X-ray study of the triagonal --> hexagonal phase transition in metamorphic kalsilite

University of Hannover, Institute of Mineralogy, Hannover, Federal Republic of Germany

High-temperature annealing experiments on metamorphic kalsilite (KAlSiO ₄ ) indicate that there is an irreversible phase transition from the P31c to P6 ₃ structure at approximately 200 degrees C. Powder X-ray investigations indicate that the transformation is incomplete. The proportion of P6 ₃ phase increases as a function of temperature, attaining 70% for the powdered sample at approximately 500 degrees C. At higher temperatures, the fraction of sample transforming to P6 ₃ phase seems to be independent of temperature, and probably depends on the structural state of the starting material as influenced by mechanical grinding. Transformation proceeds by tetrahedral rotation such that successive (001) sheets undergo opposite-sense rotations. The transition behavior differs for single crystals, which had not been mechanically ground; at 500 degrees C single crystals transformed completely into the P6 ₃ phase. Single-crystal structure refinements of the P6 ₃ phase indicate that the structure is similar to that of volcanic kalsilite. Both apical and basal O atoms show large anisotropic displacement parameters, but the displacement of the apical oxygen does not exhibit the trigonal distribution that has been observed in volcanic kalsilite. The more pronounced positional disorder in volcanic kalsilite is due to the presence of sodium.

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