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Left: A heavily oxidised
(rusting) pyrite nodule within a chalk cliff-face. Right:
A spectacular pyrite ammonite (Echioceras) from
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 odour, 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.
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How does pyrite form (pyritisation)?
One of the most important chemical processes in organic-rich
marine sediments is decomposition of organic matter in bacterial
sulphate reduction. Bacterial sulphate reduction produces
bisulphide. Bisulphite can be partially oxidised or can react with
organic matter 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
The amount of pyrite formation in marine sediments is
largely determined by the availability of sulphate, reactive iron
and reactive organic matter during the formation of the sediments.
Click image above to view hi-res.
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.
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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. Marcasite has a much greater tendency
to react to the air and therefore crumbles and turns into a yellow/green
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Right: A family hold their prized ammonite at Beachy Head.
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