Ion substitutions and non-stoichiometry of carbonated apatite-(CaOH) synthesised by precipitation and hydrothermal methods

Abstract

Apatite-(CaOH), either carbonate-free or with different concentrations of carbonate ions of the predominantly (not less than 60%) B type, was synthesised by precipitation from solutions, by hydrothermal methods from solid compounds and by hydrothermal treatment of calcite. In B type apatite, the concentration of CO 3 2 - ions could be up to 20.5 wt.%. The reverse precipitation method and hydrothermal treatment of calcite obtained monophase samples with a maximal concentration of CO 3 2 - ions of 10.3–10.5 wt.%. In apatites of a mixed type, the total concentration of CO 3 2 - ions did not exceed 8 wt.%. The synthesised apatites could be divided into three groups according to the degree and origin of non-stoichiometry: 1. Apatite of nearly stoichiometric composition. Apatite of this type was synthesised by all methods. 2. Calcium-deficient apatite. The Ca deficit was mainly due to CO 3 2 - incorporation. Monophase samples with maximal concentrations of CO 3 2 - ions were synthesised by the reverse precipitation method and by hydrothermal treatment of calcite. 3. Calcium-deficient apatite. The Ca deficit was mainly due to a deficit of OH − ions and resulted in the incorporation of water into channels of the crystal structure. This group was synthesised by reverse precipitation and hydrothermal methods. This apatite was a crystal chemical analogue of the apatite formed in the hard tissues of the human body such as teeth, bone and different stones. Apatites formed by the precipitation methods revealed higher variations in composition (including variations in the concentration of CO 3 2 - ions), compared to hydrothermally-synthesised samples. The observed effect of aqueous solution composition on the ion substitution and apatite composition was stronger than previously reported.