Minerals of the apatite group are very important accessory minerals in all types of rocks. As a major constituent of bones and teeth it is the object of much research in the medical field, and is commercially exploited for a number of important uses. The relative chemical and structural simplicity of apatite and its occurrence in a very wide range of rock types make it potentially ideal as s mineralogical "monitor" of the relative activities of fluorine, chlorine and water in the environment in which it crystallized. It is proposed to calibrate this "monitor" by determining the solid solution mixing behavior and by evaluation of endmember thermodynamic properties with experiments at 0.5 Kb pressure, and temperatures between 600o and 900oC. The information to be derived from this study will have important applications in the study of igneous processes and volcanic phenomena, formation of porphyry ore deposits, and composition of metamorphic fluids. The fundamental thermodynamic data obtained may potentially have applications in other sciences and chemical engineering.
