Several Problems in Crystalchemistry of Minerals

Recent developments in crystal chemistry of minerals are reviewed and several problems are suggested to be investigated as extensions of the presented accomplishments. Electron-density distributions in inorganic crystals have been extensively investigated for these twenty years, and valuable information has been obtained on electronic states of crystals with simple structures. It is supposed to be most promising for more complex crystals to collect diffraction data with extremely small single-crystals by making use of intense X-rays of SR. Single-crystal X-ray diffraction data afford not only electron-density distributions, but also anharmonic thermal parameters for atoms in the crystals. The knowledge on anharmonic thermal vibrations obtained at various temperatures and pressures might be useful to check the varidity of the interatomic potentials assumed in molecular dynamics calculations for simulating the states of materials in the Earth's interior. Consideration of anharmonic thermal vibrations is also important to get correct positional parameters of atoms. In the case of LiTaO3, the z parameter of Ta shifted by 0.066 A relative to Li and O on introduction of anharmonic thermal parameters into the parameters to be refined in the least-squares calculations. The author and his collaborators succeeded to grow a low temperature form crystal of BaB2O4 at a temperature above the phase-transition point from a supercooled melt of pure BaB2O4. A similar crystal growth of a low temperature form is expected for rock-forming silicate minerals, since silicate melts are easily supercooled.