Porous materials at low temperature and high pressure: a comparative study of cancrinite

Cancrinite - (Na,Ca)7-8(Si6Al6O24)(CO3)1.2-1.7•2H2O - is the parent member of a group of zeolite-type minerals, the “cancrinite group” [1,2]. The crystal structure of cancrinite is built up on an open-tetrahedral framework consisting of interconnected (and parallel) 6-membered rings with an •••ABABAB••• stacking sequence and two “secondary building units”: 6- and 4-membered tetrahedral rings (hereafter 6mR and 4mR, respectively) [1,2]. The topological symmetry of the idealised CAN framework is P63/mmc [3], with unit-cell constants: a ~ 12.5 and c ~ 5.3Å, and framework density: 16.6 T/1000Å3. One large 12-membered ring (12mR) channel runs along [0001], with a “free-diameter” of about 5.9 Å. The so-called “cancrinite-cages” (also known as ε-cages or undecahedral cages) lie around the 12mR (Fig. 1). In natural cancrinites, the Si/Al-distribution in the tetrahedral framework is fully ordered [2], reducing the symmetry to P63. Several superstructures have been found in natural cancrinites, due to substitutional (or positional) ordering of the extra-framework content and/or to a periodic variation in the stacking sequence of the building-block units [1]. In cancrinites with •••ABABAB••• stacking sequence, the extra-framework content that lies in the large 12-membered rings channels is represented by one independent Na-site (Na2) and two independent, and statistically distributed, CO3 groups [2]. The H2O molecules and a further Na-site (Na1) lie in the cancrinite-cages [2]. The oxygen site (OW) of the H2O molecules lies off from the 3-fold axis, giving rise to a statistical configuration with three symmetrically equivalent and mutually exclusive molecules [2]. The low-T and high-P behaviour of cancrinite were never investigated. The thermo-elastic behaviour down to 100 K and the low-T induced structural evolution of a natural cancrinite were investigated by a series of single-crystal intensity data collections and structure refinements. No evidence of a phase transition, or anomalous elastic behaviour, was observed within the temperature range investigated (i.e. 100 < T(K) < 293). The volume thermal expansion coefficient between 100-293 K is: αV = 3.7(7)•10-5 K-1 [with VT=V0exp(αVT)]. The structure refinement based on the data collected at room temperature after the low-T experiment reveals that the deformation process is completely reversible. The extra-framework population does not show any significant variation within the T-range investigated. The strong positional disorder of the carbonate groups along the c-axis (already observed at room-T) is maintained down to 100 K. The main deformation mechanisms at low-T are mainly governed by the framework re-arrangement, and in particular by the deformation of the secondary building units through: 1) the ditrigonalization of the 6mR[0001], 2) the contraction of the 4mR joint units, 3) the decrease of the rings corrugation in the (0001) plane and 4) the flattening of the cancrinite cages. The high-P behavior of cancrinite is under investigation through a series of in-situ single-crystal intensity data collections. The preliminary data appear to show a change in the elastic behavior between 1.5-2.6 GPa, with a decrease in compressibility. At high-P, cancrinite shows an anisotropic elastic behaviour, with βa=βb<βc. The structure refinements show that, under hydrostatic compression, the main deformation mechanisms are similar to those previously described at low-T, though different in magnitude. Within the P-range investigated, the configuration of the extra-framework population is preserved, without any significant change of the bonding scheme. It appears that the geometrical configuration of the CAN framework-type governs the main deformation mechanisms under low-T and high-P conditions; the extra-framework population seems to play a secondary role. The high symmetry of the cancrinite structure allows only a few degree of freedom of the framework deformational mechanisms, which accommodate the anisotropic contractions at both low-T/high-P. References: [1] Bonaccorsi E, Merlino S (2005) In: Ferraris G, Merlino S (eds) Micro- and Mesoporous mineral phases. Reviews in Mineralogy and Geochemistry, Vol. 57, pp. 241-290. [2] 48. Della Ventura G, Gatta GD, Redhammer GJ, Bellatreccia F, Loose A, Parodi GC (2009) Single-crystal polarized FTIR spectroscopy and neutron diffraction refinement of cancrinite. Phys Chem Minerals 36:193-206. [3] Baerlocher Ch, Meier WM, Olson DH (2001) Atlas of zeolite framework types. Structure Commission of the International Zeolite Association. Elsevier, Amsterdam.