2000 Volume 22 Issue 1 Pages 11-20
Synthetic single crystal forsterites have been shocked up to 82GPa and examined by profile analysis of Raman spectra. Raman bands due to Si–O external vibrations of translational and rotational modes are shifted to higher frequencies with the increase of shock pressure without band broadening. At the internal vibration of Si–O stretching modes, the bands show no shift up to 46GPa, while above that pressure they are shifted to the higher frequencies with distinguishable band broadening. Although Mg(M2)–O translational band is not shifted, its band broadening increases linearly with increasing shock pressure. Over the shock pressure of 46GPa, the Mg–O band is shifted gradually to the lower frequencies as opposed to that all of the internal bands of υ 1 to υ 4 have tendency to move to higher frequencies, whereas the Mg–O band width is doubled. These variations indicate that the lattice deformation resides heterogeneously in the crystal structure of the shocked forsterite, in which the SiO4 tetrahedron is compressed differently between the pressure ranges below and above the 46GPa in the plastic region. In the lower shock region, plastic deformation is caused mainly by the translational and rotational strain, while over the shock pressure of 46GPa, abrupt plastic changes occur in both tetrahedral and octahedral polyhedra, causing positive and negative residual stresses in the structure, respectively. The transition pressure of 46GPa coincides nearly with the onset pressure of phase transition, 50GPa, on the pressure-density Hugoniot of shocked forsterite (Syono et al., 1981).