Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO₃ orthopyroxene

Using the laser-heated diamond anvil cell, we investigate the stability and equation of state of the postperovskite (ppv, CalrO₃-type) phase synthesized from a natural pyroxene composition with 9 mol.% FeSiO₃. Our measured pressure-volume data from 12-106 GPa for the ppv phase yield a bulk modulus of 219(5) GPa and a zero-pressure volume of 164.9(6) ų when K′₀ = 4. The bulk modulus of ppv is 575(15) GPa at a pressure of 100 GPa. The transition pressure is lowered by the presence of Fe. Our x-ray diffraction data indicate the ppv phase can be formed at P > 109(4) GPa and 2,400(400) K, corresponding to ≈400-550 km above the core-mantle boundary. Direct comparison of volumes of coexisting perovskite and CalrO₃-type phases at 80-106 GPa demonstrates that the ppv phase has a smaller volume than perovskite by 1.1(2)%. Using measured volumes together with the bulk modulus calculated from equation of state fits, we find that the bulk sound velocity decreases by 2.3(2.1)% across this transition at 120 GPa. Upon decompression without further heating, it was found that the ppv phase could still be observed at pressures as low at 12 GPa, and evidence for at least partial persistence to ambient conditions is also reported.