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Trabajo presentado en el 14th International Symposium on Biomineralization (BIOMIN XIV) from Molecular and Nano-structural Analyses to Environmental Science, celebrado en Tsukuba (Japón), del 9 al 13 de octubre de 2017, Fossil carbonate skeletons of marine organisms are proxies for understanding paleoenvironment development and evolution. However, the correct assessment of past environment dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. In order to understand the effect of different biomineral microstructures (fibrous, prismatic, columnar, nacreous) on the extent and mode of diagenetic alteration of the pristine skeleton we subjected modern hard tissues of the coral Porites sp., the bivalves Arctica islandica and Mytilus edulis and that of the gastropod Haliotis ovina to laboratory-based, hydrothermal alteration series. Experiments were conducted with Mg-rich fluids simmulating burial water, at temperatures between 100 °C and 175 °C that lasted between one and 35 days. Carbonate phase analysis, microstructure characterisation and biomineral grain size evolution was determined with XRD, Rietveld analysis, electron backscatter diffraction (EBSD) and statistical grain size evaluation. Alteration at 100 °C for 28 days destroys most biopolymer matrices, while shell, coral aragonite and calcite as well as the characteristic biomineral microstructures are still preserved. Below 175 °C there are no signs of replacement of bioaragonite by calcite. This starts at 175 °C, after a dormant time period of four to 7 days and proceeds at different rates for the different biomineral microstructures. While for Arctica islandica (Casella et al. 2017, Biogeocsiences) the aragonite to calcite replacement is complete after 10 days, in Mytilus edulis, Haliotis ovina and Porites. sp. only a small amount of bioaragonite is transformed to calcite, even after 35 days of alteration. Most resistant are the two nacreous microstructures, columnar nacre in Haliotis ovina and sheet nacre in Mytilus edulis. Our results show that the bioaragonite to calcite transition occurs via an interface-coupled dissolution-reprecipitation mechanism. Statistical grain size evaluation reveals that pristine bioaragonite grains amalgamate, prior to new calcite formation, to larger units.