An interatomic potential-function model for Mg, Ca and CaMg olivines

The structures of Mg(forsterite), Ca and CaMg(monticellite) olivines have been investigated by lattice energy calculations, where the silicate ions are treated as rigid bodies. The lattice energy was approximated to be the sum of electrostatic and repulsive terms. The net charges on the Si and O atoms in the silicate ion and the repulsive radii of the atoms concerned were derived using the two known crystal structures, Mg and Ca olvines, by a least-squares fit. The results are as follows: q_O(|e|) = -1.569, q_Si = +2.276, R_O (Å) = 1.998, R_Mg = 1.164, and R_Ca = 1.581. The repulsive interactions of the Si atoms were neglected, since the Si atom would be screened by the outermost four O atoms of the silicate ion. The reliability and the transferability of the potential model presented were tested by the calculation of the energy-minimized structure for CaMg olivine, with the result that the observed structure was very well reproduced.