Cancrinite-group minerals behavior at non-ambient conditions

Cancrinite-group minerals occur in the late stages of alkaline (SiO2)-undersaturated magmatism and in related effusive or contact rocks. So far only few studies have been devoted to the description of the thermo-elastic behaviour, phase-stability and P/T-structure evolution (at the atomic scale) of this mineral group. Cancrinite-group minerals have an open-framework structure characterized by the [CAN]-topology. The [CAN]-framework shows large 12-ring channels, parallel to the c crystallographic axis, bound by columns of cages, the so called can units. While very limited chemical variation is observed in the framework composition (the composition is almost always [Si6Al6O24]) a remarkable chemical variability is reported for the extraframework components in the cancrinite-group minerals. 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. We aimed to model the thermo-elastic behavior and the mechanisms of the (P,T)-induced structure evolution of cancrinite-group minerals, with 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 sensu stricto {[(Na,Ca)6(CO3)1:21:7][Na2(H2O)2][Al6Si6O24]}, vishnevite {[(Na,Ca,K)6(SO4)][Na2(H2O)2][Al6Si6O24]}, balliranoite {[(Na,Ca)6(CO3)1:21:7][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. The high-T behavior of cancrinite has also been studied by means of in-situ single-crystal X-ray diffraction with a resistive heater. Cancrinite minerals share a similar volume compressibility and thermal expansivity at ambient conditions (cancrinite has KV0 = 45(2) GPa and V;293K = 4.88(8)105 K1; vishnevite has KV0 = 49(2) GPa; balliranoite has KV0 = 48(3) GPa and V;293K = 4.6(4)105 K1; davyne has KV0 = 46.5(11) GPa and V;293K = 4.2(4)105 K1). However, these minerals show different thermo-elastic anisotropy schemes, more pronounced in the cancrinite subgroup minerals. This behavior is governed by different deformation mechanisms of the crystal structure, which likely reflect the different coordination environments of the cage-cations between the minerals of the cancrinite and davyne-subgroups (i:e: Na+ and Ca2+, respectively). In addition, a P-induced re-organization of the extraframework population is observed, in vishnevite, at P 3.5 GPa, suggesting that the channel-constituents can also affect the elastic and structural behavior and the phase stability of these minerals 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. PL, GDG and MM acknowledge the Italian Ministry of Education, MIUR-Project: “Futuro in Ricerca 2012 - ImPACT- RBFR12CLQD”. MA acknowledges the ERC starting grant N. 307322 to FN.