Quantification of water content and speciation in natural silicic glasses (phonolite, dacite, rhyolite) by confocal microRaman spectrometry

The determination of total water content (H₂O_T: 0.1-10 wt%) and water speciation (H₂O_molecular/OH) in volcanic products by confocal microRaman spectrometry are discussed for alkaline (phonolite) and calcalkaline (dacite and rhyolite) silicic glasses. Shape and spectral distribution of the total water band (H₂O_T) at ∼3550 cm⁻¹ show systematic evolution with glass H₂O_T, water speciation and NBO/T. In the studied set of silicic samples, calibrations based on internal normalization of the H₂O_T band to a band related to vibration of aluminosilicate network (TOT) at ∼490 cm⁻¹ vary with glass peraluminosity. An external calibration procedure using well-characterized glass standards is less composition-dependent and provides excellent linear correlation between total dissolved water content and height or area of the H₂O_T Raman band. Accuracy of deconvolution procedure of the H₂O_T band to quantify water speciation in water-rich and depolymerized glasses depends on the strength of OH hydrogen bonding. System confocal performance, scattering from embedding medium and glass microcrystallinity have a crucial influence on accuracy of Raman analyses of water content in glass-bearing rocks and melt inclusions in crystals.     

The solution behavior of H2O in peralkaline aluminosilicate melts at high pressure with implications for properties of hydrous melts

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, X_H2O=0.20+0.0020f_H2O-0.7X_Al+0.9(X_Al)², where X_H2O is the mole fraction of H₂O (calculated with O = 1), f_H2O the fugacity of H₂O, and X_Al = 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.     

Water speciation and diffusion in haploandesitic melts at 743-873 K and 100 MPa

Water is an important volatile component in andesitic eruptions and deep-seated andesitic magma chambers. We report an investigation of H₂O speciation and diffusion by dehydrating haploandesitic melts containing ?2.5 wt.% water at 743-873 K and 100 MPa in cold-seal pressure vessels. FTIR microspectroscopy was utilized to measure species [molecular H₂O (H₂O_m) and hydroxyl group (OH)] and total H₂O (H₂O_t) concentration profiles on the quenched glasses from the dehydration experiments. The equilibrium constant of the H₂O speciation reaction H₂O_m+O?2OH, K = (X_OH)²/(X_H2OmX_O) where X means mole fraction on a single oxygen basis, in this Fe-free andesite varies with temperature as ln K = 1.547-2453/T where T is in K. Comparison with previous speciation data on rhyolitic and dacitic melts indicates that, for a given water concentration, Fe-free andesitic melt contains more hydroxyl groups. Water diffusivity at the experimental conditions increases rapidly with H₂O concentration, contrary to previous H₂O diffusion data in an andesitic melt at 1608-1848 K. The diffusion profiles are consistent with the model that molecular H₂O is the diffusion species. Based on the above speciation model, H₂O_m and H₂O_t diffusivity (in m²/s) in haploandesite at 743-873 K, 100 MPa, and H₂O_t ? 2.5 wt.% can be formulated as

     

Water diffusion in dacitic melt

H₂O diffusion in dacitic melt was investigated at 0.48-0.95 GPa and 786-893 K in a piston-cylinder apparatus. The diffusion couple design was used, in which a nominally dry dacitic glass makes one half and is juxtaposed with a hydrous dacitic glass containing up to ∼8 wt.% total water (H₂O_t). H₂O concentration profiles were measured on quenched glasses with infrared microspectroscopy. The H₂O diffusivity in dacite increases rapidly with water content under experimental conditions, similar to previous measurements at the same temperature but at pressure <0.15 GPa. However, compared with the low-pressure data, H₂O diffusion at high pressure is systematically slower. H₂O diffusion profiles in dacite can be modeled by assuming molecular H₂O (H₂O_m) is the diffusing species. Total H₂O diffusivity D_H2Ot within 786-1798 K, 0-1 GPa, and 0-8 wt.% H₂O_t can be expressed as: where D_H2Ot is in m²/s, T is temperature in K, P is pressure in GPa, K = exp(1.49 − 2634/T) is the equilibrium constant of speciation reaction (H₂O_m+O?2OH) in the melt, X = C/18.015/[C/18.015 + (100 − C)/33.82], C is wt.% of H₂O_t, and 18.015 and 33.82 g/mol correspond to the molar masses of H₂O and anhydrous dacite on a single oxygen basis. Compared to H₂O diffusion in rhyolite, diffusivity in dacite is lower at intermediate temperatures but higher at superliquidus temperatures. This general H₂O diffusivity expression can be applied to a broad range of geological conditions, including both magma chamber processes and volcanic eruption dynamics from conduit to the surface.     

Spectroscopic analysis (FTIR, Raman) of water in mafic and intermediate glasses and glass inclusions

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 (H₂O_T = OH⁻ + H₂O_m) 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 H₂O_T 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.     

Structure of H2O-saturated peralkaline aluminosilicate melt and coexisting aluminosilicate-saturated aqueous fluid determined in-situ to 800 °C and ∼800 MPa

The structure of H₂O-saturated silicate melts and of silicate-saturated aqueous solutions, as well as that of supercritical silicate-rich aqueous liquids, has been characterized in-situ while the sample was at high temperature (to 800 °C) and pressure (up to 796 MPa). Structural information was obtained with confocal microRaman and with FTIR spectroscopy. Two Al-bearing glasses compositionally along the join Na₂O•4SiO₂-Na₂O•4(NaAl)O₂-H₂O (5 and 10 mol% Al₂O₃, denoted NA5 and NA10) were used as starting materials. Fluids and melts were examined along pressure-temperature trajectories of isochores of H₂O at nominal densities (from PVT properties of pure H₂O) of 0.85 g/cm³ (NA10 experiments) and 0.86 g/cm³ (NA5 experiments) with the aluminosilicate + H₂O sample contained in an externally-heated, Ir-gasketed hydrothermal diamond anvil cell.Molecular H₂O (H₂O°) and OH groups that form bonds with cations exist in all three phases. The OH/H₂O° ratio is positively correlated with temperature and pressure (and, therefore, fugacity of H₂O, f_H2O) with (OH/H₂O°)^melt > (OH/H₂O°)^fluid at all pressures and temperatures. Structural units of Q³, Q², Q¹, and Q⁰ type occur together in fluids, in melts, and, when outside the two-phase melt + fluid boundary, in single-phase liquids. The abundance of Q⁰ and Q¹ increases and Q² and Q³ decrease with f_H2O. Therefore, the NBO/T (nonbridging oxygen per tetrahedrally coordination cations), of melt is a positive function of f_H2O. The NBO/T of silicate in coexisting aqueous fluid, although greater than in melt, is less sensitive to f_H2O.The melt structural data are used to describe relationships between activity of H₂O and melting phase relations of silicate systems at high pressure and temperature. The data were also combined with available partial molar configurational heat capacity of Qⁿ-species in melts to illustrate how these quantities can be employed to estimate relationships between heat capacity of melts and their H₂O content.     

Redox equilibria of iron and silicate melt structure: Implications for olivine/melt element partitioning

Olivine/melt partitioning of ΣFe, Fe²⁺, Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, and Ni²⁺ has been determined in the systems CaO-MgO-FeO-Fe₂O₃-SiO₂ (FD) and CaO-MgO-FeO-Fe₂O₃-Al₂O₃-SiO₂ (FDA3) as a function of oxygen fugacity (f_O2) at 0.1 MPa pressure. Total iron oxide content of the starting materials was ∼20 wt%. The f_O2 was to used to control the Fe³⁺/ΣFe (ΣFe: total iron) of the melts. The Fe³⁺/ΣFe and structural roles of Fe²⁺ and Fe³⁺ were determined with ⁵⁷Fe resonant absorption Mössbauer spectroscopy. Changes in melt polymerization, NBO/T, as a function of f_O2 was estimated from the Mössbauer data and existing melt structure information. It varies by ∼100% in melts coexisting with olivine in the FDA3 system and by about 300% in the FD system in the Fe³⁺/ΣFe range of the experiments (0.805-0.092). The partition coefficients ( in olivine/wt% in melt) are systematic functions of f_O2 and, therefore, NBO/T of the melt. There is a -minimum in the FDA3 system at NBO/T-values corresponding to intermediate Fe³⁺/ΣFe (0.34-0.44). In the Al-free system, FD, where the NBO/T values of melts range between ∼1 and ∼2.9, the partition coefficients are positively correlated with NBO/T (decreasing Fe³⁺/ΣFe). These relationships are explained by consideration of solution behavior in the melts governed by Qⁿ-unit distribution and structural changes of the divalent cations in the melts (coordination number, complexing with Fe³⁺, and distortion of the polyhedra).     

Disorder and the extent of polymerization in calcium silicate and aluminosilicate glasses: O-17 NMR results and quantum chemical molecular orbital calculations

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 (X_SiO2=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.     

Effect of water on the heat capacity of polymerized aluminosilicate glasses and melts

The effect of water on heat capacity has been determined for four series of hydrated synthetic aluminosilicate glasses and supercooled liquids close to albite, phonolite, trachyte, and leucogranite compositions. Heat capacities were measured at atmospheric pressure by differential scanning calorimetry for water contents between 0 and 4.9 wt % from 300 K to about 100 K above the glass transition temperature (T_g). The partial molar heat capacity of water in polymerized aluminosilicate glasses, which can be considered as independent of composition, is (J/mol K). In liquids containing at least 1 wt % H₂O, the partial molar heat capacity of water is about 85 J/mol K. From speciation data, the effects of water as hydroxyl groups and as molecular water have tentatively been estimated, with partial molar heat capacities of 153 ± 18 and 41 ± 14 J/mol K, respectively. In all cases, water strongly increases the configurational heat capacity at T_g and exerts a marked depressing effect on T_g, in close agreement with the results of viscosity experiments on the same series of glasses. Consistent with the Adam and Gibbs theory of relaxation processes, the departure of the viscosity of hydrous melts from Arrhenian variations correlates with the magnitude of configurational heat capacities.     

Solution behavior of reduced COH volatiles in silicate melts at high pressure and temperature

The solubility and solution mechanisms of reduced COH volatiles in Na₂OSiO₂ melts in equilibrium with a (H₂ + CH₄) fluid at the hydrogen fugacity defined by the iron-wüstite + H₂O buffer [f_H2(IW)] have been determined as a function of pressure (1-2.5 GPa) and silicate melt polymerization (NBO/Si: nonbridging oxygen per silicon) at 1400 °C. The solubility, calculated as CH₄, increases from ∼0.2 wt% to ∼0.5 wt% in the melt NBO/Si-range ∼0.4 to ∼1.0. The solubility is not significantly pressure-dependent, probably because f_H2(IW) in the 1-2.5 GPa range does not vary greatly with pressure. Carbon isotope fractionation between methane-saturated melts and (H₂ + CH₄) fluid varied by ∼14‰ in the NBO/Si-range of these melts.The (C..H) and (O..H) speciation in the quenched melts was determined with Raman and ¹H MAS NMR spectroscopy. The dominant (C..H)-bearing complexes are molecular methane, CH₄, and a complex or functional group that includes entities with CCH bonding. Minor abundance of complexes that include SiOCH₃ bonding is tentatively identified in some melts. There is no spectroscopic evidence for SiC or SiCH₃. Raman spectra indicate silicate melt depolymerization (increasing NBO/Si). The [CH₄/CCH]^melt abundance ratio is positively correlated with NBO/Si, which is interpreted to suggest that the (CCH)-containing structural entity is bonded to the silicate melt network structure via its nonbridging oxygen. The ∼14‰ carbon isotope fractionation change between fluid and melt is because of the speciation changes of carbon in the melt.     

Peralkalinity, Al{rightleftharpoons}Si Substitution, and Solubility Mechanisms of H2O in Aluminosilicate Melts

The solubility mechanisms of H₂O in peralkaline sodium aluminosilicatequenched melts (anhydrous NBO/T = 0.5) have been studied withRaman spectroscopy as a function of Al/(Al + Si) (0–0–3)and H²O content (0–7.5 wt.%). The coexisting structuralunits in the anhydrous quenched melts are TO2 (Q4), T2O5(Q3),and TO3 (Q2). In Al-free Na₂Si₄O₉ (NS4) melt, H₂O forms complexes with Na⁺(Na–OH bonds) and with Si⁴⁺ (Si–OH bonds). MolecularH₂O is also detected. TO3 structural units are not detectedin this composition. In the H₂O concentration range between0 and 4 wt.%, there is an approximately 20% increase in NBO/Tresulting from the increased abundance ratio, T₂O₅/TO2. Withfurther increments in water activity, the NBO/T of hydrous NS4melt is reduced. The depolymerization results from hydroxylationof the silica tetrahedra, whereas polymerization is due to formationof complexes with Na–OH bonding. In Al-bearing compositions on the Na₂Si₄O₉–Na₂(NaAl)₄O₉–join, there is evidence for Al–OH bonding in additionto Na–OH and Si–OH bonds. Among these complexes,the relative abundance of those with Si–OH bonds diminisheswith increasing Al/(A1 + Si), whereas complexes with Al–OHand Na–OH bonds become more important. Complexes withNa–OH bonds dominate for H₂O4 wt.%, whereas complexeswith Al–OH dominate at higher water content. The threestructural units, TO₃, T₂O₅, and TO², were observed in bothanhydrous and hydrous peralkaline sodium aluminosilicate melts.Their abundance varies, however, with the H₂O concentrationin the melts. The NBO/T decreases to a minimum (a 30–50%lowering of NBO/T relative to anhydrous materials) for low H₂Ocontents (3–4 wt.% H₂O), and increases as the H₂O contentis increased further.     

Solvation processes in steam: Ab initio calculations of ion-solvent structures and clustering equilibria

Reports of the high ion content of steam and low-density supercritical fluids date back to the work of Carlon [Carlon H. R. (1980) Ion content of air humidified by boiling water.J. Appl.Phys.51, 171-173], who invoked ion and neutral-water clustering as mechanism to explain why ions partition into the low-density aqueous phase. Mass spectrometric, vibrational spectroscopic measurements and quantum chemical calculations have refined this concept by proposing strongly bound ion-solvent aggregates and water clusters such as Eigen- and Zundel-type proton clusters H₃O⁺·(H₂O)_m and the more weakly bound water oligomers (H₂O)_m. The extent to which these clusters affect fluid chemistry is determined by their abundance, however, little is known regarding the stability of such moieties in natural low-density high-temperature fluids. Here we report results from quantum chemical calculations using chemical-accuracy multi-level G3 (Curtiss-Pople) and CBS-Q theory (Peterson) to address this question. In particular, we have investigated the cluster structures and clustering equilibria for the ions and H₃S⁺·(H₂O)_m(H₂S)_n, where m ? 6 and n ? 4, at 300-1000 K and 1 bar as well as under vapor-liquid equilibrium conditions between 300 and 646 K. We find that incremental hydration enthalpies and entropies derived from van’t Hoff analyses for the attachment of H₂O and H₂S onto H₃O⁺, and H₃S⁺ are in excellent agreement with experimental values and that the addition of water to all three ions is energetically more favorable than solvation by H₂S. As clusters grow in size, the energetic trends of cluster hydration begin to reflect those for bulk H₂O liquids, i.e. calculated hydration enthalpies and entropies approach values characteristic of the condensation of bulk water (ΔH^o = −44.0 kJ mol⁻¹, ΔS^o = −118.8 J K mol⁻¹). Water and hydrogen sulfide cluster calculations at higher temperatures indicate that a significant fraction of H₃O⁺, and H₃S⁺ ions exists as solvated moieties.     

Solubility and solution mechanism of H2O in alkali silicate melts and glasses at high pressure and temperature

The solubility behavior of H₂O in melts in the system Na₂O-SiO₂-H₂O was determined by locating the univariant phase boundary, melt = melt + vapor in the 0.8-2 GPa and 1000°-1300°C pressure and temperature range, respectively. The NBO/Si-range of the melts (0.25-1) was chosen to cover that of most natural magmatic liquids. The H₂O solubility in melts in the system Na₂O-SiO₂-H₂O (X_H2O) ranges between 18 and 45 mol% (O = 1) with (∂X_H2O/∂P)_T∼14-18 mol% H₂O/GPa. The (∂X_H2O/∂P)_T is negatively correlated with NBO/Si (= Na/Si) of the melt. The (∂X_H2O/∂T)_P is in the −0.03 to +0.05 mol% H₂O/°C range, and is negatively correlated with NBO/Si. The [∂X_H2O/∂(NBO/Si)]_P,T is in the −3 to −8 mol% H₂O/(NBO/Si) range. Melts with NBO/Si similar to basaltic liquids (∼0.6-∼1.0) show (∂X_H2O/∂T)_P<0, whereas more polymerized melts exhibit (∂X_H2O/∂T)_P>0. Complete miscibility between hydrous melt and aqueous fluid occurs in the 0.8-2 GPa pressure range for melts with NBO/Si ≤0.5 at T >1100°C. Miscibility occurs at lower pressure the more polymerized the melt.     

XPS spectra of uranyl minerals and synthetic uranyl compounds. II: The O 1s spectrum

The O 1s spectrum is examined for 19 uranyl minerals of different composition and structure. Spectra from single crystals were measured with a Kratos Axis Ultra X-ray Photoelectron Spectrometer with a magnetic-confinement charge-compensation system. Well-resolved spectra with distinct maxima, shoulders and inflections points, in combination with reported and measured binding energies for specific O²⁻ species and structural data of the uranyl minerals are used to resolve the fine structure of the O 1s envelope. The resolution of the O 1s spectra includes, for the first time, different O²⁻ bands, which are assigned to O atoms linking uranyl with uranyl polyhedra (UOU) and O atoms of uranyl groups (OUO). The resolved bands in the O 1s spectrum occur at distinct ranges in binding energy: bands for (UOU) occur at 529.6-530.4 eV, bands for (OUO) occur at 530.6-531.4 eV, bands for O²⁻ in the equatorial plane of the uranyl polyhedra linking uranyl polyhedra with (TO_n) groups (T = Si, S, C, P, Se) (TO) occur at 530.9-532.2 eV; bands for (OH) groups in the equatorial plane of the uranyl polyhedra (OH) occur at 532.0-532.5 eV, bands of (H₂O) groups in the interstitial complex of the uranyl minerals (H₂O_interst) occur at 533.0-533.8 eV and bands of physisorbed (H₂O) groups on the surface of uranyl minerals (H₂O_adsorb) occur at 534.8-535.2 eV. Treatment of uranyl minerals with acidic solutions results in a decrease in UOU and an increase in OH. Differences in the ratio of OH : OUO between the surface and bulk structure is larger for uranyl minerals with a high number of UOU and TO species in the bulk structure which is explained by protonation of underbonded UO, UOU and TO terminations on the surface. The difference in the ratio of H₂O_interst : OUO between the bulk and surface structures is larger for uranyl minerals with higher percentages of H₂O_interst as well as, with a higher number of interstitial H₂O groups that are not bonded to interstitial cations, resulting in easier dehydration of interstitial H₂O groups in uranyl minerals during exposure to a vacuum.     

Molecular H2O in armenite, BaCa2Al6Si9O30·2H2O, and epididymite, Na2Be2Si6O15·H2O: Heat capacity, entropy and local-bonding behavior of confined H2O in microporous silicates

Armenite, ideal formula BaCa₂Al₆Si₉O₃₀·2H₂O, and its dehydrated analog BaCa₂Al₆Si₉O₃₀ and epididymite, ideal formula Na₂Be₂Si₆O₁₅·H₂O, and its dehydrated analog Na₂Be₂Si₆O₁₅ were studied by low-temperature relaxation calorimetry between 5 and 300 K to determine the heat capacity, C_p, behavior of their confined H₂O. Differential thermal analysis and thermogravimetry measurements, FTIR spectroscopy, electron microprobe analysis and powder Rietveld refinements were undertaken to characterize the phases and the local environment around the H₂O molecule.The determined structural formula for armenite is Ba_0.88(0.01)Ca_1.99(0.02)Na_0.04(0.01)Al_5.89(0.03)Si_9.12(0.02)O₃₀·2H₂O and for epididymite Na_1.88(0.03)K_0.05(0.004)Na_0.01(0.004)Be_2.02(0.008)Si_6.00(0.01)O₁₅·H₂O. The infrared (IR) spectra give information on the nature of the H₂O molecules in the natural phases via their H₂O stretching and bending vibrations, which in the case of epididymite only could be assigned. The powder X-ray diffraction data show that armenite and its dehydrated analog have similar structures, whereas in the case of epididymite there are structural differences between the natural and dehydrated phases. This is also reflected in the lattice IR mode behavior, as observed for the natural phases and the H₂O-free phases. The standard entropy at 298 K for armenite is S° = 795.7 ± 6.2 J/mol K and its dehydrated analog is S° = 737.0 ± 6.2 J/mol K. For epididymite S° = 425.7 ± 4.1 J/mol K was obtained and its dehydrated analog has S° = 372.5 ± 5.0 J/mol K. The heat capacity and entropy of dehydration at 298 K are Δ = 3.4 J/mol K and ΔS^rxn = 319.1 J/mol K and Δ = −14.3 J/mol K and ΔS^rxn = 135.7 J/mol K for armenite and epididymite, respectively. The H₂O molecules in both phases appear to be ordered. They are held in place via an ion-dipole interaction between the H₂O molecule and a Ca cation in the case of armenite and a Na cation in epididymite and through hydrogen-bonding between the H₂O molecule and oxygen atoms of the respective silicate frameworks. Of the three different H₂O phases ice, liquid water and steam, the C_p behavior of confined H₂O in both armenite and epididymite is most similar to that of ice, but there are differences between the two silicates and from the C_p behavior of ice. Hydrogen-bonding behavior and its relation to the entropy of confined H₂O at 298 K is analyzed for various microporous silicates.The entropy of confined H₂O at 298 K in various silicates increases approximately linearly with increasing average wavenumber of the OH-stretching vibrations. The interpretation is that decreased hydrogen-bonding strength between a H₂O molecule and the silicate framework, as well as weak ion-dipole interactions, results in increased entropy of H₂O. This results in increased amplitudes of external H₂O vibrations, especially translations of the molecule, and they contribute strongly to the entropy of confined H₂O at T < 298 K.     

CO2 solubility in Martian basalts and Martian atmospheric evolution

To understand possible volcanogenic fluxes of CO₂ to the Martian atmosphere, we investigated experimentally carbonate solubility in a synthetic melt based on the Adirondack-class Humphrey basalt at 1-2.5 GPa and 1400-1625 °C. Starting materials included both oxidized and reduced compositions, allowing a test of the effect of iron oxidation state on CO₂ solubility. CO₂ contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and Fe³⁺/Fe^T was measured by Mössbauer spectroscopy. The CO₂ contents of glasses show no dependence on Fe³⁺/Fe^T and range from 0.34 to 2.12 wt.%. For Humphrey basalt, analysis of glasses with gravimetrically-determined CO₂ contents allowed calibration of an integrated molar absorptivity of 81,500 ± 1500 L mol⁻¹ cm⁻² for the integrated area under the carbonate doublet at 1430 and 1520 cm⁻¹. The experimentally determined CO₂ solubilities allow calibration of the thermodynamic parameters governing dissolution of CO₂ vapor as carbonate in silicate melt, K_II, (Stolper and Holloway, 1988) as follows: , ΔV⁰ = 20.85 ± 0.91 cm³ mol⁻¹, and ΔH⁰ = −17.96 ± 10.2 kJ mol⁻¹. This relation, combined with the known thermodynamics of graphite oxidation, facilitates calculation of the CO₂ dissolved in magmas derived from graphite-saturated Martian basalt source regions as a function of P, T, and f_O2. For the source region for Humphrey, constrained by phase equilibria to be near 1350 °C and 1.2 GPa, the resulting CO₂ contents are 51 ppm at the iron-wüstite buffer (IW), and 510 ppm at one order of magnitude above IW (IW + 1). However, solubilities are expected to be greater for depolymerized partial melts similar to primitive shergottite Yamato 980459 (Y 980459). This, combined with hotter source temperatures (1540 °C and 1.2 GPa) could allow hot plume-like magmas similar to Y 980459 to dissolve 240 ppm CO₂ at IW and 0.24 wt.% of CO₂ at IW + 1. For expected magmatic fluxes over the last 4.5 Ga of Martian history, magmas similar to Humphrey would only produce 0.03 and 0.26 bars from sources at IW and IW + 1, respectively. On the other hand, more primitive magmas like Y 980459 could plausibly produce 0.12 and 1.2 bars at IW and IW + 1, respectively. Thus, if typical Martian volcanic activity was reduced and the melting conditions cool, then degassing of CO₂ to the atmosphere may not be sufficient to create greenhouse conditions required by observations of liquid surface water. However, if a significant fraction of Martian magmas derive from hot and primitive sources, as may have been true during the formation of Tharsis in the late Noachian, that are also slightly oxidized (IW + 1.2), then significant contribution of volcanogenic CO₂ to an early Martian greenhouse is plausible.     

Solubilities of corundum, wollastonite and quartz in H2O-NaCl solutions at 800 °C and 10 kbar: Interaction of simple minerals with brines at high pressure and temperature

Solubilities of corundum (Al₂O₃) and wollastonite (CaSiO₃) were measured in H₂O-NaCl solutions at 800 °C and 10 kbar and NaCl concentrations up to halite saturation by weight-loss methods. Additional data on quartz solubility at a single NaCl concentration were obtained as a supplement to previous work. Single crystals of synthetic corundum, natural wollastonite or natural quartz were equilibrated with H₂O and NaCl at pressure (P) and temperature (T) in a piston-cylinder apparatus with NaCl pressure medium and graphite heater sleeves. The three minerals show fundamentally different dissolution behavior. Corundum solubility undergoes large enhancement with NaCl concentration, rising rapidly from Al₂O₃ molality (m_Al2O3) of 0.0013(1) (1σ error) in pure H₂O and then leveling off to a maximum of ∼0.015 at halite saturation (X_NaCl ≈ 0.58, where X is mole fraction). Solubility enhancement relative to that in pure H₂O, , passes through a maximum at X_NaCl ≈ 0.15 and then declines towards halite saturation. Quenched fluids have neutral pH at 25 °C. Wollastonite has low solubility in pure H₂O at this P and T(m_CaSiO3=0.0167(6)). It undergoes great enhancement, with a maximum solubility relative to that in H₂O at X_NaCl ≈ 0.33, and solubility >0.5 molal at halite saturation. Solute silica is 2.5 times higher than at quartz saturation in the system H₂O-NaCl-SiO₂, and quenched fluids are very basic (pH 11). Quartz shows monotonically decreasing solubility from m_SiO2=1.248 in pure H₂O to 0.202 at halite saturation. Quenched fluids are pH neutral. A simple ideal-mixing model for quartz-saturated solutions that requires as input only the solubility and speciation of silica in pure H₂O reproduces the data and indicates that hydrogen bonding of molecular H₂O to dissolved silica species is thermodynamically negligible. The maxima in for corundum and wollastonite indicate that the solute products include hydrates and Na⁺ and/or Cl⁻ species produced by molar ratios of reactant H₂O to NaCl of 6:1 and 2:1, respectively. Our results imply that quite simple mechanisms may exist in the dissolution of common rock-forming minerals in saline fluids at high P and T and allow assessment of the interaction of simple, congruently soluble rock-forming minerals with brines associated with deep-crustal metamorphism.     

Oxygen isotope partitioning during oxidation of pyrite by H2O2 and its dependence on temperature

A detailed experimental study was conducted to investigate mechanisms of pyrite oxidation by determining product yields and oxygen isotopic fractionation during reactions between powdered pyrite (FeS₂) with aqueous hydrogen peroxide (H₂O₂). Sealed silica-tube experiments utilized aliquots of pyrite that were reacted with 0.2 M H₂O₂ for 7 to 14 days at 4 to 150 °C. No volatile sulfur species were detected in any experiment. The only gaseous product recovered was elemental oxygen inferred to result from decomposition of H₂O₂. Aqueous sulfate (S_aq) was the only sulfur product recovered from solution. Solid hydrated ferric iron sulfates (i.e., water-soluble sulfate fraction, S_ws) were recovered from all experiments. Ferric oxide (hematite) was detected only in high temperature experiments.Reactants were selected with large differences in initial δ¹⁸O values. The oxygen isotopic compositions of oxygen-bearing reactants and products were analyzed for each experiment. Subsequent isotopic mass-balances were used to identify sources of oxygen for reaction products and to implicate specific chemical reaction mechanisms. δ¹⁸O of water did not show detectable change during any experiment. δ¹⁸O of sulfate was similar for S_aq and S_ws and indicated that both H₂O and H₂O₂ were sources of oxygen in sulfate. Low-temperature experiments suggest that H₂O-derived oxygen was incorporated into sulfate via Fe³⁺ oxidation, whereas H₂O₂-derived oxygen was incorporated into sulfate via oxidation by hydroxyl radicals (HO). These two competing mechanisms for oxygen incorporation into sulfate express comparable influences at 25 °C. With increasing reaction temperatures from 4 to 100 °C, it appears that accelerated thermal decomposition and diminished residence time of H₂O₂ limit the oxygen transfer from H₂O₂ into sulfate and enhance the relative importance of H₂O-derived oxygen for incorporation into sulfate. Notably, at temperatures between 100 and 150 °C there is a reversal in the lower temperature trend resulting in dominance of H₂O₂-derived oxygen over H₂O-derived oxygen. At such high temperatures, complete thermal decomposition of H₂O₂ to water and molecular oxygen (O₂) occurs within minutes in mineral-blank experiments and suggests little possibility for direct oxidation of pyrite by H₂O₂ above 100 °C. We hypothesize that a Fe-O₂ mechanism is responsible for oxygenating pyrite to sulfate using O₂ from the preceding thermal decomposition of H₂O₂.     

Experimental determination of the solubility of natural wollastonite in pure water up to pressures of 5 GPa and at temperatures of 400-800 °C

The solubility of natural, near-end-member wollastonite-I (>99.5% CaSiO₃) has been determined at temperatures from 400 to 800 °C and pressures between 0.8 and 5 GPa in piston-cylinder apparatus with the weight-loss method. Chemical analysis of quench products and optical monitoring in a hydrothermal diamond anvil cell demonstrates that no additional phases form during dissolution. Wollastonite-I, therefore, dissolves congruently in the pressure-temperature range investigated. The solubility of CaSiO₃ varies between 0.175 and 13.485 wt% and increases systematically with both temperature and pressure up to 3.0 GPa. Above 3.0 GPa wollastonite-I reacts rapidly to the high-pressure modification wollastonite-II. No obvious trends are evident in the solubility of wollastonite-II, with values between 1.93 and 10.61 wt%. The systematics of wollastonite-I solubility can be described well by a composite polynomial expression that leads to isothermal linear correlation with the density of water. The molality of dissolved wollastonite-I in pure water is then

log(m_woll)=2.2288-3418.23×T^-1+671386.84×T^-2+logρ_H2O×(5.4578+2359.11×T^-1).     

Thermodynamic properties of H4SiO4 in the ideal gas state as evaluated from experimental data

Solid phases of silicon dioxide react with water vapor with the formation of hydroxides and oxyhydroxides of silica. Recent transpiration and mass-spectrometric studies convincingly demonstrate that H₄SiO₄ is the predominant form of silica in vapor phase at water pressure in excess of 10⁻² MPa. Available literature transpiration and solubility data for the reactions of solid SiO₂ phases and low-density water, extending from 424 to 1661 K, are employed for the determination of Δ_fG⁰, Δ_fH⁰ and S⁰ of H₄SiO₄ in the ideal gas state at 298.15 K, 0.1 MPa. In total, there are 102 data points from seven literature sources. The resulting values of the thermodynamic functions of H₄SiO₄(g) are: Δ_fG⁰ = −1238.51 ± 3.0 kJ mol⁻¹, Δ_fH⁰ = −1340.68 ± 3.5 kJ mol⁻¹ and S⁰ = 347.78 ± 6.2 J K⁻¹ mol⁻¹. These values agree quantitatively with one set of ab initio calculations. The relatively large uncertainties are mainly due to conflicting data for H₄SiO₄(g) from various sources, and new determinations of would be helpful. The thermodynamic properties of this species, H₄SiO₄(g), are necessary for realistic modeling of silica transport in a low-density water phase. Applications of this analysis may include the processes of silicates condensation in the primordial solar nebula, the precipitation of silica in steam-rich geothermal systems and the corrosion of SiO₂-containing alloys and ceramics in moist environments.