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

Abstract

A single crystal of orthopyroxene with composition Ca 0.07 Mg 1.93 Si 2 O 6 (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: V 0 = 838.26(8) A 3 , K T0 = 110(1) GPa, K ’ = 6.6(4). The Ca substitution in the pure orthoenstatite Mg 2 Si 2 O 6 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 β b >> β c >> β 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.