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High-pressure phase transitions in MgSiO₃ orthoenstatite studied by atomistic computer simulation

GeoForschungsZentrum Potsdam, Department 4, Telegrafenberg, 14473 Potsdam, Germany

View larger version (17K): [in this window] [in a new window]   FIGURE 1. Evolution of the volume per formula unit of OEn during molecular dynamics simulations at constant T = 1000 K. Both compression and decompression curves are shown.

View larger version (27K): [in this window] [in a new window]   FIGURE 2. Evolution of the unit cell parameters a, b, and c of OEn during molecular dynamics simulations at constant T = 1000 K. Both compression and decompression curves are shown.

View larger version (17K): [in this window] [in a new window]   FIGURE 3. Phase stability of different orthorhombic and monoclinic enstatite phases predicted by DFT calculations at T = 0 K represented by enthalpy difference curves using OEn as the reference structure. At a given P, the phase with the lowest enthalpy, H, is the thermodynamically stable phase.

View larger version (24K): [in this window] [in a new window]   FIGURE 4. Volume compression at T = 0 K of orthorhombic and monoclinic enstatite phases predicted by DFT calculations compared to experimental results at 300 K from Angel and Hugh-Jones (1994). The curves of OEn and LCEn are almost identical. All cell volumes from the DFT calculations are scaled by a factor of 1.044 to account for the systematic LDA error.

View larger version (32K): [in this window] [in a new window]   FIGURE 5. Calculated energy dispersive X-ray diffraction patterns of (a) OEn, (b) HP-OEn1, and (c) HP-OEn2 using the DFT optimized positions at P = 15 GPa and T = 0 K. The diffraction angle was chosen to be 5.6° to obtain an energy scale similar to the experimental studies. An energy resolution of 0.5 keV is assumed, which determines the width of the peaks. (d) Experimental data at 16.8 GPa from Kung et al. (2004) are shown for comparison.

View larger version (13K): [in this window] [in a new window]   FIGURE 6. Polyhedral representation of the structural differences between OEn, HP-OEn1, and HP-OEn2. The transition between OEn and HP-OEn2 involves the rotation of one chain from O to S. In HP-OEn1, both chains are S-rotated. The layer of SiO4 chains that is not shown here remains essentially unaltered with O-rotated chains.