Apatite: The Effect of Volatiles on its Structure, Stability and Thermodynamic Properties

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A series of experiments, analytical studies, and modeling efforts have been designed to further our understanding of the structure, stability, and thermodynamic properties of apatite as a function of its OH, F, and Cl contents. The proposed work will focus specifically on (1) compositional and structural characterization of synthesized endmember, binary, and ternary apatites; (2) crystallization experiments to assess the presence or absence of a solvus within the low OH portion of the system at magmatic temperatures; and (3) exchange experiments between OH-poor apatite and molten salt to investigate thermodynamic mixing properties along the fluorapatite-chlorapatite join and to develop binary and ternary mixing models for fluorapatite (FAp) - chlorapatite (ClAp) - hydroxylapatite (OHAp) solid solutions. These results will be applicable to a variety of fields of terrestrial and planetary geoscience, materials science, and technology development.Intellectual merit. Apatite is an almost ubiquitous mineral in geological systems on Earth and is found in a variety of planetary materials. Its ability to sequester trace elements makes it important to petrogenetic and geochronological studies of igneous, metamorphic, and sedimentary rocks. However, its stability remains poorly constrained due to the fact that its stability depends on an anionic solid solution with F, Cl, and OH. Modeling of this solid solution has been hindered by difficulties in analyzing these anionic constituents, lack of stability information in the OH-poor part of the system, and lack of systematic structural data within the ternary system. In this project, laboratory synthesis experiments, detailed X-ray and NMR characterization, and intensive compositional analysis, and crystallographic models will be linked to fill this gap. Finally, anion exchange experiments using KCl- KF molten salts will provide quantitative information on the excess energies of mixing needed for development of a thermodynamic model for ternary solid solutions in the system.Broader impacts. This project will support a graduate student at Stony Brook. The thermodynamic solution model for apatite is likely to have wide applicability in many fields of research as this mineral is the basis for diverse economic, engineering, and medical applications.