Single-crystal compression and crystal structure of clinopyroxene up to 10 GPa

The hydrostatic compression of synthetic single crystals of diopside, CaMgSi2O6, and hedenbergite, CaFeSi2O6, was studied at 33 pressures up to 10 GPa by X-ray diffraction. In addition, intensity data for hedenbergite were collected at 12 pressures up to 10 GPa. For determination of the elasticity two crystals were loaded together in a diamond cell. The axial compressibilities βa, βb, and βc of diopside and hedenbergite are 2.36(4), 3.17(4), and 2.50(4)×10-3 GPa-1, and 1.93(5), 3.38(6), and 2.42(8)×10-3 GPa-1, respectively. The bulk moduli (Kτo) and their pressure derivatives (K′τo) were determined simultaneously from a weighted linear fit of a third order Birch-Murnaghan equation of state to the volume data at elevated pressures. Kτo and K′τo are 104.1(9) GPa and 6.2(3) for diopside and 117(1) GPa and 4.3(4) for hedenbergite, respectively. The unit-cell parameters decrease continuously with pressure. The larger polyhedra show more compression than the smaller ones. Between 0.1 MPa and 10 GPa the polyhedral volumes of CaO8, FeO6, and SiO4 decrease by 8.4, 6.6, and 2.9%, respectively. The longest bonds of CaO8 and FeO6 show most compression. Significant compression in the two shortest Si-O1 and Si-O2 bond lengths of the SiO4 tetrahedra was observed at relatively low pressures, resulting in a tetrahedral volume compression of 1.6% between 0.1 GPa and 4 GPa and 1.3% between 4 and 10 GPa. The compression of the unit cell can be described by the volume compression of the individual CaO8 and FeO6 polyhedra, with the SiO4 tetrahedron playing a minor role. Diopside is more compressible than hedenbergite as shown by their axial and volume compressibilities because the FeO6 octahedron is significantly more rigid than MgO6 at high pressures. This observation implies that octahedrally coordinated Fe2+ behaves differently from Mg at high pressures, in contrast to their behavior at ambient conditions.