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 \ncurve for brucite by the quench method and the fusion curves for portlandite dry \nand with excess water vapor by differential thermal analysis. Experimental brackets for \nthe fusion of portlandite are 800° to 809°C al 10 kb, 796° to 800°C at 20 kb, and \n781° to 786°C at 30 kb, and for the fusion of portlandite with excess vapor 715° to \n719°C at 10 kb, 675° to 682°C at 20 kb, 643° to 649°C at 30 kb. Experimental brackets \nfor the brucite dehydration are 920° to 940°C at 17 kb, 1050°C at 27 kb, and 1080° to \n1100°C at 33 kb; the 33 kb bracket was reversed; one DTA run at 27 kb gave 1055° ± \n7°C, overlapping the definitive quench run. We find no evidence for the previously \nreported temperature maximum on the brucite dehydration curve at 32 kb. From the \nexperimental data for the brucite reaction, values calculated for the Gibbs free energy \nfunction, G*, of water arc -15.325 kcal mole^(-1) at 17 kb, 930°C; -6.647 kcal mole^(-1) \nat 27 kb, 1050°C; and -2.997 kcal mole^(-1) at 33 kb, 1090°C.