High-pressure Raman spectroscopic study of Mn₂GeO₄, Ca₂GeO₄, Ca₂SiO₄, and CaMgGeO₄ olivines

Abstract

 We present a Raman spectroscopic study of the structural modifications of several olivines at high pressures and ambient temperature. At high pressures, the following modifications in the Raman spectra are observed:

1) in Mn₂GeO₄, between 6.7 and 8.6 GPa the appearance of weak bands at 560 and 860 cm⁻¹; between 10.6 and 23 GPa, the progressive replacement of the olivine spectrum by the spectrum of a crystalline high pressure phase; upon decompression, the inverse sequence of transformations is observed with some hysteresis in the transformation pressures; this sequence may be interpreted as the progressive transformation of the olivine to a spinelloid where Ge tetrahedra are polymerized, and then to a partially inverse spinel;

2) in Ca₂SiO₄, the olivine transforms to larnite between 1.9 and 2.1 GPa; larnite is observed up to the maximum pressure of 24 GPa and it can partially back-transform to olivine during decompression;

3) in Ca₂GeO₄, the olivine transforms to a new structure between 6.8 and 8 GPa; the vibrational frequencies of the new phase suggest that the phase transition involves an increase of the Ca coordination number and that Ge tetrahedra are isolated; this high pressure phase is observed up to the maximum pressure of 11 GPa; during decompression, it transforms to a disordered phase below 5 GPa;

4) in CaMgGeO₄, no significant modification of the olivine spectrum is observed up to 15 GPa; between 16 and 26 GPa, broadening of some peaks and the appearance of a weak broad feature at 700–900 cm⁻¹ suggests a progressive amorphization of the structure; near 27 GPa, amorphization is complete and an amorphous phase is quenched down to ambient pressure; this unique behaviour is interpreted as the result of the incompatibilities in the high pressure behaviour of the Ca and Mg sublattices in the olivine structure.