Shock Compression of Single - Crystal Forsterite

Dynamic compression results are reported for single-crystal forsteritc loaded along the orthorhombic a and c axes to pressures from 130 to 165 GPa. Hugoniot states for the two axes are well described by a single curve offset to densities 0.15-0.20 g/cm 3 lower than earlier data for single-crystal forsteritc shocked along the b axis above 100 GPa. Earlier data of Syono et al. [ 1981a] show marginal support for similar b-axis behavior in the mixed-phase region from 50 to 92 GPa. Thus shocked forsteritc is most compressible in the b direction for the mixed-phase and highpressure regimes (P>50 GPa). These data represent the highest pressures for which shock properties have been observed to depend on crystal orientation. Theoretical Hugoniots for mixed-oxide and perovskite-structure high-pressure assemblages of forsteritc calculated from recent experimental data are virtually identical and agree with the b-axis data. The a-and c-axis data are also consistent with both high-pressure assemblages because uncertainties in equation of state parameters produce a broad range of computed Hugoniots. Our 3calculated "average" Hugoniot is up to 0.13 g/cm less dense than the preferred theoretical Hugoniots, in agreement with earlier measurements on dense polycrystalline forsteritc. Interpolation between the single-crystal forsteritc Hugoniots and Hugoniots for fayalite and Fo gives Fo88 Hugoniots bracketing TWin Sisters45dunite data not previously well fit by systematics. Release paths are steep for the a and b axes but c-axis release paths are much shallower. Hugoniot elastic limits measured for the a and b axes are in good agreement with previous data of Syono et al.; however, the present data for the a axis reveal a triple wave structure: two deformational shock waves as well as the elastic shock, a feature not previously found. The second shock, with amplitude about 9 GPa and a shock temperature of about 350 øK, could perhaps be explained by the forsteritc •->•or • phase transformation.