Fossils in thin section
Molluscs: bivalves in thin section
Mostly aragonite, or mixed aragonite and calcite (often a calcite outer layer and aragonite inner layer), with nacreous or cross-laminated microstructure. Aragonite is unstable, so fine debris is often leached and filled or replaced by calcite spar, leaving only the shell outline.

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More photomicrographs
All fossil groups Carbonate classification
Sources
SEPM Strata: Skeletal Grains · Rygel & Quinton, Sedimentary Geology 9.7: Fossils in Thin Section