Abstract

The synthesis and upper thermal stability of cancrinite were investigated experimentally in the system Na 2 O - CaO - Al 2 O 3 - SiO 2 - CO 2 - H 2 O at 2 kbar and in the presence of a mixed H 2 O - CO 2 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 + nH 2 O. Partial melting and the formation of melilite was observed at the highest temperatures and for the most H 2 O-rich fluid compositions. The molecular water content of the cancrinite formed at various T-X CO2 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 0f = -14722+ or -147 kJ and S 0 = 981+ or -118 J/K at 298 K and 1 bar for synthetic cancrinite of the composition Na 6 Ca (sub 1.5) [Al 6 Si 6 O 24 ](CO 3 ) (sub 1.5) .1.1(+ or -0.4)H 2 O. This study demonstrates the important role that water plays in controlling the stability of cancrinite in igneous and metamorphic rocks.

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