Prehnite dissociation curve below 3 kilobars water pressure.

Abstract

Prehnite dissociates to form wollastonite, anorthite, and grossularite. The formation of the grossularite is considered as the result of the prehnite not being stoichiometric. Dissociation of prehnite occurs at 475ºC and 1000 bars water pressure. Kinetics prohibit obtaining experimental data below 800 bars water pressure I however the dissociation curve is extrapolated to low water pressures. This extrapolation is based on quantitative thermodynamic calculations. At low water pressures the curve becomes tangential to the H₂0-liquid H₂0-vapor phase boundary. Prehnite can form from CaO-Si0₂ metasomatism of Ca-rich plagioclase in nearly all rock types I including basalts, andesites, diorites, gabbros, diabases, graywackes, and volcanic sediments. Based on geologic evidence, prehnite in basalt amygdales must have formed at 1 atmosphere water pressure and less than 100ºC. The compositional dependence of the dissociation curve suggests that prehnite may not be a good index mineral to use in defining low-grade metamorphic facies. Further experimental work is required using mineral assemblages before prehnite can be used meaningfully as an index mineral.