An X-ray crystallographic examination of calcium phosphate formation in Ca(OH)2/H3PO4 mixtures

Precipitates formed over a 10 day period by adding variable amounts of Ca(OH)2 to H3PO4 were examined chemically and by X-ray diffraction in order to determine and to relate the crystallographic changes observed during calcium phosphate formation in this system to those seen previously in dental plaque. In the lower Ca(OH)2/H3PO4 ratio mixtures the initial precipitates were amorphous or poorly crystalline. The latter deposits showed an apatite-like diffraction pattern which changed to brushite and decreased in Ca/P ratio from approximately 1.2-1.0. Since the amorphous precipitates showed a similar decrease in the Ca/P ratio and a transient appearance of apatite, it was suggested that these initial precipitates, like the poorly crystalline deposits, may contain brushite amorphous to X-rays. Precipitates from the higher Ca(OH)2/H3PO4 ratio mixtures changed from a poorly crystalline to a more crystalline apatite and the Ca/P ratio remained fairly constant. The relation between the pH, Ca and P on the one hand and the brushite and apatite contents and the pattern of crystallization of the precipitates on the other was strikingly similar to the same realtionship observed earlier in dental plaque in situ, suggesting that the system used in this study could be used to study certain aspects of plaque mineralization in vitro.