Cancrinite-group minerals([CAN]-framework type) at non-ambient conditions

All the isotypic minerals of the cancrinite-group share the [CAN]-framework type, built up by layers of single six-membered rings of tetrahedra centered in an “A” or “B” position, according to the ABAB stacking sequence. The resulting framework has the following secondary building units: 12-membered ring channels parallel to the [0001] axis, bound by columns of base-sharing cages, and the so-called can units. A large chemical variability is shown by both natural and non-natural isotypic compounds. Among the natural species, the majority shows an alumino-silicate framework (Al6Si6O24), and two subgroups can be identified according to the extraframework content of the can units: the cancrinite- and the davyne-subgroups, showing Na-H2O and Ca-Cl chains, respectively. Several cations, anionic and/or molecular groups lie in the channels. The description of the phase-stability fields and the of the thermo-elastic behavior of the cancrinite-group minerals play a key role in the study of the natural and industrial processes where these compounds are primary constituents (a short summary of which is in [1,2]). We aim to model the thermo-elastic behavior and (P,T)-induced structure evolution of these isotypic compounds, with a focus on the influence played by the different extraframework constituents on the structure deformation mechanisms. The study is restricted to the chemical compositions commonly occurring in Nature, delimited by the (CO3)-rich and (SO4)-rich end-members within the two aforementioned subgroup: cancrinite {[(Na,Ca)6(CO3)1.2-1.7][Na2(H2O)2][Al6Si6O24]} and vishnevite {[(Na,Ca,K)6(SO4)][Na2(H2O)2][Al6Si6O24]}, balliranoite {[(Na,Ca)6(CO3)1.2-1.7][Ca2Cl2][Al6Si6O24]} and davyne {[(Na,Ca,K)6((SO4),Cl)][Ca2Cl2][Al6Si6O24]}, respectively. The high-pressure and low-temperature (T < 293 K) studies of the carbonate end-members (i.e. cancrinite and balliranoite) have been performed by means of in situ single-crystal X-ray diffraction. The results [1-4] show that, though sharing a similar volume compressibility and thermal expansivity, these minerals have a different thermo-elastic anisotropy, being more pronounced in cancrinite. This is due to different (P,T)-induced structure deformation mechanisms, likely governed by the different coordination environment of the cage population. An in situ high-temperature (293 ≤ T(K) ≤ 823(7)) single-crystal X-ray diffraction study of cancrinite, allowed the description of thermo-elastic behavior and anisotropy. An irreversible dehydration process takes place at 748(7) K. Preliminary results of the high-pressure studies of the sulfatic end-members (i.e. vishnevite and davyne) are available. A clear change of the elastic behavior of vishnevite, with an increase of compressibility, is shown between 2.47(2)-3.83(2) GPa. A similar increase of compressibility was also reported for cancrinite at 4.62-5.00(2) GPa. References. [1] Lotti, P., Gatta, G.D., Rotiroti, N., Cámara, F. (2012): Am. Mineral., 97, 872-882; [2] Gatta, G.D., Lotti, P., Kahlenberg, V. (2013): Micropor. Mesopor. Mater., 174, 44-53; [3] Gatta, G.D., Lotti, P., Kahlenberg, V., Haefeker, U. (2012): Miner. Mag., 76, 933-948; [4] Lotti, P., Gatta, G.D., Rotiroti, N., Cámara, F., Harlow, G.E. (2013): Z. Kristallogr., in press.