Peters, L., Knorr, K., Katzke, H., Knapp, M. and Depmeier, W. (2006) The transformation mechanism of the sodalite- to the melilite-topology: Thermal expansion and decomposition of bicchulite-type to melilite-type compounds. Zeitschrift Fur Kristallographie, 221 (3). pp. 198-205. DOI 10.1524/zkri.2006.221.3.198.

Abstract

The mineral bicchulite, vertical bar Ca-8(OH)(8)vertical bar [Al8Si4O24]-SOD, has the sodalite-(SOD-)type structure. At elevated temperatures it decomposes into gehlenite, Ca2Al2SiO7 (Me lite-type structure), and water (vapour). We report on the thermal expansion of two bicchulite-type compounds, the mineral bicchulite up to T = 740 K and vertical bar Ca6Eu2(OH)(8)vertical bar [Al(10)Si(2)O(24)vertical bar-SOD up to T = 1025 K. The expansion mechanism is described. In situ X-ray powder diffraction experiments were performed to evaluate the thermal decomposition of vertical bar Ca6Eu2(OH)(8)vertical bar[Al(10)Si(2)O(24)vertical bar-SOD into 4 (Ca1.5Eu0.5)T-1 [(T2O7)-O-2] and 4 H(2)0, with T-i = Al or Si in a ratio of 5: 1. A structural mechanism for the transformation from the sodalite to the melitite topology is proposed, based on geometrical considerations.

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