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.
The calcium phosphate, apatite, has many uses in society ranging from its use in detergents, to nuclear waste storage, to development of new bone graft technologies. These applications, and the development of novel new applications, require an understanding of how anions fit into the structure of the mineral. The work accomplished has produced the following important impacts to the discipline: a) It showed the importance of the z-site occupancy of natural apatites to developing understanding of big system questions, such as the water budget of the lunar interior b) The techniques developed to analyze z-site anion abundances can now be used by others in the field of terrestrial and planetary petrology to understand planetary systems without the costs assocated with SIMS c) The methodology developed to grow large OH-poor apatite crystals can now be used by many workers for a variety of structural and thermochemical studies. d) The structural study results have provided a new model for the mechanism for F-Cl substitution that suggests stability of OH-poor apatite. Impacts on other disciplines: The synthesis techniques developed can be used for a variety of structural studies on apatite in chemistry and material science Structural understanding of anionic substitution mechanisms in apatite is of application to biomaterials science where apatite is used therapeutically e.g., in bone grafts. Specific project results: a) the mineral apatite retains information on the volatile history of a planet through its z-site occupancy. For the Moon this has shown that the lunar interior was wetter than has been generally accepted since the Apollo days and requires a complete re-thinking of the large impactor hypothesis for lunar formation. b) Apatite grown at high temperature in vacuum can contain <1 mol% OH in the Z site, indicating that fluor-chlor apatites are stable. c) Chlorapatite grown at high temperature is hexagonal and at lower temperatures may undergo a symmetry change that must be considered in the development of new structural analogs. d) The mechanism of Cl accommodation (and presumably other large anions) into the structure involves the formation of a new site. This is relevant to all new technologies in which a large anion substitutes into the apatite structure.
