Thermal Expansion of Cancrinite

Ishmael Hassan
Department of Geology, Faculty of Science, University of Kuwait, P.O. Box 5969, Safat, 13060, Kuwait

Abstract: Thermal expansion coefficients were measured for a cancrinite from Bancroft, Ontario, Canada. Measurements of cell parameters and unit-cell volumes were obtained at room temperature and at heating intervals of 50°C over the temperature range from 50 to 1400°C. The unit-cell parameters for cancrinite increase non-linearly with temperature up to 1200°C and shortly thereafter, the mineral melted. The c parameter increases more rapidly than the a parameter, and the c/a ratio increases linearly with temperature. A plausible thermal expansion mechanism for cancrinite, which is based on the framework expansion that occurs as a function of cavity content, is presented. In the thermal expansion of cancrinite, the short Na-H2O in the H2O-Na—H2O chain expands to form equal distances to the two H2O molecules in the chain. This causes the Na atoms to move towards the plane of the six-membered rings and forces the tetrahedra to rotate and the rings become more planar. The Na atoms then form bonds to all six (O1 and O2) oxygen atoms in a ring; the Na-O1 bonds become shorter and the Na-O2 bonds become longer. These effects cause an increase in both a and c, and thus an increase in the c/a ratio. A similar thermal expansion mechanism operates in the sodalitegroup minerals where the six-membered rings and Na-Cl bond are involved.

Keywords: cancrinite • cancrinite-group • thermal expansion • powder X-Ray diffraction

Mineralogical Magazine; December 1996 v. 60; no. 403; p. 949-956; DOI: 10.1180/minmag.1996.060.403.09
© 1996, The Mineralogical Society
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