An experimental and thermodynamic study of cymrite and celsian stability in the system BaO-Al (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O

The stabilities and growth kinetics of celsian and cymrite have been investigated in the system BaO-Al (sub 2) O (sub 3) -SiO (sub 2) - H (sub 2) O at <= 10 kbar and <= 800 degrees C; a broad area of P-T space has been identified at low P and T in which cymrite grows rapidly and metastably in runs of short duration, being replaced by celsian in longer runs. The equilibrium cymrite reversible double half arrows celsian + H (sub 2) O has been tightly bracketed at approx 9.7 kbar, 800 degrees ; 9.0 kbar, 710 degrees C; 8.0 kbar, 580 degrees C; results at lower P and T confirm that cymrite is only stable at high P, providing a valuable metamorphic geothermometer. Cymrite is not stable in low P-T sedimentary and hydrothermal environments and documented natural occurrences in these environments must be considered to have resulted from metastable growth and persistence in the celsian stability field. The molecular H (sub 2) O contents of cymrites vary systematically with PH (sub 2) O and T of synthesis, increasing with increasing P and decreasing T to a maximum of 1 H (sub 2) O per formula unit. The incorporation of H (sub 2) O yields an enthalpy change of approx 59 kJ/mol and an entropy change of approx 133 J/K/mol; isopleths have been calculated for the equilibrium water content of cymrite as a function of PH (sub 2) O and T.