Abstract
Semi-emprical molecular orbital calculations (CNDO/2) have been performed for two large clusters with compositions of (Mg(1)3Mg(2)3O22H32)0+ and (Mg(1)3 Mg(2)SiO18H24)0+ to evaluate the atomic charges on the Mg ions in enstatite. Hydrogen (H) atoms are attached to the outer oxygens in the clusters to keep charge neutrality. The two clusters are constructed to include all the polyhedra which share edges with the central Mg(1)O6 and Mg(2)O6 octahedra, respectively. The convergence in the energy minimization has been successfully achieved for each cluster. The estimated Mg charges are 0.73–0.75 and 0.69e for the Mg(1) and Mg(2) atoms, respectively. The charge on Mg(1) exhibits a value larger than that on Mg(2), which indicates that the M(1) site is more ionic than the M(2) site in orthopyroxene. This result is consistent with the previous investigations based on experimental charge analysis by accurate single-crystal X-ray diffraction, and on electrostatic site energy calculations.
