LIMONITE

It is classed as a rock rather than a mineral and is a mixture of iron oxides and hydroxides. It is usually black, brown or yellow and is a member of the oxide group. Due to its variable composition it does not have a specific chemical formula and is amorphous which means it does not have a crystal structure. It's main constituent is goethite which is an iron oxide and is often mistaken for limonite. Although the two are almost identical chemically they can be differentiated as goethite has perfect cleavage whereas limonite has none because it is amorphous and has no planes along which to split. It may also contain some hematite, silica and clay.

It is relatively hard (5-5.5 on the mohs scale) and as it is classed as a rock may be a source of various minerals. It was once used as an industrial source of iron although not so often now because it is often polluted by phosphorus. It is also used in industry as a yellow pigment (ochre) and clay may be extracted and used for modelling.

It occurs mainly due to the chemical alteration of carbonaceous or siliceous rocks which contain iron. It may also form from bacteria in lakes or coastal areas or by direct precipitation from salt or fresh water in shallow seas, lagoons or bogs. Limonite formed by the latter method is sometimes known as bog iron ore. It may also occur in laterite deposits, which consist of insoluble oxides of iron and aluminium formed by rock weathering in tropical regions.

Deposits can be found in Brazil, Canada, Cuba, France, India and Italy.

MAGNESITE

A carbonate which contains magnesium and may be colourless, yellow, brown or grey. Sometimes magnesium is partially substituted by iron which tends to make the mineral more brown in colour. It is very soft (3-4 on the mohs scale) and is therefore not often used in jewellery. Crystals that are of high enough quality to be cut for gemstones originate from a certain area of central Brazil.

It is also known as magnesium carbonate and is a very valuable mineral useful in a wide range of industries although purification is necessary. It is used in disinfectants, jet engines parts, inks, glass and also as a lining for furnaces because it can withstand very high temperatures without melting. Magnesite is also used in the manufacture of Epsom salts and is a source of magnesium oxide and the metallic element magnesium. Magnesium oxide was discovered by Sir Humphrey Davy and can also be extracted from dolomite although magnesite is the main source. It has a range of uses e.g. as a reflective material for windscreens. Magnesium is used in fireworks and flares because it burns with a white light. Therefore, although magnesite is not a particularly attractive mineral it has many practical uses.

Magnesite is a common mineral and forms in a variety of environments. Industrially exploitable sources form when volcanic olivine or serpentine rocks are altered by waters that contain carbon dioxide. It may also form when calcium is replaced by magnesium in dolomite or limestone and by the precipitation of magnesium rich waters.

It can be found in Austria, Italy, Poland and Nevada (US).