Geology And Geophysics

How Iron Pyrite is Formed



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Iron pyrites is a metallic mineral made of iron and sulphur (FeS2). It is often found in mineral veins, and as pyrite nodules. It is a heavy stone with a yellowish sheen, or if exposed to oxygen pyrite may also take on a rusty red hue where it has started to oxidise. In its crystalline form it has shiny brass-coloured cubic crystals, with a strong resemblance to gold, giving it the common name of "fool's gold". If identification is uncertain from the appearance, it can be tested for a greenish or brownish black streak, as opposed to the yellow streak of gold. Another good way to identify it is to strike it with something hard, which will cause it to spark with a sulphurous smell, and this quality gives pyrite its name, from the Greek word "purites" meaning "of fire".

Pyrite is found in all types of rock - igneous, metamorphic and sedimentary, as well as in hydrothermal vents. It is formed in a variety of ways, with differing processes creating different forms of the rock with different sized crystals. Where it is found in igneous rocks, it may have been formed from magma in which the minerals were heated into a melted mass, and then the various minerals separated out at different temperatures, forming crystals as it cooled; slower cooling results in larger crystals. Metamorphic rocks are formed by heat and pressure, and pyrite could be formed as part of this process, or may form as a replacement mineral within the rock. In sedimentary rocks pyrite is usually found as a replacement mineral, when the iron and sulphur that is present in the sediments chemically combine to replace the original chemical composition of organic matter that is in the process of becoming fossilised. Fool's gold fossils, such as ammonites, are very popular with collectors and are often found in sedimentary rocks.

Iron pyrite is a fairly common mineral, and is mined or extracted globally for its many industrial uses, such as extraction of the sulphur for sulphur dioxide or sulphuric acid, and also as an ore for iron. It oxidises readily, making it easy to extract the component minerals, but care must be taken as this also makes it quite combustible. In the late 19th Century a newspaper story in England claimed that there was an active volcano on the cliffs of Dorset on the south coast - smoke had been seen rising from them! It turned out to be the reaction of pyrite in the rocks which started to combust as it became exposed to the atmosphere.

It is also collected as an ornamental mineral, with the larger and more perfectly formed cubic crystals being the most sought after. Notable locations for collection of pyrite include the Huaron mining district of Peru, which is very prolific, and La Rioja in Spain or the island of Elba in Italy, where the larger and higher quality crystal formations are mainly found, as well as many mining districts in the USA.

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