Comparative thermo-elastic behaviour of the isotypic cancrinite and balliranoite

The high-pressure behaviour and the P-induced structure evolution of a natural cancrinite from Cameroun (Na6.59Ca0.93[Si6.12Al5.88O24](CO3)1.04F0.41•2H2O, a = 12.5976(6) Å, c =5 .1168(2) Å, space group: P63) were investigated by in situ single-crystal X-ray diffraction under hydrostatic conditions up to 6.63(2) GPa with a diamond anvil cell[1]. The P-V data were fitted with an isothermal Birch-Murnaghan-type equation of state (BM EoS) truncated to the 3rd-order, giving the following elastic parameters: V0 = 702.0(7) Å3, KV0 = 51(2) GPa and KV' = 2.9(4). Linearized BM EoS was used to fit the a-P and c-P data, giving the following parameters: a0 = 12.593(5) Å, Ka0 = 64(4) GPa, Ka' = 4.5(9), and c0 = 5.112(3) Å, Kc0 = 36(1) GPa, Kc' = 1.9(3). A subtle change of the elastic behaviour appears to occur at P > 4.62 GPa, and so the elastic behaviour was also described on the basis of BM EoS valid between 0.0001 – 4.62 and 5.00 – 6.63 GPa, respectively. The high-pressure structure refinements allowed the description of the main deformation mechanisms responsible for the anisotropic compression of cancrinite. The low-temperature structure evolution of the same natural cancrinite was also investigated by means of in-situ single-crystal X-ray diffraction[2]. The V-T data exhibit a trend without any evident thermoelastic anomaly, with a thermal expansion coefficient αV = 38(7) •10^(-6) K^(-1) (between 100 and 293 K). Seven structure refinements showed that the same mechanisms observed at high pressure, mainly govern the low-T structure evolution. A study of a natural sample of balliranoite (Na4.47Ca2.86K0.10[Si5.96Al6.04O24](CO3)0.62(SO4)0.33Cl2.03, a = 12.680(1) Å, c = 5.3141(5) Å, S.G.: P63) at high pressure and low temperature is in progress. Preliminary P-V data up to 4.93 GPa were fitted with a BM EoS truncated to the 2nd order (II-BM EoS), giving the following refined parameters: V0 = 735.6(9) Å3, KV0 = 48.0(14) GPa. A fit with a II-BM EoS, applied to the P-V data of cancrinite within the range 0.0001-4.62 GPa, gave the following parameters: V0 = 702.5(5) Å3, KV0 = 48.8(6) GPa, showing similar volume compressibility. However, a different elastic anisotropy is observed (Ka0:Kc0 = 2.14:1 in cancrinite; Ka0:Kc0 = 1.40:1 in balliranoite). Structure refinements of balliranoite from high pressure and low temperature diffraction data will lead to the description of the P/T-induced structure evolution, allowing a comparative crystal-chemistry analysis of this class of materials. References 1.P. Lotti, G.D. Gatta, N. Rotiroti, F. Càmara Am. Mineral. (2012), 97, 872−882. 2.G.D. Gatta, P. Lotti, V. Kahlenberg, U. Haefeker Mineral Mag. (2012, in press).