Skeletons, fossils and calcium compounds

Calcium Calcium is a relatively reactive metal and so is normally found combined with other elements in compounds.

Bones and calcium The adult skeleton contains around 1 kg of calcium, as calcium phosphate Ca 3 (PO 4 ) 2. This explains why it is important to have a good source of calcium in the diet.

Calcium and eggs Calcium carbonate. It is the major component of eggshells.

Many animals also have shells composed of calcium carbonate.

Crystal form and structure. Over most of a shell crystals of calcium carbonate are columnar. If they form thin plates a very different material results, mother of pearl.

Corals also have skeletons of calcium carbonate.

Calcium carbonate and rocks. Limestone is also largely composed of calcium carbonate. Bath Stone (Greater Oolite) is a limestone that was laid down in the Jurassic period, c180 – 140 million years ago. It is composed of many tiny ooliths, egg stones, formed by chemical precipitation from warm tropical seas coating small fragments of shell in layers of lime

Many hot springs also precipitate a form of limestone, known as travertine, as their waters cool.

Bioclastic limestones Bioclastic (broken life) limestones are full of fossils. Eg; Mountain Limestone from the Mendip Hills that formed in tropical waters c340 million years ago.

Chalk Chalk is almost pure calcium carbonate, made up of the microscopic shells of plankton.

Marble Under high temperature and pressure calcium carbonate in limestone recrystallises to form marble.

Fossils Fossils are the remains of prehistoric plants and animals. Recent fossils are the actual remains of shells and bones. They have been almost unchanged by burial.

Petrification Many older fossils have been petrified, turned to rock. Mineral rich waters percolating through the fossil having replaced its minerals with new compounds. In petrified wood preservation can be so perfect that the details of each cell can be made out.

Casts and moulds. In most fossils the original material has been entirely dissolved away, leaving a cavity in the rock. These cavities act as moulds, which are infilled by other minerals to form casts.