The fossils of the patch reefs of the San Cassiano Formation (Triassic--northern Italy) still preserve aragonite in their shell. The explanation for the persistence of the metastable polymorph of CaCO 3 has been believed to be the lowering of the porosity and permeability of the sediment by early cementation and the presence of marls (Scherer 1977). The transmission electron microscope (TEM) adds to the understanding of the replacement mechanism. Recrystallization occurs primarily along crystal defects, suggesting that the kinetics of the replacement are controlled mainly by the microstructure of aragonite at least under conditions of slow diagenetic alteration. The replacive calcite in the San Cassiano fossils and the calcite crystals of the cement show a pervasive, modulated microstructure when observed with the TEM. This can be attributed either to compositional variations (calcite contains Mg and Fe), to stacking disorder, or to positional disorder of anions. We believe that these modulations account mainly for the metastability of the San Cassiano calcite. This study further demonstrates that transmission electron microscopy is a powerful technique in investigations of sedimentary rocks and their diagenesis.