STOICHIOMETRY OF K-CONTAINING AND CO32--CONTAINING APATITES PREPARED BY THE HYDROLYSIS OF OCTACALCIUM PHOSPHATE()

In this study the composition of K+- and CO32--containing hydroxyapati tes (KCAp's) obtained by the hydrolysis of octacalcium phosphate (OCP) , Ca-8(HPO4)(2)(PO4)(4) . 5H(2)O, was investigated as a function of th e carbonate concentration c(kc) and the molar K/CO3 ratio R of the aqu eous solution. OCP was hydrolyzed at 95 degrees C in K+- and CO32--con taining solutions with 2.5 less than or equal to c(kc) (mM) less than or equal to 100 and 2 less than or equal to R less than or equal to 20 . Physical and chemical analyses showed that the precipitates were car bonated apatites with a carbonate content ranging from 2.5 to 5.5 wt % . The samples also contained a significant amount of monohydrogen phos phate (1.3-2.2 wt %) and an appreciable amount of water (3.5-5.5 wt %) after drying at 25 degrees C under vacuum. The chemical composition o f the KCAp's can be explained by the occurrence of a main substitution mechanism [I: Ca2+ + PO43- + OH- <-> V-Ca + CO32- + V-OH] and a coupl ed K+ and CO32- incorporation [III: Ca2+ + PO43- <-> K+ + CO32-] where V-X stands for a vacancy in the X sublattice. The relative contributi on of mechanism III with respect to mechanism I increases with increas ing c(kc) and R.