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Structure change of MgSiO₃, MgGeO₃, and MgTiO₃ ilmenites under compression

Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan

Correspondence: ^* E-mail: yamanaka{at}hpc.cmc.osaka-u.ac.jp

Compression mechanisms of A²⁺B⁴⁺O₃ ilmenites with compositions MgSiO₃ (stable at high pressures), MgGeO₃ (stable at moderate pressures), and MgTiO₃ (stable at ambient pressure) were investigated at high pressure by single-crystal structure analysis, using both synchrotron radiation and an MoK rotating-anode X-ray generator. The distortions of AO₆ (A:Mg) and BO₆ (B: Si,Ge,Ti) octahedra under pressure were parameterized by bond length, shared-face area, site-volume ratio, and A²⁺-B⁴⁺ interatomic distance across the shared edges and shared face. The AO₆ octahedral volume is much more compressive than the BO₆ octahedral volume. Of the three samples, both the AO₆ and BO₆ octahedra are most rigid in MgSiO₃. The A²⁺-B⁴⁺ interatomic distance becomes more shortened with increasing pressure than do the A²⁺-A²⁺ and B⁴⁺-B⁴⁺ distances. The compression of Mg-Si is more remarkable than that of Mg-Ge and Mg-Ti. The A-B interatomic distance along c is more compressed with increasing pressure than A-A and B-B along a. The short A-B distance across the shared face becomes more shortened than the A-A and B-B distances across the shared edge. The cation position moves in the direction of c with pressure and tends to approach the center of the AO₆ and BO₃ octahedra with increasing pressure. The regularity of the octahedra is enhanced at higher pressure. Both quadratic elongation and bond angle variance verify the reduction of the deformation of AO₆ and BO₆ octahedra with pressure.

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				T. Okada, T. Narita, T. Nagai, and T. Yamanaka Comparative Raman spectroscopic study on ilmenite-type MgSiO3 (akimotoite), MgGeO3, and MgTiO3 (geikielite) at high temperatures and high pressures American Mineralogist, January 1, 2008; 93(1): 39 - 47. [Abstract] [Full Text] [PDF]