Abstract
ABSTRACT 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 CaCO3 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.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.