The thermal behaviour of sacrofanite

The thermal behaviour of sacrofanite, the ABCABACACABACBACBACABABACABC 28-layer member of the cancrinite sodalite supergroup has been investigated in situ using laboratory focussing-beam X-ray powder diffraction data. Thermal expansion has been analysed from 303 to 873 K. A moderate anisotropy has been observed at T > 473 K, the c-parameter being softer than the a-parameter. At ca. 800 K the unit cell starts to compress owing to (partial) dehydration. Comparison of the heat-induced properties of cancrinite sodalite supergroup minerals has shown no clear relationship between crystal chemical and structural features and their thermal behaviour. Only K-rich pitiglianoite shows a significant negative expansion that has been attributed to the occurrence, as in the case of the zeolite erionite, of a sort of “internal cation exchange” process involving the substitution by K of H2O residing at the centre of the cancrinite cage. The same behaviour has been invoked to account for the small cell compression of sacrofanite at T > 773 K.