Question

What organic materials make up limestone? What type of mineral and what percent of it determine if a rock is limestone? (Site 2)

Answer 1

Organic (biogenic) limestones are made mostly of the calcium‑carbonate shells and skeletons of marine organisms. Common constituents include:
- Shells and shell fragments of mollusks (bivalves, gastropods) and brachiopods
- Corals and coral fragments
- Foraminifera (microscopic shelled protists) and their tests
- Coccoliths (calcareous nannoplankton) and calcareous algae
- Bryozoans and echinoderm (crinoid) skeletal debris
These biogenic pieces may be cemented together or recrystallized into a fine carbonate mud (micrite).

The diagnostic mineral for limestone is calcium carbonate — typically calcite (CaCO3); aragonite is the original biogenic form in many shells but commonly alters to calcite. A rock is classified as limestone when a majority of it (generally at least ~50% by volume or weight) is composed of calcium‑carbonate minerals (calcite/aragonite). (If the rock is >50% dolomite [CaMg(CO3)2] it is called a dolostone.)