The crystal structure of chromite FeCr2O4 was investigated to 13.7 GPa and ambient temperature with single-crystal X-ray diffraction techniques. The unit-cell parameter decreases continuously from 8.3832 (5) to 8.2398 (11) angstrom up to 11.8 GPa. A fit to the Birch-Murnaghan equation of state (EoS) based on the P-V data gives: K-0 = 209 (13) GPa, K' = 4.0 (fixed), and V-0 = 588 (1) angstrom(3). The FeO4 tetrahedra and CrO6 octahedra are compressed isotropically with pressure with their Fe-O and Cr-O bond distances decreasing from 1.996 (6) to 1.949 (7) angstrom and from 1.997 (3) to 1.969 (7) angstrom, respectively. The tetrahedral site occupied by the Fe2+ cation is more compressible than the octahedral site occupied by the Cr3+ cation. The resulting EoS parameters for the tetrahedral and the octahedral sites are K-0 = 147 (9) GPa, K' = 4.0 (fixed), V-0 = 4.07 (1) angstrom(3) and K-0 = 275 (24) GPa, K' = 4.0 (fixed), V-0 = 10.42 (2) angstrom(3), respectively. A discontinuous volume change is observed between 11.8 and 12.6 GPa. This change indicates a phase transition from a cubic (space group Fd-(3) over barm) to a tetragonal structure (space group I4(1)/amd). At the phase transition boundary, the two Cr-O bonds parallel to the c-axis shorten from 1.969 (7) to 1.922 (17) angstrom and the other four Cr-O bonds parallel to the ab plane elongate from 1.969 (7) to 1.987 (9) angstrom. This anisotropic deformation of the octahedra leads to tetragonal compression of the unit cell along the c-axis. The angular distortion in the octahedron decreases continuously up to 13.7 GPa, whereas the distortion in the tetrahedron rises dramatically after the phase transition. At the pressure of the phase transition, the tetrahedral bond angles along the c-axis direction of the unit cell begin decreasing from 109.5 degrees to 106.6 (7)degrees, which generates a "stretched" tetrahedral geometry. It is proposed that the Jahn-Teller effect at the tetrahedrally coordinated Fe2+ cation becomes active with compression and gives rise to the tetrahedral angular distortion, which in turn induces the cubic-to-tetragonal transition. A qualitative molecular orbital model is proposed to explain the origin and nature of the Jahn-Teller effect observed in this structure and its role in the pressure-induced phase transition.

Solubility and solution mechanisms in silicate melts of oxidized and reduced C-bearing species in the C-O-H system have been determined experimentally at 1.5 GPa and 1400 degrees C with mass spectrometric, NMR, and Raman spectroscopic methods. The hydrogen fugacity, f(H2), was controlled in the range between that of the iron-wustite-H(2)O (IW) and the magnetite-hematite-H(2)O (MH) buffers. The melt polymerization varied between those typical of tholeiitic and andesitic melts.

In the system Na2O-Al2O3-SiO2-H2O-TiO2, the behavior of Ti-containing structural complexes has been determined in H2O-saturated silicate melts and in coexisting silicate-saturated aqueous fluids as well as in silicate-rich supercritical fluids to 900 degrees C and 2225 MPa. Titanium speciation in aqueous fluids in the system TiO2-H2O was also characterized. All measurements were carried out in situ at the desired temperature and pressure using confocal microRaman and microFTIR spectroscopy. The experiments were carried out in an Ir-gasketed hydrothermal diamond-anvil cell (HDAC) with K-type thermocouples for temperature measurement and the Raman shift of C-13 synthetic diamond to monitor pressure.

Structural characterization of silicate melts and aqueous fluids equilibrated at pressures and temperatures corresponding to the Earth's interior requires measurements in-situ while the samples are at the pressure and temperature of interest. To this end, structure and structure-property relations of melts and coexisting fluids in silicate-COH systems have been determined at temperatures up to 1000 degrees C and at pressures to similar to 2.0 GPa.

Hydrogen isotope fractionation between water-saturated silicate melt and silicate-saturated aqueous fluid has been determined experimentally by using vibrational spectroscopy as the analytical tool. The measurements were conducted in situ with samples at the high temperature and pressure of interest in an externally heated diamond cell in the 450-800 degrees C and 101-1567 MPa temperature and pressure range, respectively. The starting materials were glass of Na-silicate with Na/Si = 0.5 (NS4), an aluminosilicate composition with 10 mol% Al2O3 and Na/(Al+Si) = 0.5 (NA10), and a 50:50 (by volume) H2O:D2O fluid mixture. Platinum metal was used to enhance equilibration rate. Isotopic equilibrium was ascertained by using variable experimental duration at given temperature and pressure.

The C-O-H-N solubility and solution mechanisms in silicate melts and C-O-H-N speciation in coexisting fluid to upper mantle temperatures and pressures and with redox conditions from the MH to the IW buffer are discussed. Focus is on in-situ structural characterization of coexisting melt and fluid. In fluid + melt-CON, fluid + melt-NOH, and fluid + melt-OH systems, volatiles are dissolved in molecular form (CO2, CH4, NH3, N-2, H2O, H-2) and as complexes that form chemical bonding with the silicate network (CO3, CH3, NH2, OH). In silicate-OH systems molecular H2O (H2O) and OH-groups exist in silicate- and aluminosilicate-saturated fluids and coexisting water-saturated melts above similar to 400 degrees C and similar to 0.5 GPa with their OH/H2O-ratio positively correlated with temperature. The extent of hydrogen bonding in both fluids and melts diminishes with temperature so that above similar to 400 degrees C it cannot be detected. The Delta H of hydrogen bonding in aqueous fluid (22 +/- 1 kJ/mol) is about twice that in silicate melts (10 2 kJ/mol). Silicate speciation in silicate-saturated fluid and hydrous silicate melts comprises similar Q-species with Delta H of the solution reactions in silicate-saturated fluid, water-saturated melt, and supercritical fluid similar to 400 kJ/mol. In COH-silicate systems methane solubility in melt increases from 0.2 wt% to similar to 0.5 wt.% in the melt NBO/Si range from 0.4 to 1.0 at 1-2.5 GPa and 1400 C. The solubility increases by similar to 150% between the redox conditions of the IW and MH buffers. At the NNO buffer conditions and more oxidizing, carbon exists as carbonate complexes in melts and as CO2 in fluid. Reduced (C + H)-bearing species in melts (CH3-groups and molecular CH4) are stable at f(H2)(MW) and more reducing conditions, whereas the species in coexisting fluid are CH4, H-2, and H2O. In NOH-silicate systems, the N solubility in melt decreases from 0.98 to 0.28 wt.% in the melt NBO/Si-range from 0.4 to 1.18 at the redox conditions of the IW buffer. The solubility decreases by about 50% between the redox conditions of the IW and MH buffers. At IW, nitrogen occurs in silicate melts amine groups, NH2, bonded to the silicate network, and as molecular NH3, whereas in coexisting NOH fluids the dominant species are NH3, N-2, H-2 and H2O. The NH2-/NH3 abundance ratio varies by similar to 55 between melt compositions with NBO/Si = 1.18 and 0.4. In fluids and melts, decreasing hydrogen fugacity leads to oxidation of nitrogen to form molecular N-2 so that at the MH redox conditions, the dominant N-bearing species is N-2. The redox-dependent solution mechanisms of COHN volatile components in silicate melts affect their structure differently, which results in redox-dependent thermodynamic and transport properties of magmatic liquids in the interior of the Earth and terrestrial planets. These properties include mineral/melt minor and trace element partitioning, melt/fluid isotope fractionation, and transport and thermodynamic properties of melt saturated with variably-oxidized COHN volatile components. 2012 Elsevier B.V. All rights reserved.

Hydrogen isotope partitioning (as H2O and D2O) between silicate-saturated aqueous fluid and water-saturated aluminosilicate melt has been determined with vibrational spectroscopy (Raman and infrared) in situ with the samples at high temperature and pressure by using a hydrothermal diamond-anvil cell (HDAC) for sample containment. To assess the effects of pressure and, therefore, different silicate speciation in fluids and melts, on the D/H partitioning behavior, two pressure/temperature experimental trajectories (450-800 degrees C1155-754 MPa, and 450-800 degrees C/562-1271 MPa) were used. In these temperature and pressure ranges, the fluid/melt partition coefficients are temperature (and pressure) dependent with the average enthalpy change, Delta H = -6.6 +/- 15 kJ/mol and -10.3 +/- 1.1 kJ/mol for H2O and D2O, respectively. The Delta H-values for the lower-pressure trajectory (and, therefore, lower fluid density) were 15-20% higher than for the higher-pressure (and higher fluid density) trajectory. The (D/H) ratios of fluids and melts, (D/H)(fluid) and (D/H)(melt), are also temperature dependent with a small negative All for (D/H)(fluid) (average: -2.4 +/- 0.8 kJ/mol) and a positive Delta H-value for (D/H)(melt) (2.3 +/- 1.4 kJ/mol). The (D,H) exchange equilibrium between fluid and melt is also temperature (and pressure) dependent so that for the low-density P/T trajectory, the Delta H = -4.2 +/- 0.6 kJ/mol, whereas for the higher-density trajectory, Delta H = -5.4 +/- 0.7 kJ/mol. The difference between the H2O and D2O fluid/melt partition coefficients and the temperature- and pressure-dependent D/H fractionation behavior in and between hydrous silicate melts and silicate-saturated aqueous fluid in part is because pressure increases with increasing temperature in the HDAC experiments and the volume difference between fluid and melt differ for H2O and D2O. In addition, the silicate speciation in fluids and melts are temperature and pressure dependent, which also leads to significantly temperature- and pressure-dependent D/H fractionation within and between silicate melts and fluids at high temperature and pressure. In the Earth's deep crust and upper mantle, hydrogen isotope partitioning between condensed phases and aqueous fluid can differ substantially from that between condensed phases and pure H2O because the aqueous fluid in the Earth's interior is a concentrated silicate solution wherein the silicate speciation affects the isotope partitioning.

Raman spectroscopy of glasses and melts is a powerful technique which can provide information about the silicate network connectivity via the study of Q(n)-species distribution and also on other tetrahedrally coordinated cations such as aluminium, phosphorus, ferric iron and titanium. Recently, Raman spectroscopy of glasses and melts has focused on information obtained about the speciation of volatile compounds. After a brief introduction to the definition of the glassy state and on the relationships between glass and melt, the main steps of Raman spectral analysis are described, discussing the main approaches and the extent to which this method is quantitative. Finally, examples of the application of Raman spectroscopy in the field of Earth sciences are given.

Speciation of D-O-H-C-N volatiles in alkali aluminosilicate melts and of silicate in D-O-H-C-N fluid has been determined in situ to 800 degrees C and >2 GPa under reducing and oxidizing conditions by using an externally heated hydrothermal diamond-anvil cell with Raman spectroscopy as the structural probe. Reducing conditions were near those of the IW oxygen buffer, whereas oxidizing conditions were obtained by conducting the experiments with oxidized components only and with Pt as a catalyst. Raman bands assigned to C-H stretching in CHxDy isotopologues and CH4 groups (including CH3) were employed to determine the CH4/CHxDy ratio in fluids and melts. This ratio decreases from 1.5-2 at 500 degrees C to between 1.2 and 1 with 800 degrees C with Delta H-values of 13.6 +/- 2.1 and 5.5 +/- 1.1 kJ/mol for melt and fluid, respectively. The CHx/CHxDy fluid/melt partition coefficient ranges between similar to 16 and similar to 3 with Delta H= 33 +/- 6 kJ/mol assuming no pressure effect. This behavior of deuterated and protonated complexes is ascribed to speciation of volatile and silicate components in fluids and melts in a manner that is conceptually similar to D/H partitioning among complexes and phases in brines and hydrous silicate systems.

Nitrogen is the dominant gas in Earth atmosphere, but its behavior during the Earth' differentiation is poorly known. To aid in identifying the main reservoirs of nitrogen in the Earth, nitrogen solubility was determined experimentally in a mixture of molten CI-chondrite model composition and (Fe, Ni) metal alloy liquid. Experiments were performed in a laser-heated diamond-anvil cell at pressures to 18 GPa and temperatures to 2850 +/- 200 K. Multi-anvil experiments were also performed at 5 and 10 GPa and 2390 +/- 50 K. The nitrogen content increases with pressure in both metal and silicate reservoirs. It also increases with the iron content of the (Fe, Ni) alloy. Sieverts' formalism successfully describes the nitrogen solubility in metals up to 18 GPa. Henry's law applies to nitrogen-saturated silicate melts up to 4-5 GPa. Independently of these solubility models, it is shown that the partition coefficient of nitrogen between metal and silicate melts changes from almost 10(4) at ambient pressure to about 10-20 for pressures higher than 1 GPa. The pressure-dependence of the nitrogen partitioning can explain the recently suggested depletion of nitrogen relative to other volatiles in the bulk silicate Earth. (c) 2013 Elsevier Ltd. All rights reserved.