MARCASITE

A sulphide of iron which is often pale yellow and similar in appearance to pyrite. However, marcasite is slightly lighter when extracted but darkens when exposed to light. It also decomposes quite easily and quickly when exposed to the atmosphere. Pyrite undergoes the same reaction but much more slowly. Although they have the same chemical composition they have different crystal shapes and a different symmetry. Pyrite has cubic crystals whereas marcasite has table top like crystals. Sometimes the two minerals may only be differentiated by detailed scientific analysis. Crystals may sometimes be twinned and appear in the shape of spearheads These are known as cockscomb aggregates.

It was first classified as a mineral by Wilhelm von Haidinger, an Austrian mineralogist. Today it is sometimes used for cheap jewellery but should not be cleaned because it decomposes very easily. Due to their similarity much marcasite sold today is really pyrite. It is relatively hard (6-6.5 on the mohs scale).

Although similar to pyrite deposits are fewer and smaller. It can be found in hydrothermals which are geological veins that have been altered by hot water. Marcasite forms in cooler hydrothermals at temperatures below 450 degrees C and also in stalactites. It can be found in Czech Republic, Calais (France), Germany, Venice and Tuscany and Lazio (Italy). In the US it can be found in Kansas, Missouri and Oklahoma. In Britain the Kentish Marl chalk between Dover and Folkstone is a source.

MUSCOVITE

A silicate of potassium and aluminium and a common form of mica. It is usually a light silvery colour but may also be pale brown, green, silvery grey or colourless. It may appear in crystals, sheets (lamellae) or scattered flakes. Twinning is often evident.

There are several varieties of muscovite. Dark mica is rich in iron and magnesium but white mica contains large amounts of aluminium. Muscovite is also known as white mica or potash mica. One very fine grained type of muscovite is known as sericite and can be green, white or yellow. Hydromuscovite contains more oxygen and hydrogen and less potassium and fuchsite can be recognized by its green colouring due to its chromium oxide content (5 %).

Muscovite is very soft (2.5-3 on the mohs scale) but very resilient and completely insoluble in acids. It is used in the manufacture of insulators, lubricants, paper, paints, varnishes and artificial Christmas snow and as a substitute for glass because it is more resilient than the real thing. For this reason it is often used for transparent oven doors. Mica is particularly useful in industry due to its great resistance to heat and electricity. Enormous deposits were found in Russia, where it was used in windows and the portholes of ships. This was known as muscovy glass. Muscovy is the archaic word for Russia (Moscow in particular) and is from where muscovite derives its name.

It is widely distributed and occurs mainly in granites, pegmatites, schists and gneisses as well as in some sedimentary rocks and crystalline limestones. Due to its resistance to weathering it is a common constituent of sandstones and siltstones. Some crystals are huge. One crystal extracted from the East coast of India at Nellore weighed 85 tonnes. It has been found in Austria, Brazil, Canada, Italy, Norway, Russia and California, Connecticut, North Carolina (US)