A semi-empirical molecular orbital study was performed on the atomic charges in β-Mg₂SiO₄ to clarify the crystal chemical properties of this mantle-constituting material. A large cluster of atoms was cut out from the crystal structure for each of the independent atoms, with the atom put in the center of the cluster. Hydrogen atoms were attached to the outer oxygens to preserve the charge neutrality in the cluster. The atomic charge on the central atom was determined by the CNDO/2 method, using the minimal basis set of Slater-type orbitals for all the atoms. The SCF calculation for the Si cluster failed in convergence. The charges on the three Mg and the four O atoms were successfully determined and compared with the electrostatic site energies which were obtained from the formal point charges of these atoms.
As a result, three Mg atoms (Mg1, Mg2, and Mg3), which have nearly the same electrostatic site energies, converged to almost the same values of atomic charges of 0.68, 0.68 and 0.69e, respectively. The O1 oxygen, which is “undersaturated” in the sense of Pauling’s valence balance rule and has a very high electrostatic site energy, produced a small charge of −0.56e. On the contrary, the “oversaturated” O2 oxygen, which has a very low electrostatic site energy, exhibited a large charge of −0.66e. The O3 and O4 oxygens, which have intermediate electrostatic site energies, yielded intermediate values of −0.59 and −0.61e, respectively. These results indicate that the atomic charge depends qualitatively on the electrostatic site energy in order to gain a larger electrostatic stability and to compensate the disadvantage in breaking Pauling’s rule. The relationship found between the atomic charge and the electrostatic site energy will open the way to modify the charge values in interatomic pairwise potentials widely used in silicate minerals.

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The bottom sediments sampled from Lake Biwa, in the southwest of Japan, contain sand-size pellet grains which consist of diatom bodies, detrital quartz and small feldspar fragments with cementing material enclosing them. Vivianite grows in the fecal pellets. Vivianite content is loosely related to water depth as a whole, and there is a large fluctuation in it. The authigenic vivianite has a much higher manganese content than any other published vivianite composition, and Mn/(Mn+Fe) ratio in the vivianite reaches nearly 0.5 at its maximum. Manganese content in the vivianite increases with the water depth of sample localities.

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A new system of a cathodoluminescence microscope with high sensitivity was constructed by combining a scanning electron microscope and a digital image processor. Growth zonings in needle crystals of coated diamonds, igneous quartz and relict olivine in a chondrite were first observed by using the microscope at high incident beam current. The fact that the growth zoning due to the faint chemical variations were detected indicates the validity of the present system in the study of growth history such as morphological change of minerals.

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