X-ray diffraction and niolecular dynamics studies of the phase transitions in selica clathrote miuerals

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1. The structure of low melanophalohite was firstly determined using specimens from Mt. Hamilton (Ca), and thenrefined with intensity data measuread on a 4-circle diffractometer. The low form, which is a superstructure of the high temperature cubic one, results in the first order phase trasition from the cubic form.2. The structural changes in a wide range of temperature from -50 to 200℃ were studied with X-ray single crystal techniques, showing the trasition point at about 70℃. Si-O distances and Si-O-Si angles, both for the atomic mean positions, show a begative correlation : Si-O decreases significantly in the range of the low form, while Si-O-Si increases.3. Some of oxygen atoms show remarkable anisotropy in atomic mean swuare displacements, suggesting a possible disorder.4. The maximum entropy method was applied to analyze electron density distribution in the high form, indicating that all the cages of pentagon dodecahedron are occupied by CH4, which are possibly in six oreintations.5. Melanophlohite releases its guest molecules at a temperature as high as 1000℃. The guest-free specimen obtained by heating up to 1000℃ belongs to the cubic space group as the same as that of natural specimens, and show no structural transition with varying temperatures. This result denies a reported one based on a NMR study.6. Some trials of MD simulations for a model structure with pentagon dodecahedral cages, fully occupied by CH4 molecules, succeeded to reproduce the structure transition from the cubic form to the (2x2x1) tetragonal superstructure. This suggests that guest molecules play important roles in the structural transitions in melanophlogite.