The low-temperature behaviour of cancrinite: an in situ single-crystal X-ray diffraction study

The low-temperature structural behaviour of natural cancrinite with a formula Na6.59Ca0.93[Si6.12Al5.88O24](CO3)(1.04)F-0.41 center dot 2H(2)O has been investigated by means of in situ single-crystal X-ray diffraction and Raman spectroscopy. High quality structure refinements were obtained at 293, 250, 220, 180, 140, 100 and at 293 K again (at the end of the low-T experiments). The variation in the unit-cell volume as a function of temperature (7) exhibits a continuous trend, without any evident thermoelastic anomaly. The thermal expansion coefficient alpha(V) = (1/V)partial derivative V/partial derivative T is 3.8(7) x 10(-5) K-1 (between 100 and 293 K). The structure refinement based on intensity data collected at ambient conditions after the low-T experiment confirmed that the low-T induced deformation processes are completely reversible. The extraframework population does not show significant variations down to 100 K. The strong positional disorder of the carbonate groups along the c axis persists within the T range investigated. The structural behaviour of cancrinite at low-T is mainly governed by the continuous framework rearrangement through the ditrigonalization of the six-membered rings which lie in a plane perpendicular to [0001], the contraction of the four-membered ring joint units, the decrease of the ring corrugation in the (0001) plane, and the flattening of the cancrinite cages. A list of the principal Raman active modes in ambient conditions is provided and discussed.