The Stability Relations of Anorthite and Its Hexagonal Polymorph in the System CaAl2Si2O8-H2O

Hydrothermal experimentation indicates that pure anorthite is stable in the presence of H2O at temperatures down to 350° C. and possibly lower. A hexagonal polymorph of anorthite unknown in nature is consistently synthesized at low temperatures, and the possibility of its being stable below approximately 300° C. is discussed. The hydrous phases zoisite and lawsonite were not encountered.