Phase relations of portlandite, Ca(OH) 2 and brucite, Mg(OH) 2 to 33 kilobars

Using piston-cylinder apparatus, we have extended to 33 kb the dissociation for brucite by the quench method and the fusion curves for portlandite dry with excess water vapor by differential thermal analysis. Experimental brackets for fusion of portlandite are 800° to 809°C al 10 kb, 796° to 800°C at 20 kb, and 781° to 786°C at 30 kb, and for the fusion of portlandite with excess vapor 715° to 719°C at 10 kb, 675° to 682°C at 20 kb, 643° to 649°C at 30 kb. Experimental brackets the brucite dehydration are 920° to 940°C at 17 kb, 1050°C at 27 kb, and 1080° to 1100°C at 33 kb; the 33 kb bracket was reversed; one DTA run at 27 kb gave 1055° ± 7°C, overlapping the definitive quench run. We find no evidence for the previously temperature maximum on the brucite dehydration curve at 32 kb. From the data for the brucite reaction, values calculated for the Gibbs free energy , G*, of water arc -15.325 kcal mole^(-1) at 17 kb, 930°C; -6.647 kcal mole^(-1) 27 kb, 1050°C; and -2.997 kcal mole^(-1) at 33 kb, 1090°C.