Calcareous (/kælˈkɛəriəs/) is an adjective meaning "mostly or partly composed ofcalcium carbonate", in other words, containinglime or beingchalky. The term is used in a wide variety ofscientific disciplines.

Calcareous is used as an adjectival term applied to anatomical structures which are made primarily of calcium carbonate, in animals such asgastropods, i.e., snails, specifically in relation to such structures as theoperculum, theclausilium, and thelove dart. The term also applies to the calcium carbonatetests of, often, more-or-less microscopicForaminifera. Not all tests are calcareous;diatoms andradiolaria havesiliceous tests.

Themolluscs are calcareous organisms, as are thecalcareous sponges (Calcarea), that have spicules which are made of calcium carbonate.^[1]

Additionally, reef-building corals, orScleractinia, are calcareous organisms that form their rigid skeletal structure through the precipitation ofaragonite (i.e., apolymorph of calcium carbonate).^[2]

Calcareous grassland is a form ofgrassland characteristic of soils containing much calcium carbonate from underlying chalk orlimestone rock.

The term is used in pathology, for example incalcareousconjunctivitis, and when referring tocalcareousmetastasis orcalcareous deposits, which may both be removed surgically.

The termcalcareous can be applied to asediment,sedimentary rock, orsoil type which is formed from, or contains a high proportion of,calcium carbonate in the form ofcalcite oraragonite.

Calcareous sediments are typically deposited in shallow water closer to land, as marine organisms that precipitate calcium carbonate primarily reside within shallow water ecosystems due to an inability to precipitate calcium carbonate at depth (seecarbonate compensation depth). Generally speaking, the farther from land sediments fall, the less calcareous they are, and deviations from this expectation arise if (a) the ocean floor is shallower than the CCD or (b) storms/ocean currents transport calcareous sediments away from their origin point, leading to theinterbedding of calcareous sediments in alternative locations.

An additional form of calcareous marine sediment consists of calcareous ooze, which is a form of calcium carbonate sediment that consists of >30% biogenous material predominantly consisting of organisms such ascoccolithophores andforaminifera.^[3] These oozes form slowly under low-energy environments, and necessitate higher seawater saturation states or a deeper CCD (seesupersaturation andprecipitation vs.undersaturation anddissolution). Therefore, in shallow CCD conditions (i.e., undersaturation of calcium carbonate at depth), stable, non-calcareous sediments such assiliceous ooze orpelagic red clay will prevail in marine sediment records.

Calcareous soils are relativelyalkaline, in other words they have a highpH. They are characterized by the presence of calcium carbonate in the parent material; thecarbonate-ion is a base. Additionally, these soils may have a calcic horizon, a layer of secondary accumulation of carbonates (usually calciumcarbonate or magnesiumcarbonate) in excess of 15% calcium carbonate equivalent and at least 5% more carbonate than an underlying layer.^[4]

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Calcareous deposits can form in water pipes.^[5] An example of this isSunday stone.^[5]

Calcareous coatings, orcalcareous deposits, are mixtures ofcalcium carbonate andmagnesium hydroxide that are deposited oncathodically protected surfaces because of the increasedpH adjacent to the surface.