A single crystal of MgSiO3 orthoenstatite has been studied by single-crystal X-ray diffraction in a diamond-anvil cell up to a pressure of 8.5 GPa. From the unit-cell data, it has been shown that the volume variation with pressure is best described by two independent equations of state, with significantly different values for their room-pressure bulk modulus, K0, and its first derivative, K0: at P < 4 GPa, K0 = 95.8(3.0) GPa, and K0=14.9(2.0); at P > 4 GPa, K0 = 122.8(16.5) GPa, and K0=5.6(2.9).

A series of structural refinements carried out at pressure intervals of ~1 GPa shows that there is a change in compression mechanism at about 4 GPa that would account for this break in the equation of state of the pyroxene. Below 4 GPa the SiO4 tetrahedra are essentially incompressible, with no change in Si-O bond lengths and O-Si-O bond angles, whereas at higher pressures the Si-O bond lengths shorten in a regular way, with no angular distortion of the SiO4 tetrahedra. The linear volume compressibility of both of the tetrahedra between 4 and 8 GPa is 0.0062(1) GPa−1 [corresponding to a bulk modulus of 162(3) GPa]. By contrast, the Mg-O bond lengths decrease steadily over the whole pressure range studied; the compression of each of the MgO6 octahedra may be described by a single third-order Birch-Murnaghan equation of state: for the M1 site, K0 = 53.2(5.4) GPa, and K0=31.9(6.3); for the M2 site, K0 = 63.1(8.5) GPa, and K0=26.7(7.9).

There is a well-defined change in both the degree of kinking of the tetrahedral chains (as measured by changes in the O3-O3-O3 chain extension angle) and the amount of tetrahedral tilt toward the (100) plane at about 4 GPa. At low pressures the B chain, which is more distorted at room pressure, kinks dramatically while keeping the bases of its tetrahedra at a constant orientation of approximately 7° from the (100) plane, whereas above ~4 GPa the kinking ceases, and the tetrahedra begin to tilt steadily toward the (100) plane. The A chain behaves essentially in the reverse way: below 4 GPa the tetrahedra tilt markedly toward the (100) plane with only a little chain kinking occurring, whereas at higher pressures, the tetrahedral tilt virtually stops, and the kinking continues slowly in the opposite direction.

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