A silicate of magnesium and iron which usually forms in grains or granular aggregates and is fairly hard (6.5-7 on the mohs scale). The finest specimens are clear light green in colour although it may also be black, brown or yellow. (Sometimes oxygenation may turn it red) Gem quality specimens are known as peridot.

It was discovered in 1789 and since then other similar minerals have been found which form the olivene group. This group contains silicates in which atoms of magnesium are gradually replaced by atoms of ferrous iron. The minerals at the end of the scale are forsterite (a pure magnesium silicate and the lightest member of the group) and fayalite (a pure silicate of ferrous iron and the heaviest member of the group). Olivene lies roughly in the middle of the series as it contains almost equal quantities of magnesium and iron. Sometimes these elements are entirely replaced by manganese silicate.

It occurs in igneous or metamorphic rocks and mainly in basalt and gabbro. It may also be found in meteorites and in peridotite, which is the main component of the earths mantle. Olivene deposits can be found in Australia, Brazil, Egypt, France, Germany, Spain and Sweden.

OOLITIC LIMESTONE

A widespread sedimentary rock formation which forms in warm, shallow marine waters, It is usually white but may also be brown, red or yellow. It is mainly composed of tiny ovoid particles of its essential component calcium carbonate (CaCO3). It may also contain some dolomite, hematite, quartz and clay. The name oolitic is derived from the Greek words oon (meaning egg) and lithos (meaning stone). Oolitic limestone forms in bedding layers and may sometimes contain fossils. It is fine to medium grained and dissolves in hydrochloric acid.

The egg shaped particles form by the precipitation of carbonate chemicals in concentric layers around a central nucleus such as a shell fragment or quartz grain. Similar fragments measuring 2-10mm are known as pisoliths from the Greek word piso meaning pea. Pisolithic limestones are related to oolitic strata but are classified as a different rock formation because they form under different conditions. Sometimes spherical particles may resemble limestone but really be formed from the remains of algae. These are known as oncoids.

Ooolitic limestone is often found in a base of fine grained calcite. It is really a marine deposit and forms in shallow seas particularly where warm ocean waters flow into warmer shallow banks. It may also occur around hot springs and in caves where it is known as cave pearl. It can be found in the Cotswolds (UK). Recent formations can be seen off the coast of the Bahamas, in the Red Sea and in the Great Salt Lake at Utah, US.