Hydrothermal reversal experiments have been performed on the upper pressure stability of paragonite in the temperature range 550-740 ~ C. The reaction NaA13SiaOx o(OH)2 paragonite = NaA1Si20 6 +A12SiO 5 + H20 jadeite kyanite vapour
has been bracketed at 550~ 600 ~ 650~ and 700~ at pressures 24-26kb, 24-25.5 kb, 24-25 kb, and 23-24.5 kb respectively. The reaction has a shallow negative slope (-10 bar ~ and is of geobarometric significance to the stability of the eclogite assemblage, omphacite + kyanite.
The experimental brackets are thermodynamically consistent with the lower pressure reversals of Chatterjee (1970, 1972), and a set of thermodynamic data is presented which satisfies all the reversal brackets for six reactions in the system Na20-A120 3 -SiO2--H20.
The Modified Redlich Kwong equation for H20 (Holloway, 1977) predicts fugacities which are too high to satisfy the reversals of this study. The P-T stabilities of important eclogite and blueschist assemblages involving omphacite, kyanite, lawsonite, jadeite, albite, chloritoid, and almandine with paragonite have been calculated using thermodynamic data derived from this study.