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Droplets of an iron-bearing calcia magnesia aluminosilicate (Fe-CMAS) melt were reacted under distinctly reducing conditions ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] - 2.4 x [10.sup.-13] and 6.4 x [10.sup.-15] atm) at high temperature (~1400[degrees]C) and ambient pressure. The low [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] environment was maintained by a flowing gas mixture of CO and C[O.sub.2], with a high content of CO. Molten metallic iron alloyed with silicon and carbon formed on the surface of the melt; no metal was observed in sample interiors. A color change from brown to pale blue confined to the outer layer of the melt indicated that essentially complete reduction of [Fe.sup.3+] to [Fe.sup.2+] had occurred in this region. Analysis of the reaction kinetics, particularly in comparison to melts of similar polymerization but free of CaO, reveals that the concentration of electron holes has decreased to such an extent that ionic transport in the melt is significantly slowed and the diffusion of CO as a neutral species becomes dominant and rate-limiting: molecular CO, initially incorporated into the melt as a physically dissolved species, subsequently reacts to form chemically dissolved (bonded into the melt structure) C[O.sup.2-.sub.3] anions, consuming electron holes in the process. The chemical diffusion coefficient for CO in the reduced melt at 1400[degrees]C is estimated as [D.sub.CO] [approximately equal to] 4 x [10.sup.-4] [cm.sup.2]/s, consistent with that of other, similarly sized molecular species (e.g., [H.sub.2] and [H.sub.2]O) for similarly polymerized melts, as reported by other investigators. Upon quenching, the droplet acts as a closed system. Internal redox couples see the reduction of the carbonate so as to form bubbles of CO, the composition of which are confirmed with Raman spectroscopy. The open-system reduction and closed-system quenching dynamics are analyzed following an Ellingham-diagram approach. Keywords: Redox, silicate melts, carbon, diffusion, reactions, kinetics DOI: 10.2138/am.2010.3375

We have measured Raman spectra of the postperovskite (PPv) phase in CaIr[O.sub.3] up to 30 GPa to constrain the Gruneisen parameter ([gamma]). We identified a total of 4 strong modes between 200 and 650 [cm.sup.-1], which is in contrast with the Raman spectra of [Mn.sub.2][O.sub.3] and MgGe[O.sub.3]-PPv where at least nine different modes have been detected. We found no sign of a phase transition in the Raman spectra of PPv CaIr[O.sub.3], which supports the stability of the PPv phase up to 30 GPa and room temperature in CaIr[O.sub.3]. The spectroscopic Gruneisen parameter, [[gamma].sub.sp.0] = 1.66~1.72, constrained from our Raman data, is in excellent agreement with the thermodynamic Gruneisen parameter, [[gamma].sub.th,0] = 1.75 [+ or -] 0.05, calculated from recent XRD measurements (Martin et al. 2007) on CaIr[O.sub.3]-PPv synthesized at high pressure and temperature similar to our starting material. Our result suggests that [[gamma].sub.sp] constrained by Raman measurements provides a reasonable estimate on the [[gamma].sub.th] of the PPv phase in CaIr[O.sub.3]. Keywords: Postperovskite, CaIr[O.sub.3], Gruneisen parameter, Raman sepctroscopy, thermal equation of state

Intriguing seismic observations have been made for the bottom 400 km of Earth's mantle (the D' region) over the past few decades, yet the origin of these seismic structures has not been well understood. Recent theoretical calculations have predicted many unusual changes in physical properties across the postperovskite transition, perovskite (Pv) [right arrow] postperovskite (PPv), that may provide explanations for the seismic observations. Here, we report measurements of the crystal structure of ([Mg.sub.0.91][Fe.0.09])Si[O.sub.3]-PPv under quasi-hydrostatic conditions up to the pressure (P)-temperature (7) conditions expected for the core-mantle boundary (CMB). The measured crystal structure is in excellent agreement with the first-principles calculations. We found that bulk sound speed ([V.sub.[PHI]) decreases by 2.4 [+ or -] 1.4% across the PPv transition. Combined with the predicted shear-wave velocity ([V.sub.s]) increase, our measurements indicate that lateral variations in mineralogy between Pv and PPv may result in the anticorrelation between the [V.sub.[PHI] and Vs anomalies at the D' region. Also, density increases by 1.6 [+ or -] 0.4% and Gruneisen parameter decreases by 21 [+ or -] 15% across the PPv transition, which will dynamically stabilize the PPv lenses observed in recent seismic studies. equation of state | mantle | phase transition | bulk sound speed | Gruneisen parameter

We have measured Raman spectra of the postperovskite (PPv) phase in CaIrO3 up to 30 GPa to constrain the Grüneisen parameter (γ). We identified a total of 4 strong modes between 200 and 650 cm-1, which is in contrast with the Raman spectra of Mn2O3 and MgGeO3-PPv where at least nine different modes have been detected. We found no sign of a phase transition in the Raman spectra of PPv CaIrO3, which supports the stability of the PPv phase up to 30 GPa and room temperature in CaIrO3. The spectroscopic Grüneisen parameter, γsp,0 = 1.66~1.72, constrained from our Raman data, is in excellent agreement with the thermodynamic Grüneisen parameter, γth,0 = 1.75 ± 0.05, calculated from recent XRD measurements (Martin et al. 2007) on CaIrO3-PPv synthesized at high pressure and temperature similar to our starting material. Our result suggests that γsp constrained by Raman measurements provides a reasonable estimate on the γth of the PPv phase in CaIrO3. [ABSTRACT FROM AUTHOR]

Intriguing seismic observations have been made for the bottom 400 km of Earth's mantle (the D" region) over the past few decades, yet the origin of these seismic structures has not been well understood. Recent theoretical calculations have predicted many unusual changes in physical properties across the postperovskite transition, perovskite (Pv) → postperovskite (PPv), that may provide explanations for the seismic observations. Here, we report measurements of the crystal structure of (Mg0.91Fe0.09)SiO3-PPv under quasi-hydrostatic conditions up to the pressure (P)-temperature (T) conditions expected for the core-mantle boundary (CMB). The measured crystal structure is in excellent agreement with the first-principles calculations. We found that bulk sound speed (VΦ) decreases by 2.4 ± 1.4% across the PPv transition. Combined with the predicted shear-wave velocity (Vs) increase, our measurements indicate that lateral variations in mineralogy between Pv and PPv may result in the anticorrelation between the VΦ and Vs anomalies at the D″ region. Also, density increases by 1.6 ± 0.4% and Grüneisen parameter decreases by 21 ± 15% across the PPv transition, which will dynamically stabilize the PPv lenses observed in recent seismic studies. [ABSTRACT FROM AUTHOR]

Droplets of an iron-bearing calcia magnesia aluminosilicate (Fe-CMAS) melt were reacted under distinctly reducing conditions (fO2 = 2.4 x 10-13 and 6.4 x 10-15 atm) at high temperature (~1400 °C) and ambient pressure. The low fO2 environment was maintained by a flowing gas mixture of CO and CO2, with a high content of CO. Molten metallic iron alloyed with silicon and carbon formed on the surface of the melt; no metal was observed in sample interiors. A color change from brown to pale blue confined to the outer layer of the melt indicated that essentially complete reduction of Fe3+ to Fe2+ had occurred in this region. Analysis of the reaction kinetics, particularly in comparison to melts of similar polymerization but free of CaO, reveals that the concentration of electron holes has decreased to such an extent that ionic transport in the melt is significantly slowed and the diffusion of CO as a neutral species becomes dominant and rate-limiting: molecular CO, initially incorporated into the melt as a physically dissolved species, subsequently reacts to form chemically dissolved (bonded into the melt structure) CO32- anions, consuming electron holes in the process. The chemical diffusion coefficient for CO in the reduced melt at 1400 °C is estimated as DCO ≈ 4 x 10-4 cm²/s, consistent with that of other, similarly sized molecular species (e.g., H2 and H2O) for similarly polymerized melts, as reported by other investigators. Upon quenching, the droplet acts as a closed system. Internal redox couples see the reduction of the carbonate so as to form bubbles of CO, the composition of which are confirmed with Raman spectroscopy. The open-system reduction and closed-system quenching dynamics are analyzed following an Ellingham-diagram approach. [ABSTRACT FROM AUTHOR]