A diopside composition silicate glass containing 8 wt% Fe2O3 was prepared from melt equilibrated at 1500 degrees C and different redox conditions in the range logf(O2) = -0.7 (air) to logf(O2) = -6. The Fe2+/Fe-T was measured using Mossbauer spectroscopy. The Mossbauer data were used to calibrate Raman-scattering intensity variations of the same samples as a function of oxidation state, providing a simple empirical method to determine the redox ratio of this glass. This new Micro-Raman-based method has been used to quantify redox profiles across partially oxidized samples. No significant Fe2+/Fe-T, gradients were found (values were constant from the surface to the center), although the average oxidation state was observed to increase as a function of time. The former result contrasts with O self-diffusion profiles measured with the ion microprobe on diopside glasses prepared at similar experimental conditions, for which strong isotopic gradients were found at the sample scale (corresponding to a self-diffusion coefficient for O at 1450 degrees C of 1 x 10(-11) m(2)/s). Local oxidation of Fe in the melt therefore appears to occur independently of long-range diffusion of O from the sample surface. A mechanism capable of explaining this observation is proposed based upon the fact that redox gradients result in the generation of electromotive forces. This results in a powerful driving force to wipe out redox gradients through fast electron transfer. However, migration of electrons alone would result in unfavorable charge gradients, in particular at the surface of the sample. At the temperature of our experiments, the local mobility of O is apparently sufficient to compensate the migration of electrons. Despite rapid charge transfer, the bulk oxidation state of our sample is nevertheless limited by the addition of external O. The time dependence of the bulk oxidation state of our samples can be modeled by a constant rate of O diffusion across the interface of 2.1 10(-7) m/s. However, the bulk oxidation state of the liquid is also found to be concordant with variations calculated assuming that diffusion of O is the rate-limiting mechanism. This apparent paradox may be explained if the characteristic time-scales of O self-diffusion in the sample volume and of O incorporation at the sample surface are similar. We suggest that this is indeed the case, given that both of these processes are likely to be limited by the frequency of bond-breaking and bond-forming events in the liquid.

The solubility and solution mechanisms of nitrogen in silicate melts have been examined via nitrogen analyses and vibrational spectroscopy (Raman and FTIR). Pressure (P), temperature (T), hydrogen fugacity (f(H2)), and silicate melt composition (degree of melt polymerization) were independent variables in experiments in the 1-2.5 GPa pressure and 1300-1500 degrees C temperature ranges. The f(H2) was controlled at values defined by the magnetite-hematite (MH), Mn3O4-MnO (MM), NiO-Ni (NNO), magnetite-wustite (MW), and iron-wustite (IW) buffers together with H2O.

The solubility and solution mechanisms of reduced C-O-H volatiles in Na(2)O-SiO(2) melts in equilibrium with a (H(2) + CH(4)) fluid at the hydrogen fugacity defined by the iron-wustite + H(2)O 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 degrees C. The solubility, calculated as CH(4), increases from similar to 0.2 wt% to similar to.5 wt% in the melt NBO/Si-range similar to 0.4 to similar 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 (H(2) + CH(4)) fluid varied by similar to 14%. in the NBO/Si-range of these melts.

The influence of ferrous and ferric iron on the low-temperature heat capacity and vibrational entropy of silicate glasses has been determined by adiabatic calorimetry. Two pairs of samples based on sodium disilicate and calcium Tschermak molecule compositions have been studied. Along with previous data for another Fe-bearing glass, these results have been used to complement the available set of composition independent partial molar relative entropies of oxides in silicate glasses with S-298 - S-0 values of 56.7 and 116 J/mol for FeO and Fe2O3, respectively. The calorimetric data indicate that the fraction of fivefold coordinated Al is significant in the CaO-"FeO"-Al2O3-SiO2 system and that association of Ca2+ and Na+ with Fe3+ in tetrahedral coordination for charge compensation does not entail significant changes in coordination for these two cations. At very low temperatures, however, the heat capacity is no longer an additive function of composition because of unexpectedly high positive deviations from Debye laws. These anomalies are stronger for the reduced than the oxidized glasses and considerably larger than for iron-free glasses, but their origin cannot be established from the present measurements. (C) 2009 Elsevier Ltd. All rights reserved.

The structure of H2O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na2O center dot 4SiO(2)-H2O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H2O at nominal densities, rho(fluid), (from EOS of pure H2O) of 0.90 and 0.78 g/cm(3). With rho(fluid) = 0.78 g/cm(3), water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 degrees C) and pressure (677 MPa), whereas with rho(fluid) = 0.90 g/cm(3), a homogeneous single-phase liquid phase exists through the temperature and pressure range (25-800 degrees C, 0.1-1033 MPa). Less than 5 vol% quartz precipitates near 650 degrees C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na2O center dot 4SiO(2) starting material.

Solubility and solution mechanism(s) of reduced (N+H)- and H-containing N-O-H volatile components in Na2O-SiO2 composition melts in equilibrium with NH3+H-2+N-2 and H2O+H-2 fluid and H- and N-isotope concentrations ill these melts were determined experimentally at 1.5 GPa and 1400 degrees C as a function of hydrogen fugacity,fa, and melt polymerization (composition), NBO/Si (NBO/Si = 0.4-1.18). This NBO/Si-range is similar to that between dacite and olivine tholeiite melt (NBO/Si similar to 0.4-1). The f(H2) was controlled between that of the iron-wftstite + H2O [logf(H2)(IW) similar to 3.42 (bar)] and that of the magnetite-hematite + H2O [logf(H2)(MH) similar to-0.91 (bar)] buffer.