(i) Effect of crystal structure:

In plasticity, correlation between density and strength is not very obvious, rather the crystal structure plays an important role (Frost and Ashby, 1982; Karato, 1989). Notable examples are the B1-B2 (fcc to bcc) transition in NaCl (Meade and Jeanloz, 1988b), the transition of pyroxene to garnet (Karato et al., 1994), and the structural phase transformations in perovskites (Fujino et al., 1993). The low temperature yield strength of a B2 phase (the CsCl structure) is much smaller than that of a B1 phase (the NaCl structure), which can be explained by the classical Peierls theory, in which the yield strength of a crystalline material is related to crystal structure (Takeuchi and Suzuki, 1988). The effect of crystal structure on high temperature creep is more difficult to interpret because of the complex nature of the controlling mechanisms (see e.g., Frost and Ashby, 1982). However, crystal structure appears to play an important role in controlling the resistance to dislocation glide (Karato, 1989).

The strengths of the -phase (a high pressure polymorph of olivine) and garnet remain controversial subjects. Bussod et al. (1993) estimated that -phase is significantly stronger than olivine. In contrast, Young et al. (1992) inferred it to be significantly weaker than olivine. Tingle et al. (1991) measured the high temperature creep strength of a spinel phase (-phase) of MgGeO and found a slightly higher strength than olivine phase. Based on the studies on analogue materials, Karato et al. (1994) emphasized a high strength of garnets in general, whereas Doukhan et al. (1994) emphasize the ductility of high pressure garnets based on TEM (transmission electron microscopy) observations of dislocation structures in naturally deformed specimens. In contrast, based on the TEM observations, Ingrin and Madon (1994) found evidence for higher strength of garnet than co-existing spinel being consistent with Karato et al. (1994).

The only available data on the strength of perovskite are either room temperature measurements of the yield strength of MgSiO perovskite (Meade and Jeanloz, 1990; Karato et al., 1990) and the results on analogue materials. The applicability of the analogue materials approach in plasticity of perovskite has been controversial. Poirier and his group found a large diversity of plastic behavior of perovskites and emphasized the limitation of the use of the analogue materials (Poirier et al., 1989; Beauchesne and Poirier, 1990; Wright et al., 1992). In contrast, Wang et al. (1993) argued that the analogue materials approach will be useful for the hard slip system that presumably controls the strength of an aggregate. Karato and Li (1992) found an importance of diffusion creep in CaTiO perovskite and predicted that the lower mantle will be seismological isotropic if diffusion creep is important in the lower mantle.