Olivines Geometric structures

No ab initio SCF calculations have yet been performed on the geometries of distorted Si04 environments in olivines. Ab initio SCF calculations on Si04 with the olivine geometry (Tossell, unpublished results) do not give larger bond overlap populations for the shorter Si-O distances so this result must be considered an artifact of the non-self-con-sistent EHMO calculations. 

There have been a number of atomistic simulations, most of which have focused upon unit-cell parameters and fractional coordinates in olivines or upon elastic properties. Many such simulations are reviewed by 

Price and Parker (1984), who found that simulations with full ionic charges could satisfactorily reproduce either the structure or the elastic properties of olivines but not both. They presented a simulation with fractional charges and a semiempirical Morse potential for the Si-0 bond and found that both structural and elastic properties could be adequately modeled (see Chapter 7 for further discussion). Their calculated Si-O distances, shown in Table 5.2, are in quite good agreement with experiment. The atomic charges used in this simulation are also in reasonable agreement with the effective charges inferred from x-ray diffraction studies (Fujino et al., 1981). 

It is apparent from this description that both ionic and covalent forces are probably important in determining the detailed geometries of olivines. The balance of forces determining the structures seems complex, and it is not clear whether the observed distortions can be attributed to any single qualitative factor. 

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