PRESSURE-INDUCED STRUCTURAL MODIFICATIONS AND AMORPHIZATION IN OLIVINE COMPOUNDS

(Mg,Fe)2SiO4 olivine, recovered from temperatures lower than 700-degrees-C and pressures exceeding 30 GPa displays some quenchable phase transformations. If the maximum pressure of the experiment is > 70 GPa, the recovered phase is largely amorphous. This phenomenon is interpreted as a metastable pressure-induced amorphization. If the maximum pressure of the experiment is between 30 and 70 GPa, a phase is recovered which displays a partial amorphous character. It actually consists of a quasi ideally close-packed oxygen sublattice and an amorphous cation "sublattice" indicated by diffuse rings in the diffraction patterns. We interpret this phase as the result of a pressure-induced disordering of the cation sublattice in the olivine structure. Similar observations have been made in numerous other compounds with the olivine structure (Ca2GeO4, Mg2GeO4 and Mn2GeO4). In parallel, we present numerical simulations of the forsterite structure and lattice dynamics at high pressure which show that a dynamical instability of the structure appears at ambient temperature and 30 GPa. The very same instability is shown to appear in simulations of the melting of forsterite at ambient pressure. This reinforces the idea that the phenomena observed at high pressure are simply interpreted as pressure-induced metastable "premelting" and "melting" processes.