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-CO2 as expected for a dehydration reaction of the form cancrinite = nepheline + calcite + n(H2O). 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-Xo, 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 Delta H-l(0) = -14722 +/- 147 kJ and S-0 = 981 +/- 118 J/K at 298 K and 1 bar for synthetic cancrinite of the composition Na6Ca1.5[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.