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Free Energy of Formation of Carbonate Apatites in Some Phosphate Rocks¹

ABSTRACT

A constant ion-activity product or solubility-product constant for carbonate apatite (K_CA) 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 K_CA 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 (G°_{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 K_CA values and lower G°_{f ,CA} values than the apatites in phosphate rocks pre-extracted with alkaline ammonium citrate solution. Empirical equations developed to relate the G°_{f ,CA} values to the number of moles of carbonate/mole of apatite make possible the estimation of K_CA for other phosphate rocks for which the empirical molecular formula of the apatite is known.

¹ Journal Paper No. J-7934 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA 50011, Project 1868. Contribution from the Dep. of Agronomy.

² Former Graduate Assistant and Professor. The senior author is now a Research Chemist, International Fertilizer Devel. Ctr., Florence, Alabama.

Received for publication March 24, 1975. Accepted for publication December 17, 1975.

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