PRESSURE-INDUCED STRUCTURAL MODIFICATIONS IN MG2GEO4-OLIVINE - A RAMAN-SPECTROSCOPIC STUDY

Raman spectra of Mg2GeO4-olivine were obtained from ambient pressure u p to 34 GPa at ambient temperature. Under quasi-hydrostatic pressure c onditions, the following modifications in the Raman spectra occur as p ressure increases: 1) near 11 GPa, two sharp extra bands appear in the 600-700 cm-1 frequency range, and increase in intensity with respect to the olivine bands; 2) above 22 GPa, these two bands become very int ense, and the number, position and relative intensity of the other vib rational bands drastically change; 3) the intensity of sharp bands pro gressively decreases above 25 GPa. The transformation occurs at lower pressures under non-hydrostatic conditions. During decompression to at mospheric pressure, the high-pressure phase partially reverts to olivi ne. These observations can be interpreted as the progressive metastabl e transformation from the olivine structure to a crystalline phase wit h four-fold coordinated Ge, in which the GeO4 tetrahedra are polymeriz ed. We propose that the metastable high-pressure phase is a structural ly disordered spinelloid close to the hypothethical omega- or epsilon -phase, and forms by a shear mechanism assisted by the development of a dynamical instability in the olivine structure. Implications for the transformations undergone by olivines under far-from-equilibrium cond itions (e.g. in subducting lithospheric slabs and in shocks) are discu ssed.