The elasticity of natural hypersthene and the effect of Fe and Al substitution

Abstract

ABSTRACTCompressional behavior of a natural hypersthene has been investigated at pressures up to 10.1 GPa at room temperature using in situ synchrotron X-ray diffraction in a diamond anvil cell. No phase transition was observed within the experimental pressure range. The pressure–volume data have been fitted with a third-order Birch–Murnaghan equation of state, resulting in K0 = 113.6(6.2) GPa and for the bulk modulus and its pressure derivative with a fixed unit-cell volume V0 = 854.0(6) Å3. The unit-cell parameters, a, b, and c, decrease nonlinearly with elevated pressure and show an anisotropic axial compressibility with a ratio βa: βb: βc: =1:2.05:1.3. The comparison with previous results of orthopyroxene reveals that Al substitution causes a decrease in unit-cell volume and an increase in the bulk modulus, whereas the incorporation of Fe has opposite effects. The relation between unit-cell volume and En/Ts contents has been obtained, V0 (Å3) = 873.7(1.1) − 40.8(1.44) × En-90.6(14.7) × Ts, which permits the prediction of unit-cell volume for Fe- and Al-bearing orthopyroxene system.