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What is pyrite?

The mineral pyrite, or iron pyrite, is iron disulfide, FeS2. It has isometric crystals that appear as cubes or pyritohedrons (twelve irregular, pentagonal or five-sided faces). Pyrite takes its name from the Greek word "pyros" meaning "fire" because sparks flew from it when hit with another mineral or a metal.

Pyrite is commonly referred to as "Fool's Gold". There are other similar gold-like minerals, but pyrite is by far the most common and the most often mistaken for gold. The notable difference is that Pyrite is much harder and more brittle than gold and cannot therefore be cut or shaped. In addition, gold has no odor, but pyrite gives off a sulphurous smell (like rotten eggs).

Although pyrite is common and contains a high percentage of iron, it has never been used as a significant source of iron. However nodules also contain a high sulfur content and were mined for this during World War II and used in the production of sulfuric acid.

How does pyrite form (pyritisation)?

 
One of the most important chemical processes in organic-rich marine sediments is decomposition of OM (organic matter) in bacterial sulphate reduction.  Bacterial sulphate reduction produces bisulphide.  Bisulphite can be partially oxidised or can react with OM and reactive metal species.  All these reactions may be bacterially mediated.  The reaction of reduced sulphur with reactive dissolved iron and iron minerals, if available, results in the formation of iron sulphides. The most common iron sulphide in pyrite.

The amount of pyrite formation in marine sediments is largely determined by the availability of sulphate, reactive iron and reactive OM during the formation of the sediments.
 

Pyrite and fossils

Pyrite occurs in many fossil specimens, ranging from isolated crystals to encompassing the entire fossil.  In the latter case there are effectively two forms.  In the first instance replacement of the original object can occur, whereby the entire fossil such as an ammonite is preserved.  In the second instance, infilling can occur, where only the internal structure is preserved.

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Pyrite can cause serious problems when trying to preserve certain specimens. Once exposed to the moisture in the air, unstable pyrite can disintegrate due to oxidation, resulting is partial or complete destruction of the fossil. As a result Discovering Fossils is currently researching chemical solutions to this problem.

Pyrite and Marcasite

Pyrite is often mistaken for the mineral marcasite due to their similar characteristics. In fact marcasite is a polymorph of pyrite, which means it has the same chemistry as pyrite but a different structure, resulting in different symmetry and crystal shapes. Marcacite has a much greater tendency to react to the air and therefore crumbles and turns into a yellow/green powder.

 

 

  

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