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The effect of Ca substitution on the elastic and structural behavior of orthoenstatite

Fabrizio Nestola^*, G. Diego Gatta and Tiziana Boffa Ballaran

Bayerisches Geoinstitut, Universitaet Bayreuth, Universitaetstrasse 30, D-95447, Bayreuth, Germany

A single crystal of orthopyroxene with composition Ca_0.07Mg_1.93Si₂O₆ (space group Pbca) has been investigated at high pressure and room temperature by in-situ X-ray diffraction using a diamond anvil cell. The unit-cell parameters have been determined at ten different pressures up to 10.16(5) GPa. In the pressure range investigated no evidences of phase transitions have been found. The pressure-volume data have been fitted with a third-order Birch-Murnaghan equation of state resulting in the following parameters: V₀ = 838.26(8) ų, K_T0 = 110(1) GPa, K’ = 6.6(4). The Ca substitution in the pure orthoenstatite Mg₂Si₂O₆ structure causes a slight increase in K_T0 and a decrease in K’. The compressibility of a, b, and c unit-cell parameters is strongly anisotropic with a compressibility scheme ß_b >> ß_c >> ß_a. The structure evolution as a function of pressure has been determined at five different pressures up to 6.25(5) GPa. The M2 polyhedron undergoes the largest volume variation (~7.7%), whereas the volume variation of M1 is ~6.1%. The TA and TB tetrahedral volumes decrease by about 3% and 1.2%, respectively, without a discontinuity in the pressure range investigated.

Key Words: Orthopyroxene • high-pressure • compressibility • equation of state

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