J. inorg, nucl. Chem., 1967, Vol. 29, p. 2127. Pergamon Press Ltd. Printed in Northern Ireland
LETTER TO THE EDITORS
Defect apatites THE RECENTpaper by ]JERRYm provides further confirmation of results previously published by other investigators. ~-~ Unfortunately, Berry misinterpreted and misquoted our published results, thus giving the impression that there is a serious disagreement between his and Winand's data on the one hand and ours on the other. In order to dispel any doubts of the validity of the defect apatite formula and to further its general acceptance a clarification appears to be desirable. We have shown that a defect apatite prepared by Dallemagne's method (Ca/P = 1.50) had the formula Cao(HPO4)(PO4)5OH rather than Cag(HPO,)2(PO4)4(OH)2 and was not a mixture ofhydroxylapatite and dicalcium phosphate. The thermal reaction of the HPO~-group was an important factor in establishing this formula. Berry's contention that a calcination at 560°C for 48 hr does not convert all HPO4 into P20¢ is correct and has been taken into account. Minor, but significant amounts of H P O 4 2 - a r e converted into PaO105- and (PO3-)~. Prolonged calcination eventually yields P~O~'- by reaction of these intermediates with PO48- as explained in our paper. ~4~ However, the decomposition of HPO~ 2- groups is complete after a 48-hr calcination at 560°C. Berry's analytical method did not permit the determination of P80105- and (PO3-)~, but only that of P2074-; he was, therefore, forced to calcine for a longer time, after which all originally present HPO4 ~- was converted into P2074-. The separation of the phosphate species by paper chromatography and their individual determination allowed us to calculate the percentage of pyrophosphate being formed by a prolonged calcination. The results showed that 16.7 per cent of the total phosphorus was present as HPO42- before the calcination, thus giving the above mentioned formula. Further investigation of a great number of defect apatite samples having a wide range of Ca/P ratios provided sufficient evidence to firmly establish the general formula: Cal0_~(HPO,)~(PO,)6_~(OH)2_~, which was published by us at the 36th Annual Meeting of the International Association for Dental Research in March, 1959, (see also Ref. 2). The great majority of our products belonged to this series of defect apatites, and we are in complete agreement with the results later obtained by Winard and recently by Berry. In contrast to the many samples belonging to this regular series of defect apatites, we obtained a small, but significant, number of products whose composition did not agree with the above mentioned formula. As interpretation for these results we suggested the presence of "second order defects" giving rise to the extended formula: Calo. . . . (HPO,),(PO4)6-,(OH)2-,-,,, where y _< 1 -- x[2. These findings were reported in a separate short paragraph, and it should be understood that they are the exception from the rule, i.e. that y is normally equal to 0. We feel sure, nevertheless, that the results are real, especially since we found similar deviations from the general formula of carbonate defect apatites, m G. KOHL Mobil Oil Corporation Central Research Division Princeton, New Jersey 08540 m E. E. BERRY,J. inorg, nacl. Chem. 29, 317 (1967). ~ W. I-I. NEBERGALLand G. K0nL, Naturwissenschaften 47, 254 (1960). ~8~L. W~AND, Annls Chim. 6, 941 (1961). t4~ G. KOHL and W. H. NEaERGALL,Z. anorg, allg. Chem. 324, 313 (1963). 2127