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Tetrahedral compression in (Mg,Fe)SiO3 orthopyroxenes |
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| Authors: | Demelza Hugh-Jones Ann Chopelas Ross Augel |
| Institution: | Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK, GB Max-Planck-Institut für Chemie, Saarstra?e 23, D-55122 Mainz, Germany, DE Bayerisches Geoinstitut, Universit?t Bayreuth, D-95440 Bayreuth, Germany, DE |
| Abstract: | Single-crystal structure determinations at pressure have shown that the structural response of synthetic (Mg0.6Fe0.4)SiO3 orthopyroxene to compression is the same as that previously observed in MgSiO3 orthoenstatite. At pressure below ～4?GPa there is no significant compression of the SiO4 tetrahedra, while above ～4?GPa the tetrahedra decrease in volume as a result of Si?O bond shortening. A study of the compressional behaviour of synthetic FeSiO3 orthoferrosilite also shows the same behaviour below 4?GPa, but studies at higher pressures are precluded due to the transformation of the sample to the higher density C2/c high-clinoferrosilite polymorph. A further single-crystal study to 6?GPa of a Ca2+-containing natural orthopyroxene shows that chemical substitution of, primarily, Al3+ and Ca2+ into the structure of orthopyroxene inhibits the initial rapid compression of the M2?O3 bonds observed in the synthetic samples, and no significant tetrahedral compression is observed in this sample. Raman data collected from synthetic MgSiO3 orthoenstatite show that there is a change in the rate of change of frequency with pressure, δν/δP, between 3.5 and 6.0?GPa, but no changes in the number of observed bands. These observations indicate a non-symmetry-breaking change in the properties of the orthoenstatite, which is associated with the change in compression mechanism observed using X-ray diffraction techniques at this pressure. |
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