The crystal structure of apophyllite has been refined by a least squares method, using three dimensional X-ray diffraction data. The structure determined by Taylor and Náray-Szabó is essentially correct. The water molecules are found to be hydrogen-bonded to two oxygen atoms in the Si8O20 sheets. The two-stage dehydration in apophyllite is interpreted as the result of the transformation from KCa4Si8O20(F, OH)·8H2O to KCa4Si8O20·4H2O at 250°C.

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