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 behavior,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)1.2−1.7][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. 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 αV,293K = 4.88(8)·10−5 K−1; vishnevite has KV0 = 49(2) GPa; balliranoite has KV0 = 48(3) GPa and αV,293K = 4.6(4)·10−5 K−1; davyne has KV0 = 46.5(11) GPa and αV,293K = 4.2(4)·10−5 K−1). 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.