Free Energy of Formation of Carbonate Apatites in Some Phosphate Rocks

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Abstract
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.

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