Low-temperature, hydrothermal transformation of aragonite to hydroxyapatite

A Porites coral skeleton (aragonite polymorph of CaCO3) was converted hydrothermally to carbonate-containing hydroxyapatite (HA) by replacement of the CaCO3 in acidic phosphate solutions, kept at 160–200 °C for 4–7 h in a PTFE-lined, stainless-steel bomb. The pressure inside the reactor was self-generated by water vapour. The phases of the products obtained were characterized by XRD and FT-IR, and their morphologies by SEM. Under the present experimental conditions, it was found that CaCO3 topotaxially reacted with H2PO4− ions in the solution to form CaHPO4 single-crystal plates (10–30 μm), stacked with their flat sheets parallel to each other. The stacks of CaHPO4 plates were randomly orientated, inherited from the interlocking of the original, nanocrystalline aragonite crystals (0.1–0.5 μm). Despite the great difference in the crystal size of the CaHPO4 and the aragonite plates, the observed morphology might help to retain the original coral structure. In addition to the platy crystals, there were clusters of CaHPO4 whiskers with hexagonal cross section (∼0.2 μm in diameter) growing on the surfaces of some plates, which indicated that recrystallization had taken place. Further hydrothermal treatment of the CaHPO4-converted coral with Ca(OH)2 solution to increase the Ca/P molar ratio close to that of HA, transformed the CaHPO4 to rod-shaped HA crystals of nanoscale order (<0.5 μm). In the case of CaHPO4 plates, the c-axes of HA rods formed were oriented perpendicular to the c-axes of the plates, and in the case of CaHPO4 whiskers, the c-axes of HA rods were radiating out from the stems of the whiskers. From this observation, we suggest that the transformation of aragonite to HA via CaHPO4 route, to some extent, involves a dissolution–recrystallization mechanism.