INTERLAYERING OF CRYSTALLINE OCTACALCIUM PHOSPHATE AND HYDROXYLAPATITE

Hydroxylapatite (OHAp), Ca10(P04)6(OH)2, and octacalcium phosphate (OCP), Ca8H2(PO4)6·5H2O, form interlayered mixtures in which some of their h00 X-ray diffraction peaks interact to form combination peaks. Calculations reveal that the d200(OCP)-d100(OHAp) peak shifts toward lower spacings from 9.34 Å, the d200 of OCP, to 8.16 Å, the d100 of OHAp, as the Ca/P ration increases from 1.33 to 1.67. The d500(OCP)-d200(OHAp) and the d700(OCP)-d300(OHAp) combination peaks shift to higher spacings as the Ca/P ratio of the interlayered mixture increases. The calculated shifts in the d200(OCP)-d100(OHAp) peak are corroborated by experimental data in the literature. Although shifts in the X-ray diffraction peaks are common in clays, this is the first time this phenonmenon has been observed in calcium phosphates; it provides a new approach to the study of tooth and bone mineral in their formative stages when OCP is most likely to be present. © 1979 American Chemical Society.