One, an older "morphological" unit cell, was inferred by measuring angles between faces of crystals, typically with a goniometer, and looking for the smallest numbers that fit.

[4][10] The diagnostic properties of calcite include a defining Mohs hardness of 3, a specific gravity of 2.71 and, in crystalline varieties, a vitreous luster.

Color is white or none, though shades of gray, red, orange, yellow, green, blue, violet, brown, or even black can occur when the mineral is charged with impurities.

[14] Single calcite crystals display an optical property called birefringence (double refraction).

[17] An experiment was conducted by adding activators such as ions of Mn, Fe, Co, Ni, Cu, Zn, Ag, Pb, and Bi to the calcite samples to observe whether they emitted heat or light.

[17] By changing the temperature and observing the glow curve peaks, it was found that Pb2+and Mn2+acted as activators in the calcite lattice, but Pb2+ was much less efficient than Mn2+.

When conditions are right for precipitation, calcite forms mineral coatings that cement rock grains together and can fill fractures.

When conditions are right for dissolution, the removal of calcite can dramatically increase the porosity and permeability of the rock, and if it continues for a long period of time, may result in the formation of caves.

Continued dissolution of calcium carbonate-rich formations can lead to the expansion and eventual collapse of cave systems, resulting in various forms of karst topography.

[26] Ancient Egyptians carved many items out of calcite, relating it to their goddess Bast, whose name contributed to the term alabaster because of the close association.

Identifying the sun's location would give seafarers a reference point for navigating on their lengthy sea voyages.

[36] Furthermore, due to its particular crystal habit, such as rhombohedron, hexagonal prism, etc., it promotes the production of PCC with specific shapes and particle sizes.

[36] Calcite, obtained from an 80 kg sample of Carrara marble,[37] is used as the IAEA-603 isotopic standard in mass spectrometry for the calibration of δ18O and δ13C.

However, artificial calcite is the preferred material to be used as a scaffold in bone tissue engineering due to its controllable and repeatable properties.

Lakes and rivers can lead to cyanobacteria blooms due to eutrophication, which pollutes water resources.

[40] As an active capping material, calcite can help reduce P release from sediments into the water, thus inhibiting cyanobacteria overgrowth.

[40] Calcite is a common constituent of sedimentary rocks, limestone in particular, much of which is formed from the shells of dead marine organisms.

It also occurs in deposits from hot springs as a vein mineral; in caverns as stalactites and stalagmites; and in volcanic or mantle-derived rocks such as carbonatites, kimberlites, or rarely in peridotites.

Their death releases these biominerals into the environment, which subsequently transform to calcite via a monohydrocalcite intermediate, sequestering carbon.

[41][42] Calcite is often the primary constituent of the shells of marine organisms, such as plankton (such as coccoliths and planktic foraminifera), the hard parts of red algae, some sponges, brachiopods, echinoderms, some serpulids, most bryozoa, and parts of the shells of some bivalves (such as oysters and rudists).

Calcite is found in spectacular form in the Snowy River Cave of New Mexico as mentioned above, where microorganisms are credited with natural formations.

Trilobites, which became extinct a quarter billion years ago, had unique compound eyes that used clear calcite crystals to form the lenses.

[47] Calcite formation can proceed by several pathways, from the classical terrace ledge kink model[48] to the crystallization of poorly ordered precursor phases like amorphous calcium carbonate (ACC) via an Ostwald ripening process, or via the agglomeration of nanocrystals.

A neutral starting pH during mixing promotes the direct transformation of ACC into calcite without a vaterite intermediate.

But when ACC forms in a solution with a basic initial pH, the transformation to calcite occurs via metastable vaterite, following the pathway outlined above.

Lineages evolved to use whichever morph of calcium carbonate was favourable in the ocean at the time they became mineralised, and retained this mineralogy for the remainder of their evolutionary history.

[54] The evolution of marine organisms with calcium carbonate shells may have been affected by the calcite and aragonite sea cycle.

[56] Climate change is exacerbating ocean acidification, possibly leading to lower natural calcite production.

[57] Calcifying organisms in the sea, such as molluscs foraminifera, crustaceans, echinoderms and corals, are susceptible to pH changes.

As ocean acidification causes pH to drop, carbonate ion concentrations will decline, potentially reducing natural calcite production.