CRINOIDAL LIMESTONE

A sedimentary rock with coarse grains which contains the fossilized remains of sea creatures (crinoids). It may be distinguished from other limestones due to the presence of large quantities of fossilized creatures. It is white or grey and contains calcium carbonate which is derived from remains or from sea water. Some calcium carbonate is broken down to form a cement which binds the larger crystals and broken bits of crinoids together. Although it is mainly made up of crinoidea it may also contain some brachiopods, corals or molluscs along with some chert, mud and silt. Crinoids are related to starfish and sea urchins. Today there are over 700 species but those found in calcium carbonate lived 230-600 million years ago.

Crinoidal Limestones are sometimes found in deposits of coral limestones. Coral lives in warm coastal waters therefore suggesting that crinoidal limestones formed in a shallow marine environment where the bodies of dead sea creatures accumulated.

Crinoidal limestone may form as a bed or as a reef formation.

It may be found in Austria, Nile Valley (Egypt), Derbyshire, Durham, North Yorkshire and Somerset (UK).

DIOPSIDE

A silicate of calcium and magnesium which may sometimes contain impurities of iron oxide. It is usually pale green but may also be brown, grey or yellow. It is a member of the pyroxene group and is related to jadeite and augite. There are two different types of pyroxene which have different crystal structures (clinopyroxenes and orphopyroxenes). Clinopyroxenes always contain calcium, whereas orphopyroxenes contain little or no calcium. (Diopside is a clinopyroxene)

It is moderately hard (5.5-6.5 on the mohs scale) and is therefore limited in it's use as a gemstone. The finest samples are used in jewellery.

It derives it's name from the Greek meaning two appearances. It was named Diopside because it appears two different colours depending on the angle from which it is viewed. This is known as pleochroism.

Diopside, along with other pyroxenes has an unusual characteristic. It has two angles of cleavage which intersect at right angles to each other.

It is widespread and is mainly found in metamorphic rock. It may be found in countries such as Austria, Brazil, Canada, Finland, India, Sweden and USA (California and Montana). It may also be found in meteorites.