Phase Relationships in the Ternary System CaO─P 2 O 5 ─H 2 O at 25°C

The portion of the stable phase diagram for the ternary system CaO‐P 2 O 5 ‐H 2 O involving calcium phosphate phases more basic than Ca(H 2 PO 4 ) 2 has been constructed. Construction involves plotting concentrations as their fifth roots to allow phases having very different solubilities to be represented on a single diagram. Hydroxyapatite is shown to be congruently soluble. The stability region for hydroxyapatite includes compositions having Ca/P ratios extending between 1.5 and 1.67. Calcium‐deficient hydroxyapatite forms a stable invariant point with CaHPO 4 and a metastable invariant point with CaHPO 4 . 2H 2 O. Inspection of the diagram indicates that the compounds CaHPO 4 and CaPHO 4 .2H 2 O dissolve incongruently. Incongruent dissolution of either CaHPO 4 or CaHPO 4 .2H 2 O results in the formation of hydroxyapatite during the dissolution process. Such incongruency between these compounds and hydroxyapatite influences their microstructural relationships.