HR: 0800h
AN: MR31A-0139    [Abstracts]
TI: Thermodynamics of CaMgSi2O6-KAlSi2O6 clinopyroxene solid solution: Quantum mechanical and static lattice energy calculations
AU: * Vinograd, V L
EM: v.vinograd@kristall.uni-frankfurt.de
AF: University of Frankfurt, Institute of Geosciences, Altenhöferallee 1, Frankfurt a.M., 60438, Germany
AU: Safonov, O G
EM: oleg@iem.ac.ru
AF: Geological faculty, Moscow State University, Vorob'ev y Gory, Moscow, 119192, Russian Federation
AU: Wilson, D J
EM: d.wilson@kristall.uni-frankfurt.de
AF: University of Frankfurt, Institute of Geosciences, Altenhöferallee 1, Frankfurt a.M., 60438, Germany
AU: Gale, J D
EM: julian@ivec.org
AF: Nanochemistry Research Institute, Curtin University of Technology, PO Box U1987, Perth, WA 6845, Australia
AU: Perchuk, L L
EM: llp@geol.msu.ru
AF: Geological faculty, Moscow State University, Vorob'ev y Gory, Moscow, 119192, Russian Federation
AU: Winkler, B
EM: b.winkler@kristall.uni-frankfurt.de
AF: University of Frankfurt, Institute of Geosciences, Altenhöferallee 1, Frankfurt a.M., 60438, Germany
AB: K-enriched clinopyroxenes are often found in mineral assemblages formed at high and ultrahigh pressures, such as inclusions in diamonds, eclogitic and peridotitic xenoliths in kimberlites and lamproites. Thermodynamic properties of the solid solution between diopside and potassium jadeite cannot be studied experimentally due to impossibility to synthesize crystals with more than 25% of KAlSi2O6. Here we show that the missing thermodynamic information can be obtained with the aid of computer simulations. A set of empirical interatomic potentials has been used for calculation of static lattice energies of 800 different structures in a 2x2x4 supercell of C2/c pyroxene with compositions intermediate between diopside (Di) and K-jadeite (KJd), as well as with different ordering states of the exchangeable K/Ca and Mg/Al cations. Excess static energies of these structures were cluster expanded in a basis set of 37 pair-interaction parameters. These parameters were used to constrain Monte Carlo simulations of temperature-dependent properties in the range of 273-2023 K and to calculate a T-X phase diagram for the solid solution. The simulations predicted the formation of stable intermediate compounds at 1/3, 5/12, 1/2, 7/12 and 2/3 of the KJd mole fraction. The compound at 1/2 with a space group P2/b is analogous to omphacite in the diopside - jadeite system. However, the cation distribution in this phase is inverted: K and Ca in K-omphacite occupy positions which would be Ca-rich and Na-rich, respectively, in Na-omphacite. The standard enthalpies and volumes of KJd and K-omphacite were estimated from first principles calculations, while the standard entropies, thermal expansion coefficients and bulk moduli were predicted on the basis of the force field lattice dynamics. The activity-composition relations in the disordered C2/c phase were approximated with respect to Di and KJd end-members with a Redlich-Kister polynomial. Using these results and thermodynamic data available in literature, the isopleths of KJd in CaMgSi2O6-KAlSi2O6 clinopyroxene in assemblages with sanidine + coesite, Si-wadeite + kyanite + coesite, Si-wadeite + kyanite + stishovite and hollandite + stishovite were computed. The predicted isopleths permit an estimation of the pressure or temperature of equilibration of K-clinopyroxenes in assemblages with sanidine and hollandite.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting