抄録
Semi-empirical molecular orbital study (CNDO/2) has been performed for a large cluster of atoms (Mg(1)3Mg(2)2Si2O22H26)0+ to evaluate cation charges of forsterite. Hydrogen (H) atoms were attached to outer oxygens of the cluster to keep local neutrality. The cluster included all the polyhedra which share edges with the Mg(1)O6 and Mg(2)O6 octahedra and the SiO4 tetrahedron under consideration for cation charges. The convergence in the energy minimization was successfully achieved for this cluster. The obtained cation charges were 0.62–0.63, 0.80–0.81 and 1.24–1.34e for Mg(1), Mg(2) and Si atoms, respectively. The charge of Mg(2) showed a larger value than that of Mg(1), which indicated that M(2) site of olivine was more ionic than M(1) site. This result is consistent with the previous works of experimental charge determination by X-ray method and calculation of electrostatic site potentials. The Si atom showed a higher valence value comparing with Mg atoms.
