The effect of dissolved silica on the PVT properties of H2O and structure of silica-saturated aqueous fluids in equilibrium with quartz in the SiO2-H2O system has been determined in situ with the materials at temperature (up to 800 degrees C) and pressure (up to 1350 MPa) in a constant-volume hydrothennal diamond anvil cell. Pressure was measured with the Raman shift of C-13 synthetic diamond and was also calculated from the PVT properties of pure H2O. The two sets of pressures thus obtained differ by <= 50 MPa at T < 500 degrees C. At higher temperatures (and pressures), the pressure difference increases and reaches about 350 MPa at 800 degrees C.

The structure of phosphorus-bearing, H2O-saturated silicate melts, silicate-saturated aqueous fluids, and silicate-rich single phase (supercritical) liquids has been characterized in situ to 800 degrees C and 1486 MPa in an Ir-gasketed hydrothermal diamond-anvil cell (HDAC) with the aid of both confocal microRaman and FTIR spectroscopy. Temperature and pressure in the HDAC were recorded with thermocouples (+/- 1 degrees C uncertainty) and pressure- and temperature-dependent Raman shift of C-13 diamonds (+/- 40 MPa uncertainty). Starting materials were aluminum-free Na2O center dot 4SiO(2) (NS4) and with 10 mol% Al2O3 (NA10) substituting for SiO2, both with 5 mol% P2O5.

Probing the pressure-induced structural changes and the extent of disorder in aluminosilicate glasses and melts at high pressure remains a challenge in modern physical and chemical sciences. With an aim of establishing a systematic relationship between pressure, composition, and glass structures, we report Al-27 and O-17 3QMAS NMR spectra for sodium aluminosilicate glasses [Na2O:Al(2)O3:SiO2 = 1.5:0.5:2n with n = 1 (NAS150520, X-SiO2 = 0.5), 2 (NAS150540, X-SiO2 = 0.67), and 3 (NAS150560, X-SiO2 = 0.75)] quenched from melts at pressures up to 8 GPa. We also explore the stability of the Al-[4]-O-Al-[4] cluster in the highly depolymerized, NAS150520, glass at high pressure. For given glass composition, the Al-[5,Al-6] peak intensity increases with increasing pressure. The population of Al-[5,Al-6] increases linearly with X-SiO2 from NAS150520 (X-SiO2 = 0.5) to NAS150560 glass (X-SiO2 = 0.75) at both 6 and 8 GPa. The [5,6]Al/X-SiO2 ratio also tends to increase with pressure, indicating a possible relationship between Al-[5,Al-6] fraction and X-SiO2 that depends on pressure. The effect of pressure on the network connectivity in the sodium aluminosilicate glasses is manifested in the increase in Si-[4]-O-Al-[5,Al-6] peak intensity and the decrease in the nonbridging oxygen (NBO) fraction with increasing pressure. The fraction of Si-[4]-O-Al-[5,Al-6] in NAS150520 is smaller than in NAS 150560. Taking into consideration the pressure-induced Al coordination transformation in the fully polymerized glass (albite, Na2O:Al2O3:SiO2= 1:1:6, NBO/T = O), the fraction of Al-[5,Al-6] at a given pressure varies nonlinearly with variations of NBO/T. Al-[5,Al-6] fraction at 8 GPa increases with decreasing degree of melt polymerization from similar to 8% for fully polymerized albite melt (NBO/T = 0) to similar to 37% for partially depolymerized melt (NAS150560, at NBO/T = 0.29). Then it gradually decreases to similar to 15% for NAS150520 with further increase in NBO/T of 0.67. This observed trend in the densification behavior at a given pressure indicates competing densification mechanisms involving steric hindrance vs changes of NBO fraction in the silicate melts. The NMR results also suggest that both NBO and BO, particularly Si-[4]-O-Si-[4], interact with Na+, and thus the Na+ distribution is likely to be homogeneous around both NBO and BO at high pressure without spatial segregation of silica-rich and alkali-rich domains for the glass compositions studied here. The presence of the Al-[4]-(OAl)-Al-[4] cluster is distinct in the NMR spectrum for NAS150520 glass at both 6 and 8 GPa. A new scheme of pressure-induced structural transitions in silicate melts involving Al-[4]-O-Al-[4] includes the formation of Al-[4]-O-Al-[4].

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.