Calcite

Calcite is calcium carbonate, one of the most common minerals in the crust and the main constituent of limestone, marble and chalk. It is soft, hardness 3, and splits cleanly in three directions into rhombs, so a broken fragment is itself a tidy rhombohedron. Its optical importance is real: clear calcite, the transparent variety known as Iceland spar, is strongly birefringent, splitting a single image into two, and it was used in nineteenth- and twentieth-century polarising instruments; some historians have argued, on disputed evidence, that a spar sunstone could have helped Norse navigators locate the sun through cloud. Calcite occurs in many colours through trace impurities and is a frequent subject of the modern crystal trade, where the different colours are assigned to different chakras, a twentieth-century scheme, not a traditional one.

Much of what is called Egyptian alabaster in museum collections and in the vessels and canopic jars of Egyptian burial practice is in fact calcite, specifically the banded travertine form long quarried at Hatnub in Middle Egypt, and not true alabaster, which is gypsum; the two very different minerals share only a milky translucence and a shared trade name that has caused confusion since antiquity. Pure white calcite marble was likewise the material of choice for Mediterranean sculpture, quarried above all at Carrara in Italy, a quarry still active and still the source most closely associated with the tradition running from Roman statuary to Michelangelo.

Calcite is calcium carbonate, and by any measure of how much of the earth's surface it accounts for and how much of it was made by living things, it is the most consequential mineral there is.

Limestone is calcite. So is chalk, which is calcite in the form of the microscopic plates of coccolithophores that settled on a Cretaceous sea floor in numbers beyond counting. Marble is limestone recrystallised by heat and pressure. Stalactites and stalagmites are calcite deposited from solution. Most shells, most coral skeletons and a great deal of the sediment on the ocean floor are calcite. The mineral is the principal reservoir in the carbon cycle over geological time.

It is identified in seconds. Calcite fizzes in dilute acid, releasing carbon dioxide, and it has a hardness of three, meaning a copper coin will scratch it and a steel knife certainly will. Those two tests separate it from almost everything it might be confused with.

That softness and solubility govern everything built from it. Limestone and marble buildings dissolve slowly in rain, faster in acidified rain, and the weathering of carved detail on a medieval cathedral is calcite going into solution. Karst landscapes, sinkholes and cave systems are the same process at scale.

Its optical behaviour, the strong double refraction that makes a line viewed through a clear rhombohedron appear twice, is treated in the other article on this stone. It is worth noting only that the optical property and the geological importance belong to the same crystal structure.