HYDROTHERMAL DIAMOND-ANVIL CELL STUDY OF MELTS - EUTECTIC MELTING OF THE ASSEMBLAGE CA(OH)(2)-II PHASE(CACO3 WITH EXCESS H2O AND LACK OF EVIDENCE FOR PORTLANDITE)

Eutectic melting temperatures for the assemblage Ca(OH)(2) + CaCO3 in the presence of excess H2O were determined along six isochores of H2O in a hydrothermal diamond-anvil cell. These temperatures range from 61 0 degrees C at 1204 bar to 583 degrees C at 7203 bar; they agree very well (within 3 degrees C) with data obtained from high-pressure differ ential thermal analysis (HP-DTA) and are about 20 degrees C lower than those obtained from quench experiments. The additional DTA peaks near 630 degrees C for the melting experiments of pure portlandite reporte d by Harker (1964) and Bai et al. (1994) may be the result of a small amount of melting, caused by CaCO3 contamination, and not the result o f a solid phase transition between portlandite and ''portlandite II.''