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Lars Peters, Karsten Knorr, Hannelore Katzke, Michael Knapp, Wulf Depmeier

The transformation mechanism of the sodalite- to the melilite-topology: Thermal expansion and decomposition of bicchulite-type to melilite-type compounds

Keywords: Sodalite, Melilite, Thermal expansion, thermal decomposition, Reaction mechanism

The mineral bicchulite, |Ca₈(OH)₈| [Al₈Si₄O₂₄]-SOD, has the sodalite-(SOD-)type structure. At elevated temperatures it decomposes into gehlenite, Ca₂Al₂SiO₇ (melilite-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 |Ca₆Eu₂(OH)₈| [Al₁₀Si₂O₂₄]-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 |Ca₆Eu₂(OH)₈|[Al₁₀Si₂O₂₄]-SOD into 4 (Ca_1.5Eu_0.5)T¹[T₂²O₇] and 4 H₂O, with Tⁱ = Al or Si in a ratio of 5 : 1. A structural mechanism for the transformation from the sodalite to the melilite topology is proposed, based on geometrical considerations.

Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0044-2968
Volume: 221, 03/2006
Pages: 198 - 205

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