The possible oriented growth of one crystalline phase on the surface of another is especially important in systems containing both phosphate and sulfate salts of calcium. Whether the overgrowth results from a true epitaxial relationship is dependent on factors such as the thermodynamic driving forces and the free energies of the surfaces. Despite the fact that calcium sulfate dihydrate (CSD, gypsum) and calcium hydrogen phosphate dihydrate (DCPD, brushite) show many crystallographic and structural analogies, their surface reactions are quite different. The nucleation and growth of gypsum on brushite surfaces has been investigated in supersaturated solutions of calcium sulfate dihydrate at 25.0 degreesC using the constant composition (CC) method. During the kinetics experiments, the harvested solid phases were examined by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDS). Induction periods, tau, preceding the initial formation of gypsum crystals at the brushite surfaces, varied markedly with relative supersaturation, sigma. A thin layer wicking method was used to investigate the interfacial foe energies of the growing phases, and these data were also calculated from the kinetics results. The interfacial free energy, gamma, estimated from initial growth rates was 8.4 mJ m(-2), while that calculated from the induction times was 8.9 mJ m(-2). These values were in agreement with those determined directly using thin layer wicking. (C), 2001 Published by Elsevier Science B.V.