The hydrostatic compression of synthetic single crystals of diopside, CaMgSi 2 O 6 , and hedenbergite, CaFeSi 2 O 6 , 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 beta a , beta b , and beta c of diopside and hedenbergite are 2.36(4), 3.17(4), and 2.50(4)X10 (super -3) GPa (super -1) , and 1.93(5), 3.38(6), and 2.42(8)X10 (super -3) GPa (super -1) , respectively. The bulk moduli (K (sub tau o) ) and their pressure derivatives (K' (sub tau o) ) were determined simultaneously from a weighted linear fit of a third order Birch-Murnaghan equation of state to the volume data at elevated pressures. K (sub tau o) and K' (sub tau o) 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 CaO 8 , FeO 6 , and SiO 4 decrease by 8.4, 6.6, and 2.9%, respectively. The longest bonds of CaO 8 and FeO 6 show most compression. Significant compression in the two shortest Si-O1 and Si-O2 bond lengths of the SiO 4 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 CaO 8 and FeO 6 polyhedra, with the SiO 4 tetrahedron playing a minor role. Diopside is more compressible than hedenbergite as shown by their axial and volume compressibilities because the FeO 6 octahedron is significantly more rigid than MgO 6 at high pressures. This observation implies that octahedrally coordinated Fe (super 2+) behaves differently from Mg at high pressures, in contrast to their behavior at ambient conditions.

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