COMPARATIVE CRYSTAL CHEMISTRY OF SILICATES BEYOND 20 GPA. Li. Zhang, H. Ahsbahs, A. Kutoglu, S. S. Hafner, Center of Materials Sciences, University of Marburg, Germany

The crystal chemistry of Mg²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ca²⁺, Fe³⁺ and Al³⁺ at high pressure in relatively incompressible silicates is not well studied. Present knowledge is largely based on limited experimental results at relatively low pressure. As a result, a systematic description of the high pressure crystal chemistry for the above cations is not available. Mg²⁺, Fe²⁺, Ca²⁺, and Al³⁺ cations are major constituents in minerals of the Earth's deep mantle. For understanding especially the chemical processes and the physical properties in that region it is essential to obtain information about the high pressure behavior of the above cations in the mantle minerals.

The high pressure crystal chemistry of those cations was studied by considering their electronic structures, polyhedral coordination as well as the crystal structure. Experiments were performed on synthetic endmember garnets, Mg₃Al₂(SiO₄)₃ Fe₃Al₂(SiO₄)₃ Mn₃Al₂(SiO₄)₃ Ca₃Fe₂(SiO₄)₃ Ca₃Al₂(SiO₄)₃, olivines, Mg₂SiO₄, Fe₂SiO₄, Mn₂SiO₄, CO₂SiO₄ and pyroxenes, CaMgSi₂O₆, CaFeSi₂O₆. The crystals were loaded in diamond anvil cell with rare gas solids, argon, neon and helium as well as mixtures of ethanol-methanol serving as pressure transmitting media. The unit-cell parameters determined up to 212 GPa show that polyhedral coordination of individual cation controls primarily their crystal chemical behavior. The 6-fold coordinated polyhedra with cations of similar electronic structures like Co²⁺, Fe²⁺, Mn²⁺ do possess compression behavior which obeys prediction of bulk modulus - volume systematics, whereas those with cations of different electronic structure like e.g. Fe²⁺, Mg²⁺ do not follow such predictions. This is evidenced in olivine and pyroxene. The high pressure crystal chemistry of these cations in garnet, olivine and pyroxene as well as its implication to the bulk modulus - volume systematics will be discussed.