Calcite
Calcium carbonate — the mineral of limestone, marble, chalk and most shells, and the reference point for hardness 3.
Calcite is calcium carbonate, and it is the single most important mineral in the built environment — not in itself, but because limestone and marble are made of it, and because burning it produces the lime that makes mortar and cement possible.
It is also the mineral that most reliably betrays itself: a drop of dilute acid on calcite fizzes vigorously as carbon dioxide is released. Few identification tests in mineralogy are that unambiguous.
How to identify it
Three tests, any one of which is close to conclusive. It effervesces in cold dilute hydrochloric acid. It cleaves into rhombs — break it and the fragments are skewed boxes, not random shapes. And it is soft enough that a copper coin will scratch it.
Clear calcite also shows strong double refraction: text viewed through a cleaved rhomb appears doubled, an effect striking enough that it was noticed and described in the seventeenth century.
How it forms
Most calcite is biological in origin. Marine organisms — corals, molluscs, foraminifera, coccolithophores — build shells and skeletons of calcium carbonate, and when they die those remains accumulate on the seabed and lithify into limestone. Chalk is almost entirely the plates of microscopic algae.
Calcite also precipitates directly from solution: as travertine around springs, as the stalactites and flowstone of caves, and as the cement binding many sandstones. Recrystallised under heat and pressure, limestone becomes marble, which is calcite with the fossils destroyed and the grains grown coarse.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is a component of
- Limestone — rock · 50–100%
- Marble — rock · 90–100% · recrystallised calcite; the interlocking texture is what lets marble take a polish
- Sandstone — rock · as cement between the grains in calcareous sandstones
- Paper — material · ground calcite as a filler, for opacity and printability
- Skarn — rock · 5–40% · relict from the limestone the skarn replaced, surviving where the reaction ran short of silica
- Carbonatite — rock · 40–90% · the dominant mineral of a calcite carbonatite, and the reason the rock was mistaken for marble for so long
- Shale — rock · as a cement between the grains, and abundant enough in the calcareous shales to grade into marl
- Precious coral — material · high-magnesium calcite, and this is the mineralogical line between precious coral and the reef corals, which build in aragonite
- Chalk — rock · 90–99% · 95 per cent or more, and almost all of it as coccoliths — interlocking calcite plates a few micrometres across, secreted by single-celled algae
is commonly confused with
- Quartz — mineral · both common, colourless and vitreous; separated instantly by hardness and by acid
- Precious coral — material · dyed calcite and dyed howlite are sold as coral constantly; real coral shows the fine longitudinal growth striations of a branch, and dyed stone shows colour concentrated in cracks
is a polymorph of
- Aragonite — mineral · both CaCO3; orthorhombic against trigonal, and aragonite converts to calcite over geological time Wikidata
is used as
- Pigment — application · ground calcite is the white filler and extender in most paint
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain