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 + n H₂O . 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 ΔH 0 f = -14722 ± 147 kJ and S 0 = 981 ± 118 J/K at 298 K and 1 bar for synthetic cancrinite of the composition Na 6 Ca 1.5 [Al 6 Si 6 O 24 ](CO 3 ) 1.5 ·1.1(±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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Sirbescu et al. (1999) studied this question.
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