CANCRINITE-GROUP MINERALS AT NON-AMBIENT CONDITIONS: A MODEL OF THE ELASTIC BEHAVIOR AND STRUCTURE EVOLUTION

The minerals of the cancrinite group are zeolite-like compounds, sharing the [CAN]-topology of the framework. Their structure shows large 12-ring channels along [0001], bound by columns of cages, the so-called can units. Natural and synthetic compounds exhibit a remarkable chemical variability. Among the natural species, the majority shows an aluminosilicate framework. 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. The channels are stuffed by cations, anions and molecules. In Nature, cancrinite-group minerals occur in the late/hydrothermal stages of alkaline (SiO2)-undersaturated magmatism and in related effusive or contact rocks. Cancrinite-group compounds have been proposed as stable storage form for alkaline wastes. The characterization of the phase-stability fields, thermo-elastic behavior and structure response to applied (P,T) is needed to evaluate and predict their behavior in natural and industrial processes. The present study aimed to model the thermo-elastic behavior and the mechanisms of (P,T)-induced structure evolution of cancrinite-group minerals, with a special interest on the role played by the extraframework population. The study was restricted to the following (CO3)-rich and (SO4)-rich end-members: cancrinite {[(Na,Ca)6(CO3)1.2-1.7][Na2(H2O)2][Al6Si6O24]}, 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]}. Their high-P and low-T (T < 293 K) behavior was investigated by means of in situ single crystal X-ray diffraction, using diamond-anvil cells and (N2)-cryosystems, respectively. Though sharing a similar volume compressibility and thermal expansivity, these minerals have a different thermo-elastic anisotropy, more pronounced in the cancrinite-subgroup compounds. This behavior is governed by different deformation mechanisms, which reflect the different coordination environments of the cage population between the minerals of the two subgroups. The davyne sample studied at high-P showed a displacive phase transition from the P63/m to the P63 space group after the load of pressure [0.0001 ≤ P (GPa) ≤ 0.38(2)]. In vishnevite, a P-induced re-organization of the extraframework population took place at P > 3.5 GPa, coupled with a significant increase in compressibility, suggesting that the channel-constituents can also play an active role at non-ambient conditions. Besides common features likely ascribable to the [CAN]-topology, the nature of the extraframework population appears to control significantly the (P,T)-induced structure evolution and thermo-elastic behavior of the cancrinite-group compounds.