High-temperature behaviour of fluorcarletonite, KNa4Ca4Si8O18(CO3)4(F,OH)⋅H2O, from the Murun Alkaline Complex, Russia, appraised by experimental and theoretical methods

Abstract

The thermal behaviour of fluorcarletonite, KNa4Ca4Si8O18(CO3)4(F,OH)⋅H2O, from the charoitites of the Severny district at the Malyy Murun massif, Murun complex, NW Aldan Shield, Siberia, Russia, has been investigated in order to understand the temperature-induced changes in the crystal structure of this rare silicate. The study has been carried out combining in situ high-temperature single-crystal X-ray diffraction (T range 25–550°C), ex situ high-temperature Fourier-transform infrared spectroscopy (25–700°C) and ab initio calculations. An increasing trend of lattice parameters and cell volume was observed in the 150–550°C temperature range, when the mineral underwent a progressive dehydration process. At 550°C ~40% water loss was detected. If compared with the fluorcarletonite structure at room temperature, the partially dehydrated fluorcarletonite shows: the same space group (P4/mbm); increased distances between the oxygens of the H2O molecules (O11w and O12w) and their Na-centred octahedral cations (Na1 and Na2, respectively); distortion of the four- and six-member tetrahedral rings of the double silicate layer. The dehydration process mainly involves the oxygen at the O11w site which has a different local environment with respect to the oxygen at the O12w site. At T > 600°C, the complete dehydration is accompanied by deprotonation of the OH groups substituting for the F atoms and by the collapse of the structure when the CO2 is released. The adopted approach allowed definition of the temperature thresholds at which modifications occur in the fluorcarletonite crystal structure when subjected to controlled heating conditions. Our findings contribute to assessment of stability, reactivity and, more generally, the thermal behaviour of sheet silicates with fluorcarletonite-like topology.