Кораловий вапняк

Coral reefs and their skeletons

Coral reefs are formed of colonies of coral polyps held together by calcium carbonate. Unlike sea anemones, corals secrete hard carbonate exoskeletons that support and protect the coral. Coral reefs first appeared 485 million years ago, at the dawn of the Early Ordovician.

A fossilized reef in Florida

The Key Largo Limestone is a geologic formation in Florida and a fossilized coral reef. The limestone includes fossils of corals, mollusks and bryozoans.

Formation

Limestone forms when calcite and aragonite precipitate out of water containing dissolved calcium. This can happen through biological and nonbiological processes, but biological ones, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera, which secrete structures of aragonite or calcite and leave them behind when they die. Coral grains are more common in high-energy environments, characterized by strong currents and turbulence.

Composition

Calcite in limestone is divided into low-magnesium and high-magnesium calcite, with the dividing line placed at a composition of 4% magnesium. Coquina is a poorly consolidated limestone composed of abraded pieces of coral, shells, or other fossil debris. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.

Uses

Limestone is used as a chemical feedstock for the production of lime used for cement, as aggregate for the base of roads, and as white pigment or filler in products such as toothpaste or paint. It is also used as a soil conditioner and as a popular decorative addition to rock gardens.

Explore all seven extraits in the Discovery Set.

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