THERMODYNAMIC PREDICTIONS OF BIOAPATITE STABILITY IN ENVIRONMENTAL SYSTEMS

Bone is a composite material consisting of organics (collagen) and inorganics (mineral). The mineralized fraction of bone, structurally and chemically, closely resembles a carbonated hydroxylapatite phase, here termed bioapatite. In a living organism, bioapatite undergoes dynamic dissolution/precipitation processes, and provides a bioavailable store of Na, Ca, and P. Once removed from a living organism and deposited in an environment, organic matter degradation opens pore spaces within bone to fluid migration, which leads to mineral dissolution and to potential preservation of bioapatite eventually as a fossil. We developed dynamic thermodynamic models of apatite mineral stabilities, including hydroxylapatite (HAP), fluorapatite (FAP), and a carbonated fluorapatite (CO₃-FAP), based on the prevailing geochemistry encountered from a range of terrestrial depositional environments (e.g., fluvial, paludal, lacustrine) from which fossils are routinely discovered. Solutions from representative fluvial and paludal systems, across a range pH, temperatures, and total concentration of phosphate ([P]), were undersaturated with respect to HAP and FAP. [P] is limited in most freshwater systems, and is usually bound to organic compounds or sorbed to oxide surfaces. However, under conditions of high dissolved [P] (here, 0.0484 mmol/L), such as in anaerobic pore fluids with changing pH, modeled apatite stability shifted to predicted supersaturation. Thermodynamic predictions suggest that in the waters modeled, a CO₃-FAP phase would be the most stable, even under low [P] and at low temperatures. The results of these models suggest that unless bioapatite undergoes early substitutions forming phases that approach a CO₃-FAP phase, preservation of bone over geologic time should not occur.