HR: 0800h
AN: MR11A-0891    [Abstracts]
TI: Volume of {\it C}2/{\it c} Pyroxenes at Mantle P, T, and {\it x}
AU: * Thompson, R M
EM: thompson@geo.arizona.edu
AF: Department of Geosciences, University of Arizona, Tucson, AZ 85721-0077
AU: Downs, R T
EM: downs@geo.arizona.edu
AF: Department of Geosciences, University of Arizona, Tucson, AZ 85721-0077
AB: The variations in unit cell volumes of mantle minerals as functions of P and T are important parameters in the description of the interior of the Earth and the behavior of materials. Recently, Thompson and Downs (2004) presented a model for the crystal structures of pyroxenes parameterized in terms of the O3-O3-O3 angle and the oxygen radius. This model has proven useful in the analysis of compression and expansion mechanisms in pyroxenes, providing an understanding of the O3-O3-O3 angle and the oxygen radius as functions of P and T. However, it did not provide a basis for analyzing changes in some properties that are highly dependent on composition. In this paper, we show that ambient cell volumes of the {\it C}2/{\it c} pyroxenes are strongly correlated with M1 cation radius. This relationship can be exploited to estimate ambient condition cell volumes. From this starting point, pyroxene cell volume variation with P and T can be modeled as a function of O3-O3-O3 angle (itself a function of P and T), and the oxygen radius, r(P,T). This relationship is investigated for diopside, hedenbergite, acmite, jadeite, and kosmochlor. The model reproduces observed cell volumes of these phases recorded at P to within 0.09% and at T to within 0.10%, at simultaneous P and T for jadeite to within 0.57%, and at simultaneous P and T for diopside to within 1.20%. K$_{o}$ and K' from third order Birch-Murnaghan fits to the observed volume versus pressure relationships and the volume versus pressure relationships calculated from the Thompson-Downs model are the same within error. The fit of the Thompson-Downs EOS to the observed data is compared to the fit of the third order Birch-Murnaghan. The model is used to estimate volume as a function of P and T for diopside, hedenbergite, acmite, jadeite, and kosmochlor. Thompson, R.M., and Downs, R.T. (2004) Model pyroxenes II: Structural variation as a function of tetrahedral rotation. American Mineralogist 89, 614-628.
DE: 3620 Crystal chemistry
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting