Calcite occurs as stalactites and stalagmites in caverns and is also found in mineral deposits in hot springs. It constitutes the shells of marine organisms such as bivalves and echinoderms.
Calcite is found in sedimentary rocks, primarily limestone, which is made up of the shells of dead marine organisms.
In limestone caves, rainwater trickles over rocks and picks up carbon dioxide and other minerals from limestone. This combination creates calcium bicarbonate, which is essentially a dissolved version of calcite, in the water. The water drips from the ceiling and leaves behind calcite that becomes a stalactite over time. A stalagmite forms when the stalactite drips water onto the cave floor and the water deposits calcite in a rising pile over time.
Calcite is present in the shells of clams, corals, crabs and other marine organisms. It is also present in the hard parts of red algae, some tube worms and some sponges. It is found in volcanic rocks or mantle rocks such as kimberlite, carbonatites and peridotites.
The Snowy River Cave in New Mexico contains a stream bed of white calcite, which is attributed to microorganisms and groundwater dissolving the walls of the cave over time.
The biggest calcite crystals ever recorded were discovered in Iceland and weighed about 250 tons.