The thermal behaviour of sacrofanite
The thermal behaviour of sacrofanite
European Journal of Mineralogy (May 2018) 30 (3): 507-514
- alkali metals
- cell dimensions
- compression
- crystal chemistry
- crystal structure
- erionite
- feldspathoids
- framework silicates
- high temperature
- ion exchange
- metals
- physical properties
- potassium
- Rietveld refinement
- silicates
- substitution
- temperature
- thermal expansion
- thermal properties
- unit cell
- volume
- water
- X-ray diffraction data
- zeolite group
- davyne
- afghanite
- liottite
- microsommite
- sacrofanite
- cancrinite-sodalite supergroup
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 H (sub 2) O 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.
