THE INFLUENCE OF CARBONATE AND MAGNESIUM-IONS ON THE GROWTH OF HYDROXYAPATITE, CARBONATED APATITE AND HUMAN POWDERED ENAMEL

Human dental enamel can be modelled by a defect apatite in which carbonate ions have been incorporated into the crystal lattice. This study, using the constant composition (CC) kinetics technique, investigates the growth of synthetically prepared carbonated apatites (CAP), stoichiometric hydroxyapatite (HAP), as well as powdered human enamel (HE) seed crystals. The CC technique was modified by the addition of a gas mixing device in order to allow for the quantitative introduction of known amounts of carbon dioxide into the reactant mixture. This provided a constant source of carbonate ions which could then be incorporated into the growing lattice. With all seed materials, an increase in carbon dioxide resulted in a decrease in the rate of growth. This effect was greatest for the HAP and enamel seed crystals but not as pronounced using CAP seed crystals. In the presence of magnesium ions, the growth rate for both HAP and CAP decreased markedly. Thus the addition of magnesium chloride at concentrations of 5.00.10(-3) and 5.00.10(-5) mol l-1 reduced the growth rates of HAP and CAP seed crystals from 7.19.10(-6) to 3.92.10(-7) mol m-2 min-1 and from 2.68.10(-8) to 8.80.10(-9) mol m-2 min-1, respectively.