The hydrothermal transformation of single aragonite crystals into polycrystalline calcite has been studied under hydrothermal conditions. The transformation involves a fluid-mediated replacement reaction, associated with fracturing of the initial aragonite crystal and growth of calcite throughout various parts of the reacted aragonite. The observed overall preservation of the crystal morphology is typical of an interface-coupled dissolution- reprecipitation mechanism. Analysis of the crystallographic orientation of the product calcite using electron backscatter diffraction (EBSD) showed little to no link between the two phases under the studied conditions, with calcite crystallites exhibiting dominantly different crystallographic orientations compared to those of the aragonite and of neighbouring calcite domains. The complexity of the observed textures is mainly a result of the combination of fracturing of the crystal and initiation of nucleation and growth at different points of the exposed aragonite surface. Experiments performed with solutions enriched in 18O as a tracer for oxygen exchange and monitored by Raman spectroscopy, showed that carbonate ions in the starting solution are mixed with carbonate from the dissolving aragonite, resulting in an 18O concentration in the product calcite which depended on the local fluid transport through the fractures. As replacement processes among the CaCO3 phases are relevant to a wide range of applications, understanding the mechanisms is essential for the interpretation of observations of natural and/or experimental samples. This study describes the interplay between the replacement process, the fracturing of the crystal and growth of the new phase, calcite, and provides new insights into the mechanism of the aragonite to calcite transition. The combination of the two methods, EBSD and Raman spectroscopy, demonstrates the importance of local fluid composition and transport pathways in determining the isotope and element exchange in mineral replacement reactions.