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American Mineralogist; December 1999; v. 84; no. 11-12; p. 1850-1860
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Experiments on the stability of cancrinite in the system Na 2 O-CaO-Al 2 O 3 -SiO 2 -CO 2 -H 2 O

Mona Sirbescu, and David M. Jenkins

Binghamton University, Department of Geological Sciences and Environmental Studies, Binghamton, NY, United States

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 0 f = -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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