Crystal structure and stabilization mechanism of minerals with modulated structure-melilite and Laihunite.

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l. MeliliteSingle crystals of akermanite solid-solution, (Ca, Sr)_2(Mg, Co, Zn, Fe)Si_2O_7, were grown by the floating zone method using a lamp-image furnace(N. Jb. Miner, Mh., 1989-5, 219-226, 1989). The modulated structure, the incommensurate-normal(I-N)transition and the local structure of them were studied by the electron diffraction method, the lattice image method, the X-ray diffraction method, the heat capacity measurement, the dielectric constants measurement and the EXAFS(PH)ys. Chem. Minerals, 17-5, 467471, 1990 ; ibid, 17-8, 720-725, 1991 ; ibid, submitted, 1992 ; Eur. J. Miner., submitted, 1992). The analysis of the modulated structure was performed by the optical diffraction method. The model explaining well the observed data was obtained by considering the phase relation among each atoms and both the transverse wave causing the satellites of<110>direction and the longitudinal wave causing the satellites of<100>direction.2. LaihuniteLaihunite, a nonstoichiometric olivine-type mineral with superstructure, has been produced by heating powders and single crystals of synthetic Fe-Mn olivines in air at 500, 600 and 700ﾟC. It was shown that the vacancies and trivalent cations of laihunite formed from tephroite component rich olivine are less than those formed from fayalite component rich olivine(N. Jb. Miner. Mh., 1989-6, 245254, 1989). The formation of lanunite has been examined by heating powders and single crystals of syntheic FO-(Mg, Co, Mn, Ca)olivines in air and under- a controlled atmosphere of oxygei)partial pressure at 300-800ﾟC. It was found that the polymorph(2C, 3C_1, 3C_2)of laihunite depends on the composition of the original olivine(N. Jb. Miner. Mh., 1989-8, 345-356, 1989).