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 heden bergite 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 beta(a), beta(b), and beta(c), of diopsi de and hedenbergite are 2.36(4), 3.17(4), and 2.50(4) x 10(-3) GPa(-1) , and 1.93(5), 3.38(6), and 2.42(8) x 10(-3) GPa(-1), respectively. Th e bulk moduli (K-tau 0) and their pressure derivatives (K-tau 0) were determined simultaneously from a weighted linear fit of a third order Birch-Murnaghan equation of state to the volume data at elevated press ures. K tau(0) and K-tau 0' are 104.1(9) GPa and 6.2(3) for diopside a nd 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 GP a 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-01 and Si- 02 bond lengths of the SiO4 tetrahedra was observed at relatively low pressures, resulting in a tetrahedral volume compression of 1.6% betwe en 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 indiv idual CaO8 and FeO6 polyhedra, with the SiO4 tetrahedron playing a min or role. Diopside is more compressible than hedenbergite as shown by t heir axial and volume compressibilities because the FeO6 octahedron is significantly more rigid than MgO6 at high pressures. This observatio n implies that octahedrally coordinated Fe2+ behaves differently from Mg at high pressures, in contrast to their behavior at ambient conditi ons.