Calcium carbonate

Crystal structure of calcite
Calcite is the most stable polymorph of calcium carbonate. It is transparent to opaque. A transparent variety called Iceland spar (shown here) was used to create polarized light in the 19th century.
Calcium carbonate chunks from clamshell
Surface precipitation of CaCO3 as tufa in Rubaksa, Ethiopia
Tufa at Huanglong, Sichuan
500-milligram calcium supplements made from calcium carbonate
Travertine calcium carbonate deposits from a hot spring
Electron micrograph of needle-like calcium carbonate crystals formed as limescale in a kettle

Chemical compound with the formula CaCO3.

- Calcium carbonate

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Calcium

Chemical element with the symbol Ca and atomic number 20.

Structure of the polymeric [Ca(H2O)6]2+ center in hydrated calcium chloride, illustrating the high coordination number typical for calcium complexes.
One of the 'Ain Ghazal Statues, made from lime plaster
Travertine terraces in Pamukkale, Turkey

The most common calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life; gypsum, anhydrite, fluorite, and apatite are also sources of calcium.

Agricultural lime

Soil additive made from pulverized limestone or chalk.

A bulk lime spreader operating at Canterbury Agricultural College, 1949

The primary active component is calcium carbonate.

Thermal decomposition

Chemical decomposition caused by heat.

Processes in the thermal degradation of organic matter at atmospheric pressure.

Calcium carbonate (limestone or chalk) decomposes into calcium oxide and carbon dioxide when heated. The chemical reaction is as follows:

Cave

Natural void in the ground, specifically a space large enough for a human to enter.

Lechuguilla Cave, New Mexico, United States
Speleothems in Hall of the Mountain King of Ogof Craig a Ffynnon, a solutional cave in South Wales.
Exploring a lava tube in Hawaii.
Painted Cave, a large sea cave, Santa Cruz Island, California
Salt cave in Mount Sodom, Israel.
Glacier cave in Big Four Glacier, Big Four Mountain, Washington, c. undefined 1920
Domica Cave in Slovak Karst (Slovakia)
An entrance of the Torhola Cave in Lohja (Finland)
Townsend's big-eared bats in a cave in California
Olms in a Slovenian cave
Taíno petroglyphs in a cave in Puerto Rico

Limestone caves are often adorned with calcium carbonate formations produced through slow precipitation.

Eggshell

Outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats.

A broken wild bird eggshell
Butterfly embryo / caterpillar visible through transparent eggshell
This chicken egg has been soaked in vinegar for a few days and has become translucent and flexible.
Anatomy of a chicken egg.
Chicken egg with irregular calcification
Structure revealed by light

The chicken eggshell is 95-97% calcium carbonate crystals, which are stabilized by a protein matrix.

Antacid

Substance which neutralizes stomach acidity and is used to relieve heartburn, indigestion or an upset stomach.

Calcium carbonate antacid tablets
Structural depiction of tetracycline metal chelation, where 'M' is a metal such as those found in antacids

Rarely, long-term use of calcium carbonate may cause kidney stones.

Travertine

Form of terrestrial limestone deposited around mineral springs, especially hot springs.

Travertine terraces at Mammoth Hot Springs, Yellowstone National Park, in 2016
Calcium-carbonate-encrusted, growing moss in a low-temperature freshwater travertine formation (coin for scale)
Badab-e Surt's stepped travertine terrace formations. This travertine owes its red color terraces to iron carbonate.
Mausoleum submerged in a travertine pool at Hierapolis hot springs, Turkey
Soda Dam, Jemez Mountains, New Mexico
Burghausen Castle, Europe's longest castle, is 1,000 years old and built mainly with travertine.
Travertine in a 400-year-old wall
Travertine vessels found in El Tapesco del Diablo Cave in Ocozocoautla, Chiapas, Mexico (600–900 CE)

It is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave.

Seashell

Hard, protective outer layer usually created by an animal that lives in the sea.

Seashells washed up on the beach in Valencia, Spain; nearly all are single valves of bivalve mollusks, mostly of Mactra corallina
Hand-picked molluscan seashells (bivalves and gastropods) from the beach at Clacton on Sea in England
A group of seashells
Mixed shells on a beach in Venezuela
These are some different shells that vary in size, form and pattern combination.
Seashells hand-picked from beach drift in North Wales at Shell Island near Harlech Castle, Wales, bivalves and gastropods, March/April 1985
Shells on the seashore
Single valves of the bivalve Senilia senilis, plus two gastropods, washed up on the beach at Fadiouth, Senegal
Numerous Turritella gastropod shells washed up on a beach at Playa Grande, Costa Rica
Loose valves or plates from Chiton tuberculatus from the beachdrift on the southeast coast of Nevis, West Indies
Cuttlebone from a Sepia sp.
Shells of 3 species of Nautilus
An ocellated (spotted) octopus using a clamshell as a shelter
Marine hermit crab Diogenes pugilator, using a shell of the dog whelk Nassarius reticulatus
A group of purchased (mostly marine) shells includes the shell of a large tropical land snail (upper right), and a shiny freshwater apple snail shell (center)
1742 drawing of shells of the money cowry, Monetaria moneta
A sacred chank shell on the flag of Travancore, India
Spatha shell. From Naqada tomb 1539, Egypt. Naqada I period. The Petrie Museum of Egyptian Archaeology, London
Hindu priest sounding a ritual trumpet made from Turbinella pyrum
Korean military procession with Charonia trumpets
Use of gastropod shells, specifically cowries, in traditional dress of the Kikuyu people of Kenya, Africa
A Pearly King and Queen in London
Enormous seashell sculpture at Akkulam, Thiruvananthapuram, India
Large sculpture of a scallop on the beach at Aldeburgh, by Maggi Hambling, 2003
Aphrodite, 1st century BC, 13 cm, 5 in
Illustration from an 18th-century book, edited by Albertus Seba. These decorative arrangements were a popular way to display seashells at the time
Portrait of the Shell Collector Jan Govertsen van der Aer, by Hendrick Goltzius (1603)
The moulted carapace of a lady crab found on the beach at Long Beach, Long Island, New York State
Shell of horseshoe crab on a beach
Sea urchin test
A whole animal of the brachiopod Lingula anatina from Australia with the shell showing on the left
Dish with beachworn coral pieces, marine gastropod shells, and echinoderm tests, from the Caribbean and the Mediterranean
An x-ray photograph of a gorgonian
Marine diatoms form hard silicate shells

A seashell is usually the exoskeleton of an invertebrate (an animal without a backbone), and is typically composed of calcium carbonate or chitin.

Calcite

Crystal structure of calcite
Demonstration of birefringence in calcite, using 445 nm laser
One of several calcite or alabaster perfume jars from the tomb of Tutankhamun, d. 1323 BC
Calcite with mottramite
Trilobite eyes employed calcite
Calcite crystals inside a test of the cystoid Echinosphaerites aurantium (Middle Ordovician, northeastern Estonia)
Rhombohedrons of calcite that appear almost as books of petals, piled up 3-dimensionally on the matrix
Calcite crystal canted at an angle, with little balls of hematite and crystals of chalcopyrite both on its surface and included just inside the surface of the crystal
Thin section of calcite crystals inside a recrystallized bivalve shell in a biopelsparite
Grainstone with calcite ooids and sparry calcite cement; Carmel Formation, Middle Jurassic, of southern Utah, USA.
Several well formed milky white casts, made up of many small sharp calcite crystals, from the sulfur mines at Agrigento, Sicily
Reddish rhombohedral calcite crystals from China. Its red color is due to the presence of iron
Cobaltoan, the cobalt-rich variety of calcite
Sand calcites (calcites heavily included with desert sand) in South Dakota, USA
Calcite, butterfly twin, {{nobr|4,0 × 3,3 × 1,6 cm}}. José María Patoni, San Juan del Río, Durango (Mexico)

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3).

Aragonite

Aragonite from Salsigne mine, Salsigne, Aude, France; size: 30×30×20 cm
Aragonite crystals from Cuenca, Castile-La Mancha, Spain
Aragonite crystal cluster from Spain
Remnant biogenic aragonite (thin, rainbow-colored shell) on the ammonite Baculites (Pierre Shale, Late Cretaceous, South Dakota)
Scanning electron microscope image of aragonite layers in the nacre of a blue mussel (Mytilus edulis)
Fluorescence of aragonite
Aragonite crystal from Los Molinillos, Ceunca, Spain

Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, CaCO3 (the other forms being the minerals calcite and vaterite).