Amorphous calcium phosphate (ACP) is one of the major components of biological mineralisation and plays a crucial role in several biomaterials. However, due to their instability, the thermochemical characteristics of ACP and of the derived metastable phases are still unknown. The thermal transformations affecting ACP, with a Ca/P atomic ratio close to 1.50, were studied in the range 373-1173 K. The samples were examined by X-ray diffraction, FTIR spectroscopy and BET measurements. The thermal behaviour was analysed by thermogravimetric and differential thermal analysis. The dissolution enthalpies of the products, in 9 wt% aqueous nitric acid solution, were determined using an isoperibolic calorimeter. The samples remained amorphous up to 773 K. The evolution of the dissolution enthalpy between 298 and 573 K was attributed to the irreversible loss of adsorbed water. The adsorption enthalpy at 298 K of liquid water on ACP was estimated at 212.3 kJ mol 21 . At 673 K, the variation of the dissolution enthalpy corresponded to the occurrence in the amorphous phase of a more stable ionic organisation. The nucleation of the crystalline phase was believed to begin at 773 K. At 873 K and above, the ACP crystallised mainly into metastable a-tricalcium phosphate (a-TCP). Though the enthalpy of dissolution of the formed a-TCP varied with temperature, it remained lower than that of the most stable tricalcium phosphate crystalline structure: b-TCP. This phase was formed at 1173 K. The thermal measurements confirm the high instability of ACP compared to other calcium phosphates, including apatites. These properties, plus the very high specific surface area, explain the very fast transformation of ACP into apatite in aqueous media and justify its use in new types of self-setting cements.

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Labile environments of carbonate and HPO4 present in synthetic poorly crystalline apatites, analogous to bone mineral crystals, can be easily and reversibly exchanged. Such reactions depend on the maturation stage of the apatites and are related to both the short range order and organization of the crystals and their specific surface area. Similar reactions may also occur in bone mineral altering its properties and function as an ion reservoir in vivo. INTRODUCHONPoorly crystalline apatites are the major constituent of mineralized tissues of vertebrates. Their reactivity governs several biological processes such as the adsorption of mineral ions, matrix and serum proteins, adhesion of cells (example osteoclasts), and the subsequent resorption of bone. The reactivity of poorly crystalline apatites and consequently their biological functions are directly related to the structure, short range order and compositional characteristics of the crystals.Poorly crystalline apatites contain significant amounts of carbonate and phosphate groups in non-apatitic environments which can be demonstrated by FTIR and 31-P MAS-NMR. Many of the CO32-and HPO42-groups are labile and in non-apatitic environments. These are present at higher concentrations in newly formed crystals and in the bone of young animals, and gradually diminish with time (maturation). The aim of this work was to study the role of non-apatitic labile environments in the reactivity of poorly crystalline apatites. MATERIALS AND METHODSPoorly crystalline apatites were prepared according to the methods published in previous papers 1. Their composition was determined by chemical analysis 1 of calcium, phosphate and carbonate ions. The exchange experiments were made by exposing the poorly crystalline apatites (100 mg) in ammonium bicarbonate or ammonium hydrogen phosphate 1 molar (10 ml) . The samples were then centrifuged, washed with distilled water and lyophilized. The environments of mineral ions were analyzed by FTIR spectroscopy