Summary: Phase equilibria involving calcite, apatite, portlandite, liquid, and vapour were determined at 1000 bars. Calcite, apatite, and portlandite melt at 654 °C to form a liquid of composition (wt %), Ca(OH)₂ 53 %, CaCO₃ 46 %, Ca₃(PO₄)₂ 1 %. With water present melting occurs at 635 °C to form a similar liquid with a low water content. The X-ray diffraction patterns of the equilibrium apatites were not significantly different from synthetic hydroxyapatite.