OH incorporation in nearly pure MgAl2O4 natural and synthetic spinels

Four nearly pure MgAl₂O₄ spinels, of both natural and synthetic occurrence, have been studied by means of X-ray single crystal diffraction and FTIR spectroscopy in order to detect their potential OH content. Absorption bands that can be assigned to OH incorporated in the spinel structure were only observed in spectra of a non-stoichiometric synthetic sample. The absorption intensity of two bands occurring at 3350 and 3548 cm⁻¹ indicate an OH content of 90 ppm H₂O. Based on correlations of OH vibrational frequencies and O-H?O distances, the observed absorption bands correspond to O-H?O distances of 2.77 and 2.99 Å, respectively, which is close to the values obtained by the structure refinements for ^VIO-O_unsh (2.825 Å) and ^IVO-O (3.001 Å). This indicates that one probable local position for hydrogen incorporation is the oxygens coordinating a vacant tetrahedral site. The present spectra demonstrate that the detection limit for OH in Fe-free spinels is in the range 10-20 ppm H₂O. However, at appreciable Fe²⁺ levels, the detection of OH bands becomes hampered due to overlap with strong absorption bands caused by electronic d-d transitions in Fe²⁺ in the tetrahedral position.     

Interaction between sulphide and H2O in silicate melts

Reaction between dissolved water and sulphide was experimentally investigated in soda-lime-silicate (NCS) and sodium trisilicate (NS3) melts at temperatures from 1000 to 1200 °C and pressures of 100 or 200 MPa in internally heated gas pressure vessels. Diffusion couple experiments were conducted at water-undersaturated conditions with one half of the couple being doped with sulphide (added as FeS or Na₂S; 1500-2000 ppm S by weight) and the other with H₂O (∼3.0 wt.%). Additionally, two experiments were performed using a dry NCS glass cylinder and a free H₂O fluid. Here, the melt was water-saturated at least at the melt/fluid interface. Profiling by electron microprobe (sulphur) and infrared microscopy (H₂O) demonstrate that H₂O diffusion in the melts is faster by 1.5-2.3 orders of magnitude than sulphur diffusion and, hence, H₂O can be considered as a rapidly diffusing oxidant while sulphur is quasi immobile in these experiments.In Raman spectra a band at 2576 cm⁻¹ appears in the sulphide - H₂O transition zone which is attributed to fundamental S-H stretching vibrations. Formation of new IR absorption bands at 5025 cm⁻¹ (on expense of the combination band of molecular H₂O at 5225 cm⁻¹) and at 3400 cm⁻¹ was observed at the front of the in-diffusing water in the sulphide bearing melt. The appearance and intensity of these two IR bands is correlated with systematic changes in S K-edge XANES spectra. A pre-edge excitation at 2466.5 eV grows with increasing H₂O concentration while the sulphide peak at 2474.0 eV decreases in intensity relative to the peak at 2477.0 eV and the feature at 2472.3 eV becomes more pronounced (all energies are relative to the sulphate excitation, calibrated to 2482.5 eV). The observations by Raman, IR and XANES spectroscopy indicate a well coordinated S²⁻ - H₂O complex which was probably formed in the glasses during cooling at the glass transition. No oxidation of sulphide was observed in any of the diffusion couple experiments. On the contrary, XANES spectra from experiments conducted with a free H₂O fluid show complete transformation of sulphide to sulphate near the melt surface and coexistence of sulphate and sulphide in the center of the melt. This can be explained by a lower H₂O activity in the diffusion couple experiments or by the need of a sink for hydrogen (e.g., a fluid which can dissolve high concentration of hydrogen) to promote oxidation of sulphide by H₂O via the reaction S²⁻ + 4H₂O = SO₄²⁻ + 4H₂. Sulphite could not be detected in any of the XANES spectra implying that this species, if it exists in the melt, it is a subordinate or transient species only.     

On the presence of hydrous defects in differently coloured wulfenites (PbMoO4): an infrared and optical spectroscopic study

Several samples of wulfenite, PbMoO₄, varying in colour from colourless to yellow, orange and red, have been characterised by means of IR and optical absorption spectroscopy and by microprobe analyses. A distinct pleochroic band group with absorption maxima centred at 3,380 and 3,150 cm^?1 can be seen in the IR spectra of wulfenite single-crystals, indicating the presence of hydroxyl groups. The pleochroic and thermal behaviour of the OH stretching bands along with deuteration experiments, as well as results obtained from synthetic flux-grown samples, exclude the presence of submicroscopic hydrous mineral inclusions as their primary origin. The pleochroic scheme and the band positions were used to postulate a model for the OH incorporation mode, based on the assumption of vacancies on Mo and Pb sites in the structure of this ‘nominally anhydrous mineral’. Optical absorption spectra of coloured natural samples show a broad and polarised band around 23,000–24,000 cm^?1, preceding the fundamental UV absorption edge, which has been identified as the reason for the colour of the mineral. The comparison with synthetic PbMoO₄ single-crystals, doped with variable amounts of Cr⁶⁺, yielded conclusive evidence that trace amounts of the CrO₄ ^2? anion group, substituting for MoO₄ ^2?, determine the variable colour. Besides, in one sample, trace amounts of Nd³⁺ have been spectroscopically identified.     

An experimental investigation of hydroxyl solubility in jadeite and Na-rich clinopyroxenes

The solubility and incorporation mechanisms of water in synthetic, water-saturated jadeite and Na-rich clinopyroxenes have been experimentally investigated. Infrared spectra for water-saturated jadeite synthesised from 2.0 to 10 GPa show two prominent sharp peaks at 3,373 and 3,613 cm^–1 together with several weaker features in the OH-stretching region, indicating that there are at least 5 distinct modes of hydrogen incorporation in the structure. Water solubility in pure jadeite reaches a maximum of about 450 ppm by weight at 2 GPa and slowly decreases with increasing pressure to about 100 ppm at 10 GPa. Solubility can be described by the function c_OH=A f_H2O^0.5 exp (–PV_Solid/RT), where c_OH is water solubility in ppm H₂O by weight, A is 7.144 ppm/bar^0.5, f_H2O is water fugacity, and V_Solid=8.019 cm³/mol is the volume change of the clinopyroxene upon incorporation of OH. Jadeite provides a good model for understanding hydrogen incorporation mechanisms in more complex omphacite compositions. Assignment of absorption bands in IR spectra verifies the importance of cation vacancies on the M2 site in providing mechanisms for hydrogen incorporation. However, results also suggest that substitution of lower valency cations onto the M1 site may also be important. Solid solution of jadeite with diopside and in particular, with Ca-Eskola component leads to a drastic increase of water solubility, and the bulk composition has a more important effect on the capacity of omphacite to store water than pressure and temperature. Omphacite is expected to be the major carrier of water in a subducted eclogite after the breakdown of hydrous minerals.Editorial responsibility: W. Schreyer     

Influence of glass polymerisation and oxidation on micro-Raman water analysis in alumino-silicate glasses

The development of an accurate analytical procedure for determination of dissolved water in complex alumino-silicate glasses via micro-Raman analysis requires the assessment of the spectra topology dependence on glass composition. We report here a detailed study of the respective influence of bulk composition, iron oxidation state and total water content on the absolute and relative intensities of the main Raman bands related to glass network vibrations (LF: ∼490 cm⁻¹; HF: ∼960 cm⁻¹) and total water stretching (H₂O_T: ∼3550 cm⁻¹) in natural glasses. The evolution of spectra topology was examined in (i) 33 anhydrous glasses produced by the re-melting of natural rock samples, which span a very large range of polymerisation degree (NBO/T from 0.00 to 1.16), (ii) 2 sets of synthetic anhydrous basaltic glasses with variable iron oxidation state (Fe³⁺/Fe_T from 0.05 to 0.87), and (iii) 6 sets of natural hydrous glasses (C_H2OT from 0.4 to 7.0 wt%) with NBO/T varying from 0.01 to 0.76.In the explored domain of water concentration, external calibration procedure based on the H₂O_T band height is matrix-independent but its accuracy relies on precise control of the focusing depth and beam energy on the sample. Matrix-dependence strongly affects the internal calibrations based on H₂O_T height scaled to that of LF or HF bands but its effect decreases from acid (low NBO/T, SM) to basic (high NBO/T, SM) glasses. Structural parameters such as NBO/T (non-bridging oxygen per tetrahedron) and SM (sum of structural modifiers) describe the matrix-dependence better than simple compositional parameters (e.g. SiO₂, Na₂O + K₂O). Iron oxidation state has only a minor influence on band topology in basalts and is thus not expected to significantly affect the Raman determinations of water in mafic (e.g. low SiO₂, iron-rich) glasses. Modelling the evolution of the relative band height with polymerisation degree allows us to propose a general equation to predict the dissolved water content in natural glasses:

     

O and H diffusion in uraninite: Implications for fluid-uraninite interactions, nuclear waste disposal, and nuclear forensics

Diffusion coefficients for oxygen and hydrogen were determined from a series of natural uraninite-H₂O experiments between 50 and 700 °C. Under hydrous conditions there are two diffusion mechanisms: (1) an initial extremely fast-path diffusion mechanism that overprinted the oxygen isotopic composition of the entire crystals regardless of temperature and (2) a slower volume-diffusive mechanism dominated by defect clusters that displace or eject nearest neighbor oxygen atoms to form two interstitial sites and two partial vacancies, and by vacancy migration. Using the volume diffusion coefficients in the temperature range of 400-600 °C, diffusion coefficients for oxygen can be represented by D = 1.90e⁻⁵ exp (−123,382 J/RT) cm²/s and for temperatures between 100 and 300 °C the diffusion coefficients can be represented by D = 1.95e⁻¹⁰ exp (−62484 J/RT) cm²/s, where the activation energies for uraninite are 123.4 and 62.5 kJ/mol, respectively. Hydrogen diffusion in uraninite appears to be controlled by similar mechanisms as oxygen. Using the volume diffusion coefficients for temperatures between 50 and 700 °C, diffusion coefficients for hydrogen can be represented by D = 9.28e⁻⁶ exp (−156,528 J/RT) cm²/s for temperatures between 450 and 700 °C and D = 1.39e⁻¹⁴ exp (−34518 J/RT) cm²/s for temperatures between 50 and 400 °C, where the activation energies for uraninite are 156.5 and 34.5 kJ/mol, respectively.Results from these new experiments have implications for isotopic exchange during natural UO₂-water interactions. The exceptionally low δ¹⁸O values of natural uraninites (i.e. −32‰ to −19.5‰) from unconformity-type uranium deposits in Saskatchewan, in conjunction with theoretical and experimental uraninite-water and UO₃-water fractionation factors, suggest that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals. The low δ¹⁸O values have been interpreted as resulting from the low temperature overprinting of primary uranium mineralization in the presence of relatively modern meteoric fluids having δ¹⁸O values of ca. −18‰, despite petrographic and U-Pb isotope data that indicate limited alteration. Our data show that the anomalously low oxygen isotopic composition of the uraninite from the Athabasca Basin can be due to meteoric water overprinting under reducing conditions, and meteoric water or groundwater can significantly affect the oxygen isotopic composition of spent nuclear fuel in a geologic repository, with minimal change to the chemical composition or texture. Moreover, the rather fast oxygen and hydrogen diffusion coefficients for uraninite, especially at low temperatures, suggest that oxygen and hydrogen diffusion may impart characteristic isotopic signals that can be used to track the route of fissile material.     

A computer simulation study of OH defects in Mg2SiO4 and Mg2GeO4 spinels

Classical atomistic simulation techniques have been used to investigate the energies of hydrogen defects in Mg₂SiO₄ and Mg₂GeO₄ spinels. Ringwoodite (γ-Mg₂SiO₄) is considered to be the most abundant mineral in the lower part of the transition zone and can incorporate large amounts of water in the form of hydroxyls, whereas the germanate spinel (γ-Mg₂GeO₄) corresponds to a low-pressure structural analogue for ringwoodite. The calculated defect energies indicate that the most favourable mechanisms for hydrogen incorporation are coupled either with the reduction of ferric iron or with the creation of tetrahedral vacancies. Hydrogen will go preferentially into tetrahedral vacancies, eventually leading to the formation of the hydrogarnet defect, before associating with other negatively charged point defects. The presence of isolated hydroxyls is not expected. The same trend is observed for germanate, and thus γ-Mg₂GeO₄ could be used as a low-pressure analogue for ringwoodite in studies of water-related defects and their effect on physical properties.     

On the mechanisms for H and Al incorporation in stishovite

The solubility and incorporation mechanisms of hydrogen in synthetic stishovite as a function of Al₂O₃ content have been investigated. Mechanisms for H incorporation in stishovite are more complex than previously thought. Most H in stishovite is incorporated via the Smyth et al. (Am Mineral 80:454–456, 1995) model, where H docks close to one of the shared O–O edges, giving rise to an OH stretching band in infrared (IR) spectra at 3,111–3,117 cm⁻¹. However, careful examination of IR spectra from Al-stishovite reveals the presence of an additional OH band at 3,157–3,170 cm⁻¹. All H is present on one site, with interstitial H both coupled to Al³⁺ substitutional defects on adjacent octahedral (Si⁴⁺) sites, and decoupled from other defects, giving rise to two distinct absorption bands. Trends in IR data as a function of composition are consistent with a change in Al incorporation mechanism in stishovite, with Al³⁺ substitution for Si⁴⁺ charge-balanced by oxygen vacancies at low bulk Al₂O₃ contents, and coupled substitution of Al³⁺ onto octahedral (Si⁴⁺) and interstitial sites at high bulk Al₂O₃ contents. Trends in OH stretching frequencies as a function of Al₂O₃ content suggest that any such change in Al incorporation mechanism could alter the effect that Al incorporation has on the compressibility of stishovite, as noted by Ono et al. (Am Mineral 87:1486–1489, 2002).     

Hydrogen incorporation in enstatite in the system MgO–SiO2–H2O–NaCl

The incorporation of hydrogen in enstatite in a hydrous system containing various amounts of NaCl was investigated at 25 kbar. The hydrogen content in enstatite shows a clear negative correlation to the NaCl-concentration in the system. The most favourable explanation is the reduction of water fugacity due to dilution. Other reasons for the limited hydrogen incorporation at high NaCl levels, such as a significant influence of Na⁺ on the defect chemistry or an exchange between OH^- and Cl⁻in enstatite, appear much less important. A partition coefficient D _Na ^En/Fluid = 0.0013 could be determined, demonstrating that Na is less incompatible in enstatite than H. The new results support the idea that dissolved components have to be considered when the total hydrogen storage capacity in nominally anhydrous minerals is estimated, especially in geological settings with high levels of halogens, such as subduction zones.     

Behaviour of Li and its isotopes during metasomatism of French Massif Central lherzolites

Li behaviour and distribution in the mantle were investigated by ion microprobe in situ measurements on co-existing olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and amphibole (amp) in xenoliths from the French Massif Central. The fertile spinel lherzolites of this study record increasing degrees of mantle metasomatism, from unmetasomatised anhydrous samples through cryptically metasomatised samples to highly metasomatised amphibole-rich samples. In anhydrous lherzolites, Li is preferentially incorporated into olivine (1.1-1.4 ppm, average values) compared to pyroxenes (0.2-0.9 ppm). The hydrous samples clearly show enrichment of Li in ol (1.5-5.0 ppm), opx (1.1-2.4 ppm) and cpx (2.4-5.4 ppm), while amphibole incorporates less Li than the co-existing phases (0.8-1.3 ppm). Average δ⁷Li values range from +7.6 to +14.5‰ in ol, from 5.1 to +13.7‰ in opx and from 8.8 to +10.3‰ in cpx from the anhydrous lherzolites. A layered peridotite sample (Sdi) shows higher Li content in all phases, with lighter isotopic composition in opx and cpx (−0.6 and −2‰ average δ⁷Li values, respectively). In the hydrous lherzolites average δ⁷Li values both overlap and extend beyond these ranges in ol (up to 17.5 ‰) and in opx (up to 22.9‰), and vary widely in cpx (−2.7 to +9.7‰). Low δ⁷Li values are observed in some opx (−10.4‰) and cpx (−13‰) from sample Sdi, and in cpx from three hydrous samples (from −9.7 to −5.3‰). The different anhydrous phases from the hydrous samples show large intra-grain variations in Li isotopic ratios (e.g., up to 18‰) compared to the same phases from the anhydrous samples (mostly less than 6‰), excepting sample Sdi which has up to 20.4‰ variation in cpx. Similar to the anhydrous silicates, amphiboles show a wide variation of δ⁷Li values on the intra-grain scale (2-27‰). These variations are interpreted to result from fractionation processes during metasomatism by a silicate melt undergoing compositional changes as it percolates through and reacts with the peridotite phases. Thus Li abundances and isotopic in situ measurements are useful for tracing metasomatic processes but the heterogeneities observed in the samples preclude any identification of a specific mantle source by its Li signature.     

On the protonation of oxo- and hydroxo-groups of the goethite (α-FeOOH) surface: A FTIR spectroscopic investigation of surface O-H stretching vibrations

The O-H stretching region of goethite particles evaporated at different levels of acidity was investigated by Attenuated Total Reflectance (ATR)-Fourier Transform InfraRed (FTIR) spectroscopy. Two-dimensional IR Correlation Spectroscopy was used to identify correlations between different sets of discrete surface OH stretches and a Multivariate Curve Resolution analysis was used to resolve the predominant spectral components. Two dominant groups of hydroxyls were identified on the basis of their differences in proton affinity. Group I hydroxyls appear as two 3698/3541 and 3660/3490 cm⁻¹ band pairs. Group II hydroxyls are manifested through the 3648 and 3578 cm⁻¹ bands at greater levels of surface proton loading. There is consequently no correlation between O-H stretching frequencies and proton affinity. Groups I and II were assigned to mostly singly- (-OH) and doubly- (μ-OH) coordinated hydroxyls, respectively. Stretches arising from triply-coordinated (μ₃-OH) are proposed to be embedded within the dominant O-H band of bulk goethite. The possibility that these sites contribute to Group I and II hydroxyls should, however, not be entirely dismissed without further investigations.A reexamination of Temperature Programmed Desorption (TPD)-FTIR data of one goethite sample evaporated from alkaline conditions [Boily J.-F., Szanyi J., Felmy A. R. (2006) A combined FTIR and TPD study on the bulk and surface dehydroxylation and decarbonation of synthetic goethite. Geochim. Cosmochim. Acta70, 3613-3624] provided further constraints to this band assignment by providing clues to the network of surface hydrogen bonds. Important cooperative effects between hydrogen-bonded surface hydroxyls are suggested to play a crucial role on the variations of the position and intensity of discrete O-H stretching bands as a function of protonation level and temperature.     

A note on the Raman spectra of water-bearing albite glasses

The Raman spectra of albite glasses with 4.5 and 6.6 weight percent water have been obtained, and are compared with that of a dry sample. The hydrous glasses show bands near 3600 cm^?1 due to O-H stretching, and a previously unreported weak band near 1600 cm^?1 due to bending of molecular H₂O. Other weak spectral features are discussed, and the effect of dissolved water on the aluminosilicate framework vibrations is considered.     

Hydrous mineralogy of CM and CI chondrites from infrared spectroscopy and their relationship with low albedo asteroids

IR spectroscopy is one of the few techniques that can directly probe water molecules in rocks. This method has been used to characterize the mineralogy of hydrated/hydrous carbonaceous chondrites, and to link known meteorite families with spectroscopic observations of low albedo asteroids. In this paper, we present measurements of the infrared transmission spectra of matrix chunks from 3 CI and 9 CM chondrites. Spectra were measured at ambient conditions and then at different temperatures along a dehydration path toward high-T (∼300 °C) under primary vacuum. At ambient conditions, the 3-μm spectral range is always dominated by adsorbed atmospheric water molecules. Upon moderate (∼100 °C) and high (∼300 °C) heating under low pressure (P < 10⁻⁴ mbar), adsorbed water and then phyllosilicates interlayer water are removed, revealing a residual absorption band around 3 μm. This band is a characteristic IR feature of the phyllosilicate phases which dominate the mineralogical assemblage of hydrated carbonaceous chondrites. Among the CM chondrites, the high-T spectra reveal a strong variability that appears correlated with the alteration classification scheme of Rubin et al. (2007) and Howard et al. (2009a). The 3-μm band continuously evolves from a broad feature peaking at 3550-3600 cm⁻¹ for the weakly altered CMs (Murchison-type) to a sharp asymmetric peak at ∼3675 cm⁻¹ for the more extensively altered samples (Cold Bokkeveld-type). We attribute this spectral evolution to variations in the chemistry of the phyllosilicate phases from Fe-rich to Mg-rich. On the other hand, the 10-μm spectral region shows a single broad peak which does not compare with known terrestrial serpentine spectra, probably due to high structural disorder of the chondrite phyllosilicate phases. The present work clearly shows that previously published reflectance spectra of chondrites are biased by the presence of adsorbed terrestrial water molecules. Laboratory data collected under dry conditions are needed to reinterpret the chondrite-asteroid connection from the comparison of their 3-μm absorption features.     

Sulfur K-edge XANES analysis of natural and synthetic basaltic glasses: Implications for S speciation and S content as function of oxygen fugacity

XANES analyses at the sulfur K-edge were used to determine the oxidation state of S species in natural and synthetic basaltic glasses and to constrain the fO₂ conditions for the transition from sulfide (S²⁻) to sulfate (S⁶⁺) in silicate melts. XANES spectra of basaltic samples from the Galapagos spreading center, the Juan de Fuca ridge and the Lau Basin showed a dominant broad peak at 2476.8 eV, similar to the spectra obtained from synthetic sulfide-saturated basalts and pyrrhotite. An additional sharp peak at 2469.8 eV, similar to that of crystalline sulfides, was present in synthetic glasses quenched from hydrous melts but absent in anhydrous glasses and may indicate differences in sulfide species with hydration or presence of minute sulfide inclusions exsolved during quenching. The XANES spectra of a basalt from the 1991 eruption of Mount Pinatubo, Philippines, and absarokitic basalts from the Cascades Range, Oregon, USA, showed a sharp peak at 2482.8 eV, characteristic of synthetic sulfate-saturated basaltic glasses and crystalline sulfate-bearing minerals such as hauyne. Basaltic samples from the Lamont Seamount, the early submarine phase of Kilauea volcano and the Loihi Seamount showed unequivocal evidence of the coexistence of S²⁻ and S⁶⁺ species, emphasizing the relevance of S⁶⁺ to these systems. XANES spectra of basaltic glasses synthesized in internally-heated pressure vessels and equilibrated at fO₂ ranging from FMQ − 1.4 to FMQ + 2.7 showed systematic changes in the features related to S²⁻ and S⁶⁺ with changes in fO₂. No significant features related to sulfite (S⁴⁺) species were observed. These results were used to construct a function that allows estimates of S⁶⁺/ΣS from XANES data. Comparison of S⁶⁺/ΣS data obtained by S Kα shifts measured with electron probe microanalysis (EPMA), S⁶⁺/ΣS obtained from XANES spectra, and theoretical considerations show that data obtained from EPMA measurements underestimate S⁶⁺/ΣS in samples that are sulfate-dominated (most likely because of photo-reduction effects during analysis) whereas S⁶⁺/ΣS from XANES provide a close match to the expected theoretical values. The XANES-derived relationship for S⁶⁺/ΣS as a function of fO₂ indicates that the transition from S²⁻ to S⁶⁻ with increasing fO₂ occurs over a narrower interval than what is predicted by the EPMA-derived relationship. The implications for natural systems is that small variation of fO₂ above FMQ + 1 will have a large effect on S behavior in basaltic systems, in particular regarding the amount of S that can be transported by basaltic melts before sulfide saturation can occur.     

Raman spectroscopic measurements of CO2 density: Experimental calibration with high-pressure optical cell (HPOC) and fused silica capillary capsule (FSCC) with application to fluid inclusion observations

Raman spectroscopy is a powerful method for the determination of CO₂ densities in fluid inclusions, especially for those with small size and/or low fluid density. The relationship between CO₂ Fermi diad split (Δ, cm⁻¹) and CO₂ density (ρ, g/cm³) has been documented by several previous studies. However, significant discrepancies exist among these studies mainly because of inconsistent calibration procedures and lack of measurements for CO₂ fluids having densities between 0.21 and 0.75 g/cm³, where liquid and vapor phases coexist near room temperature.In this study, a high-pressure optical cell and fused silica capillary capsules were used to prepare pure CO₂ samples with densities between 0.0472 and 1.0060 g/cm³. The measured CO₂ Fermi diad splits were calibrated with two well established Raman bands of benzonitrile at 1192.6 and 1598.9 cm⁻¹. The relationship between the CO₂ Fermi diad split and density can be represented by: ρ = 47513.64243 − 1374.824414 × Δ + 13.25586152 × Δ² − 0.04258891551 × Δ³ (r² = 0.99835, σ = 0.0253 g/cm³), and this relationship was tested by synthetic fluid inclusions and natural CO₂-rich fluid inclusions. The effects of temperature and the presence of H₂O and CH₄ on this relationship were also examined.     

Mechanisms of OH defect incorporation in naturally occurring,hydrothermally formed diopside and jadeite

The IR spectrum of an alpine, hydrothermally formed diopside containing 17 wt ppm H₂O consists of three main OH absorption bands centred at 3647, 3464 and 3359 cm⁻¹. Jadeite from a Californian vein occurrence is characterised by bands at 3616 and 3557 cm⁻¹ and contains about 197 wt ppm H₂O. Based on the pleochroic scheme of the OH absorption bands in diopside, OH defect incorporation models are derived on the basis of fully occupied cation sites and under the assumption of M1 and M2 site vacancies; OH defects replacing O2 oxygen atoms are most common. The less pronounced OH pleochroism and the broad band absorption pattern of jadeite indicate a high degree of OH defect disordering. The pleochroic scheme of the main absorption bands at 3616 and 3557 cm⁻¹ implies partial replacement of O2 oxygen atoms by OH dipoles pointing to vacant Si sites. Under the assumption of M1 and M2 site vacancies, O1–H and O2–H defects are also derivable. OH incorporation modes assuming Si-vacancies should be considered for jadeite-rich clinopyroxenes formed in deep crust and upper mantle regions.     

Speciation and thermodynamic properties for cobalt chloride complexes in hydrothermal fluids at 35-440 °C and 600 bar: An in-situ XAS study

Aqueous Co(II) chloride complexes play a crucial role in cobalt transport and deposition in ore-forming hydrothermal systems, ore processing plants, and in the corrosion of special Co-bearing alloys. Reactive transport modelling of cobalt in hydrothermal fluids relies on the availability of thermodynamic properties for Co complexes over a wide range of temperature, pressure and salinity. Synchrotron X-ray absorption spectroscopy was used to determine the speciation of cobalt(II) in 0-6 m chloride solutions at temperatures between 35 and 440 °C at a constant pressure of 600 bar. Qualitative analysis of XANES spectra shows that octahedral species predominate in solution at 35 °C, while tetrahedral species become increasingly important with increasing temperature. Ab initio XANES calculations and EXAFS analyses suggest that in high temperature solutions the main species at high salinity (Cl:Co >> 2) is CoCl₄²⁻, while a lower order tetrahedral complex, most likely CoCl₂(H₂O)_2(aq), predominates at low salinity (Cl:Co ratios ∼2). EXAFS analyses further revealed the bonding distances for the octahedral Co(H₂O)₆²⁺ (^octCo-O = 2.075(19) Å), tetrahedral CoCl₄²⁻ (^tetCo-Cl = 2.252(19) Å) and tetrahedral CoCl₂(H₂O)_2(aq) (^tetCo-O = 2.038(54) Å and ^tetCo-Cl = 2.210(56) Å). An analysis of the Co(II) speciation in sodium bromide solutions shows a similar trend, with tetrahedral bromide complexes becoming predominant at higher temperature/salinity than in the chloride system. EXAFS analysis confirms that the limiting complex at high bromide concentration at high temperature is CoBr₄²⁻. Finally, XANES spectra were used to derive the thermodynamic properties for the CoCl₄²⁻ and CoCl₂(H₂O)_2(aq) complexes, enabling thermodynamic modelling of cobalt transport in hydrothermal fluids. Solubility calculations show that tetrahedral CoCl₄²⁻ is responsible for transport of cobalt in hydrothermal solutions with moderate chloride concentration (∼2 m NaCl) at temperatures of 250 °C and higher, and both cooling and dilution processes can cause deposition of cobalt from hydrothermal fluids.     

Oxygen sites in hydrous aluminosilicate glasses: the role of Al-O-Al and H2O

We describe here high-field ¹⁷O magic-angle-spinning (MAS) and triple-quantum MAS (3QMAS) NMR spectra for several alkali silicate and Na, K, and Ca aluminosilicate glasses containing up to 10 wt.% water. The H₂O site appears to have a large quadrupolar coupling constant, and its chemical shift increases from Na- to K- glasses, suggesting significant cation-H₂O interactions. In ¹⁷O one-pulse MAS and 3QMAS and ²⁷Al one-pulse NMR experiments, major differences were seen between spectra for anhydrous and hydrous calcium aluminosilicate glasses. The changes in the ¹⁷O MAS spectra can be explained by the addition of an H₂O peak and to the disappearance of an Al-O-Al peak from the ¹⁷O NMR spectrum for the hydrous glass. The ²⁷Al results are consistent with this interpretation.     

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.     

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.