Experiments on the stability of cancrinite in the system Na₂O-CaO-Al₂O₃-SiO₂-CO₂-H₂O

The synthesis and upper thermal stability of cancrinite were investigated experimentally in the system Na₂O-CaO-Al₂O₃-SiO₂-CO₂-H₂O at 2 kbar and in the presence of a mixed H₂O-CO₂ fluid. Cancrinite could only be formed under water-rich conditions in this system. The breakdown of cancrinite to nepheline + calcite occurred at decreasing temperatures with increasing X(CO(2)) as expected for a dehydration reaction of the form cancrinite = nepheline + calcite + n(H(2)O). Partial melting and the formation of melilite was observed at the highest temperatures and for the most H₂O-rich fluid compositions. The molecular water content of the cancrinite formed at various T-X(CO(2)) conditions was evaluated with a combined infrared (IR)-thermogravimetry (TG) technique. Results suggest (within analytical error) a decrease in the water content of cancrinite toward the breakdown reaction and an apparently constant water content along the breakdown curve. Thermodynamic analysis combining the compositional and phase-equilibrium data from this study was performed and yielded a value of ΔH(t)⁰= -14722 ± 147 kJ and S⁰ = 981 ± 118 J/K at 298 K and 1 bar for synthetic cancrinite of the composition Na₆Ca_1.5[Al₆Si₆O₂₄](CO₃)_1.5·1.1(±0.4)H₂O. This study demonstrates the important role that water plays in controlling the stability of cancrinite in igneous and metamorphic rocks.