruby fluorescence method [51]. Raman spectra were collected using a custom built Raman system composed of a 532 nm Coherent® Compass Laser with a maximum power of 100 mW. The laser was focused onto the sample using a 50× ultra long working distance Mitutoyo objective with a 0.42 N.A. To minimize the Raman signal and fluorescence from the diamond, a spatial filter was used. The signal was discriminated from the laser light using a Kaiser® laser bandpass filter and a Semrock® edge filter followed by a quarter wave plate to reduce the preferred polarization response of the spectrometer. The signal was collected using a 300i Acton spectrometer and a Princeton Instrument liquid nitrogen cooled back thinned CCD detector. Polarized (VV) and depolarized (VH) Raman spectra were collected at ambient pressure by inserting a 100:1 polarizing beam cube and a half-wave plate. It should be noted that the depolarized spectra were collected for twice as long as the polarized spectra to obtain similar signal to noise ratios (360 s versus 180 s). Unpolarized in-situ Raman spectra were collected in the DAC to pressures just above 10 GPa. Spectra were collected for 5 min with 6 accumulations and calibrated by a quartz spectrum. Raman spectra were fit to a series of Gaussian peaks using least squares optimization in a custom MATLAB® fitting interface to fit peak position, width, and amplitude. We chose the minimum number of peaks needed to yield a featureless difference curve. For each glass, the peak number, peak position and peak width were linked during the fitting of the ambient pressure polarized, unpolarized and depolarized spectra of each glass. The fitting resulted in seven peaks, at approximately 530–540, 580–600, 650–670, 720–750, 910–930, 960–980, and 1020–1040 cm−1 (Fig. 1, Table 2). These peak positions and peak half widths were used as starting parameters to fit the high pressure in-situ unpolarized Raman spectra. To minimize the number of free parameters, the peak width was fixed for each of the bands at ambient-pressure values and only the peak intensity and position were refined. The errors were determined using the minimization function at the 1-sigma level in the ‘compint’ function in MATLAB®. To facilitate comparisons between spectra taken at different pressures, spectra at each pressure were scaled to the peak maximum in the 800 to 1200 cm−1 range. A non-uniform Raman system response interfered below 350 cm− 1 and increased linearly from 0 to 350 cm− 1. Three bands below 500 cm−1 were fit in the dry and wet glass Raman spectra to account for this system response (shaded in Fig. 1); Raman bands in this region are not interpreted. Additionally, system response created a baseline across all Raman spectra. A zero slope constant was subtracted from individual Raman spectra to minimize the background signal. The unpolarized in-situ spectra were reversible upon decompression. Polarized spectra were not measured on samples recovered from the DAC after compression. Compression spectra of the anhydrous and hydrous glasses are shown in Figs. 2 and 3, respectively.
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Fig. 1. Ambient pressure polarized Raman spectra of (a) anhydrous glass (VV), (b) anhydrous glass (VH), (c) hydrous glass (VV) and (d) hydrous glass (VH). Peak fits (dashed lines) are shown; shaded peaks are due to system response. Peak locations and half-widths are virtually the same between polarizations. VH spectra were collected for twice as long as VV spectra to obtain similar signal-to-noise ratios. (360 s versus 180 s).
Relative volume change during compression was measured in the DAC for both anhydrous and hydrous glasses loaded in 16:4:1 methanol:ethanol:water pressure medium using the method of Amin et al. [52]. Lateral dimensions of a thin piece of glass were determined from digital microscope images using imaging software. The relatively large error bars on the measurements are the result of the poor visual contrast between the pressure medium and the sample, both of which are colorless. The relative volume data was fitted to a second order Birch-Murnaghan equation of state to determine the bulk modulus (K0). Relative volume curves and fits are shown in Fig. 4. Zero-pressure density is about 2619 g/l using the relation of Benne and Behrens [50].
Fig. 2. In situ unpolarized Raman spectra of compressed anhydrous glass from ambient pressure to approximately 10 GPa. Notable features are the growth of the 650– 670 cm−1 and the decrease in 500–600 cm−1 band intensities at pressures up to 2.5 GPa, followed by the growth of the 650–670 cm−1 and the 720–750 cm−1 bands to pressures up to 10 GPa. Additionally at pressures N2.5 GPa, 500–600 cm−1 intensities remain minimal. These observations support pressure-induced coordination of bridging oxygens to IVAl to form VAl. Loss of bridging oxygens depolymerizes network forming tetrahedra. As pressure increases from 2.5 GPa to 10 GPa, both VAl and VIAl become present, acting as network-modifying cations while not contributing to networkforming polymerization.
3. Results and discussion 3.1. Results Figs. 2 and 3 display the compression spectra of the dry and wet glass, respectively. The most notable features are the differences between 500-
Table 2 Assignment of Raman bands. Column 4 summarizes structural differences between hydrous glass and dry glass based on relative band intensities. Bands (cm−1)
Assignment
References
Hydrous glass relative to dry glass
530–540 580–600 650–670 720–750 910–930 960–980 1020–1040
T-O-T stretch of Qn, n = 1,2,3,4 BOs T-O-T stretch of Qn n = 1,2,3,4 BOs V Al-O-T stretch/bend VI Al-O-T stretch/bend T-NBO stretch of Qn, n b 3 BOs T-NBO stretch of Qn, n b 3 BOs T-NBO stretch likely Q3
Merzbacher and White [27] Merzbacher and White [27] Neuville et al. [16] Neuville et al. [16] McMillan [43] McMillan [43] McMillan [43]
More polymerized More Polymerized V Al somewhat abundant VI Al somewhat abundant Qnb3 not as abundant nb3 Q not as abundant Q3 very abundant, indicative of more polymerization
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315
1
hydrated sample dry sample
relative volume (V / Vo)
0.95
0.9
0.85
0.8
0.75
0.7 0
2
4
6
8
10
Pressure (GPa) Fig. 4. Relative volume versus pressure for the dry (filled circles) and hydrous (open squares) glass along with 3rd order Birch-Murnaghan fit to the data using pressure derivative K0′ = 4. The respective K0 are 52 (5) GPa and 16 (2) GPa. Greater compressibility in the wet glass is consistent with prior compressibility measurements but the wet-dry difference is greater than for basaltic compositions [53].
Fig. 3. In situ unpolarized Raman spectra of compressed hydrous glass (6.7 wt%, 18.2 mol% H2O). Notable features are the drop in intensity of 500–600 cm−1 region and growth of the 960–980 cm−1 band at pressures up to 2.5 GPa, followed by the growth of 500–600 cm−1 intensities and the 720–750 cm−1 band at pressures above 2.5 GPa. These observations indicate depolymerization of the network at pressures up to 2.5 GPa, followed by polymerization and the increasing presence of VIAl at higher pressures.
600 cm−1 and 600–800 cm−1 band intensities between the two glasses. The structural implications will be discussed in the following sections. Fig. 4 shows the compressibility measurements of both glasses up to about 10 GPa. The fitted bulk moduli show the hydrous glass (K 0 = 16 ± 2 GPa) to be more compressible than the dry glass (K0 = 52 ± 5 GPa) (Fig. 4). This observation is consistent with that of Whittington et al. [53] who showed higher compressibility of hydrated silicate (basaltic) glasses compared to dry compositions although the anhydrous-to-hydrous differences in compressibility are much greater in our work here, suggesting a strong compositional dependence. Raman band positions shift systematically upward in frequency with pressure for both glasses (Fig. 5). This shift is generally due to increased vibrational frequencies of bending/stretching modes as pressure decreases bond lengths and angles [54,55]. Consistency of peak positions between the glasses at ambient (Fig. 1) and elevated pressures (Figs. 2, 3) indicates the same structural elements can be assigned to each peak (Table 2). The slope in peak position versus pressure is greatest up to about 2.5 GPa, which correlates with notable changes in peak
Fig. 5. Raman peak positions versus pressure in dry (filled circles) and hydrated (open squares) glasses. The increase of frequency is consistent with pressure shortened bond lengths and smaller bending angles of vibrational modes [54,55].
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Fig. 6. Peak areas relative to the 800–1200 cm−1 band region versus pressure. (a) 650–670 cm−1 Raman band associated with VAl. (b) 720–750 cm−1 Raman band associated with VIAl. Filled circles represent the anhydrous glass; open squares represent the hydrous glass. The 720–750 cm−1 band associated with VIAl generally shows intensity growth above 2.5 GPa in both glasses. Only the anhydrous glass shows intensity growth of the 650–670 cm−1 band associated with VAl above ambient pressures.
intensity (Figs. 2, 3, 5). Peak intensity variations with pressure indicate differing proportions of structural elements (Fig. 6). 3.2. Anhydrous glass compression Compression of the dry glass above ambient pressures induces the formation of triclusters (OT3). Prior viscosity and NMR studies support the notion of tricluster formation in aluminosilicate melts as viscosity decreases with increased pressure for pressures up to about 3 GPa [11, 56–60]. Tricluster formation requires that T-O-T bridging oxygens coordinate to a third network formation cation, preferentially IVAl, to become triply coordinated O. This is evidenced by decreased 500– 600 cm−1 band intensities relative to 800–1200 cm−1 bands suggesting loss of T-O-T bending/stretching modes (Fig. 2) [12,61]. Also observed is the increased intensity of the 650–670 cm−1 band associated with VAl, where prior investigations have suggested VAl as the preferential third network forming cation in OT3 (Fig. 2, 6a, 7) [12,39,59]. At pressures above 2.5 GPa, compression induced coordination to NBO causes IVAl to become highly coordinate V,VIAl, as evidenced by intensity growth of 650–670 cm−1 and 720–750 cm−1 bands (Fig. 2, 6a,b, 8). This process of highly coordinated aluminum formation upon increasing pressure was proposed in NMR studies of An-Di dry and wet glasses [10,13,31,35,62]. Results shown here suggest that highly
coordinate Al acts as a network modifying cation as 500–600 cm− 1 band intensities, associated with T-O-T modes of the polymerized network remain minimal above ambient pressures up to 10 GPa. Growth of the 650–670 cm−1 and 720–750 cm−1 bands, associated with NBO coordination to V,VIAl, make it difficult to determine whether triclusters are formed above 2.5 GPa. 3.3. Hydrous glass compression The following structural interpretations of hydrated glass Raman spectra are based on structural properties proposed by Xue and Kanzaki [13,62] in NMR studies of An-Di glasses. At ambient pressure, hydration appears to increase the relative proportion of polymerized networkforming tetrahedra, where the presence of M-OH (M = Ca, Mg) enables further Si-O-Si bonding which elevates the degree of polymerization of the glass. This is evidenced by the relatively more intense 500– 600 cm−1 peaks in the wet glass (Fig. 1). At pressures up to 2.5 GPa the hydrous glass appears to depolymerize with the formation of AlOH groups, where protons preferentially depolymerize Al networkforming tetrahedra to Qn b 3 species (Fig. 9). This is evidenced by 500– 600 cm−1 band intensity decrease and 960–980 cm−1 band intensity increase, which is associated with more Qnb3 (Fig. 3). At pressures above 2.5 GPa, polymerization increases along with the formation of
Fig. 7. Upon compression of the anhydrous glass up to 2.5 GPa, bridging oxygens coordinate to a third network forming cation to become triply coordinated. The third network forming cation is preferentially IVAl, which becomes VAl (denoted as the pentahedral P ion in the figure) [12]. This is evidenced by decreased intensities in the 500–600 cm−1 region, interpreted as loss of T-O-T modes, as well the growth of the 650–670 cm−1 band assigned to VAl (Fig. 2, 6b).
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Fig. 8. At pressures above 2.5 GPa in the dry glass, NBO previously coordinated to network modifying (M) cations undergo pressure induced coordination to IVAl, which form high coordinate V,VIAl [13]. This is supported by growth of the 650–670 cm−1 band associated with VAl and growth of the 720–750 cm−1 band associated with VIAl (Fig. 1, 6a,b).
network-modifying VIAl. This is evidenced by increased 500–600 cm−1 regional intensity as well as growth of the 720–750 cm−1 band (Fig. 3, 6b). Increased polymerization occurs as hydroxyl groups coordinate to network modifiers Ca and Mg (forming M-OH structures), enabling the formation of Si-O-Si bridging oxygens (Fig. 9). Further supporting polymerization is that the wet glass is more compressible than its dry counterpart (Fig. 4). Prior studies have shown direct correlation between compressibility and polymerization in anhydrous silicate and aluminosilicate glass compositions [53,63,64]. Differing from the anhydrous glass is the lack of evidence for VAl (where VAl was due to the presence of tricluster formation). This is evidenced by there being no display of 650–670 cm−1 band growth above ambient pressures in the hydrous glass, nor is there any strong visual indication of the band in compression spectra (Fig. 3, 6a). It is likely that V Al exists in very low amounts in the hydrous glass, and that water enables VIAl formation at high pressures. The latter conclusion could be because VIAl may offer more sites to store the additional oxygen introduced by water.
become highly coordinate V,VIAl as proposed by Xue and Kanzaki [13] (Fig. 8). Raman spectral evidence and compressibility measurements support water dissolution and structural models of An-Di glass in which water serves to polymerize the melt through formation of highly coordinated Al and M-OH that does not form part of the polymerized network [13] (Fig. 9). Xue and Kanzaki [13] discussed compositionally dependent structural components, whereas this work presents pressure-dependent variations in these structural components. Raman spectral differences between the two glasses upon compression, especially intensity variations of the 500–600 cm− 1 and 600– 800 cm−1 regions, point to significantly different structural responses that are enabled with the presence of water. Particularly important is the tendency for the wet glass to polymerize when compressed up to 10 GPa whereas the dry glass depolymerizes. These observations, also supported by compressibility measurements, indicate the importance of water in influencing melt structure and other properties key to understanding volcanism and the deep Earth. Acknowledgements
4. Conclusions At pressures near 2.5 GPa in the dry glass, Raman spectral evidence supports the model of tricluster formation where BO in T-O-T are lost as they coordinate with IVAl to become high coordinate VAl (Fig. 7) [12,13,62]. At more elevated pressures, NBO coordinate to IVAl to
This research was supported by the NSF grant EAR 0739050 to JAT. This work was partially supported through an Undergraduate Research Internship award from the Arizona State University/NASA Space Grant program (NASA Grant NNX10AI41H). We gratefully acknowledge assistance from the ASU SIMS Facility supported by NSF EAR 1352996.
Fig. 9. Implications for hydrated glass structure using the model of Xue and Kanzaki [13]. Upon hydration, Raman spectra suggest an increase of polymerized network-forming tetrahedra as 500–600 cm−1 band intensities are greater in the hydrous glass (at ambient pressures). Compression up to 2.5 GPa influences a compression mechanism in which water depolymerizes network-forming tetrahedra, likely into Al-OH and M-OH groups. This is evidenced by the relative decrease of 500–600 cm−1 intensities and the increase of the 960–980 cm−1 band intensity in this pressure regime (Figs. 3, 5). Upon compression up to 10 GPa, polymerization increases in addition to highly coordinate Al, preferentially as VIAl-OH. This is evidenced by the increase of 500–600 cm−1 intensities and the growth of the 720–750 cm−1 band associated with VIAl.
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