The replacement of aragonite by apatite is a process that occurs naturally during diagenesis, chemical weathering and natural hydrothermal reactions and is artificially promoted in medical sciences for use of the product material as a bone implant. We have investigated the mechanism and the kinetics of this replacement by using biogenic aragonite (cuttlebone of the Sepia officinalis) as a starting material and reacting it with di-ammonium hydrogen phosphate solution. Isothermal experiments were carried out over a range of temperatures up to 190 degrees C. Quantification of each solid phase, for different reaction times, was obtained by the Rietveld analysis of powder X-ray diffraction patterns. An empirical activation energy was calculated by using two different approaches to analyze the data. Scanning electron microscopy showed that the fine structure of the cuttlebone was perfectly retained even after aragonite had been completely converted to apatite. We present a detailed investigation of the kinetics of a reaction that involves interaction of a solid phase with an aqueous fluid and leads to a pseudomorphic replacement of the initial solid phase by a new, chemically different, phase. This replacement process is described in terms of an interface-coupled dissolution-reprecipitation mechanism. (C) 2010 Elsevier B.V. All rights reserved.