Anisotropic refinements in space group I2/m using 1150-1200 reflections resulted in R factors ranging from 0.034 to 0.063. Over the temperature intervals studied, the tetrahedral distances remain statistically identical, but all other mean polyhedral distances increase significantly. Differential polyhedral expansion is accompanied by straightening of the tetrahedral chains and by increased displacement of the double chains relative to each other. The behavior of the richterite structures at elevated temperatures is generally similar to that of tremolite.--Modified journal abstract.

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