Free Energy of Formation of Carbonate Apatites in Some Phosphate Rocks

A constant ion-activity product or solubility-product constant for carbonate apatite (KCA) was found within a certain pH range in dilute HCl solutions equilibrated with each of a group of phosphate rocks that had been treated with an alkaline ammonium citrate solution to remove alkaline-earth carbonates. The KCA values generally increased with an increase in the number of moles of carbonate/mole of apatite. As predicted theoretically, the free energies of formation calculated for the carbonate apatites (ΔG°f ,CA) in various phosphate rocks increased linearly with an increase in the number of moles of carbonate/mole of apatite. The apatites in phosphate rocks extensively pre-extracted with HCl had lower KCA values and lower ΔG°f ,CA values than the apatites in phosphate rocks pre-extracted with alkaline ammonium citrate solution. Empirical equations developed to relate the ΔG°f ,CA values to the number of moles of carbonate/mole of apatite make possible the estimation of KCA for other phosphate rocks for which the empirical molecular formula of the apatite is known.