The crystal structures of a pure synthetic forsterite and a natural metamorphic manganoan hortonolite have been refined from three-dimensional intensity data obtained at 20°, 300°, 600°, and 900°C. Both structures show a zero or slightly negative expansion of silicon-oxygen distances compared with large positive expansions of the octahedral cation-oxygen distances with increasing temperature. The hortonolite refinements indicate a slight preference of Fe,for possibly as Fe3+, for the smaller Ml site. This preference appears to increase with increasing temperature.

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