STANDARD THERMODYNAMIC PROPERTIES OF MEIONITE, CA4AL6SI6O24CO3, FROM EXPERIMENTAL PHASE-EQUILIBRIUM DATA

Reversals of two reactions bounding the stability field of the scapoli te meionite, [GRAPHICS] were performed with a variety of experimental conditions and starting materials. Definitive reversals of Reaction 1 were obtained in the presence of an equimolar CO2-H2O flux (hydrous ox alic acid) with meionite synthesized at 900-degrees-C and synthetic an orthite and calcite. Reversed brackets obtained in the internally heat ed ps vessel and higher pressure reversals without a vapor flux in the piston-cylinder apparatus are consistent with the experiments of Gold smith and Newton (1977) and are higher in temperature by 60-80-degrees -C than the recent experimental results of Huckenholz and Seiberl (198 8, 1989). Fluid-absent reversals of Reaction 2 were obtained in the pi ston-cylinder apparatus for a range 900-1300-degrees-C and 20-32 kbar. The thermal expansion of the scapolite was measured by X-ray diffract ion at a series of temperatures between 20 and 800-degrees-C, yielding alpha = 1.8164 +/- 0.0148 x 10(-5)/K and V1.298 = 33.985 +/- 0.005 J/ (bar.mol). Best-fit lines through the experimental brackets of Reactio ns 1 and 2 define S1.298 (meionite) as 0.761 kJ/(K.mol). This value re quires that meionite has a high degree of disorder.