Pyritisation


What is Pyritisation ?
	The reducing (anoxic) conditions in the sea-floor sediments, (and sometimes in the lower water column near the sea-floor), were produced by the large supply of organic matter from dead sea creatures. Sulphate-reducing bacteria reduced the sulphate ions in seawater to sulphide, producing the unpleasant gas hydrogen sulphide (rotten eggs) in solution. This would then have reacted with any iron available to produce the brassy-looking ferrous (Iron) Sulphide, known as Iron Pyrite or Fool's Gold. Pyrite is easily formed by decomposing organic matter when in the presence of iron.
Can Pyritisation vary in its effect ?
	The photo above shows two ammonites that underwent different amounts of pyritisation. The ammonite on the left was pyritised but the process went on too long. This has resulted in the ammonite being completely covered by 'bubbles' of additional iron pyrite. Had the process continued for much longer, the shape of the ammonite would have been completely lost. The ammonite on the right shows some bubbles in the upper half, but the rest of the fossil is perfect, only being hidden by some dried marl.
Does Pyritisation last ?
	Depending on how 'fresh' the pyrite fossils are, they can range from either a dull-brown to a shiny-gold in colour. Unless properly treated after they have been collected, they will eventually corrode into a pile of rust, (actually into a mineral known as Melanterite, with the chemical formula FeSO₄.7H₂O).
How can I preserve Pyritised fossils ?
	The pyritised fossils should be soaked in water to remove any salt or acid, dried slowly and then coated in polyethylene glycol (PEG), or varnish, in order to preserve them. In older days, shellac dissolved in alcohol, was used. See below for a useful link to Florida Museum of Natural History, University of Florida "Fossil Preparation and Conservation".
What does raw 'Iron Pyrite' look like ?
	Iron pyrite can also form on pieces of other material, such as marl. The photo below shows just such a piece. The cubic crystalline structure of the iron pyrite can easily be seen.
Other Pyritisation references
	Pyritisation of fossil wood http://earth.leeds.ac.uk/people/lloyd/pyrite/fossilwood.htm Pyritisation in fossilisation http://servant.geol.cf.ac.uk/research/sulphide/default.asp?p=2 Preserving pyritised fossils by wax impregnation. http://www.cmnh.org/research/paleobotany/coal-balls.html PYRITE PRESERVATION http://www.korrnet.org/kgms/feb-01/feb01-8.htm Florida Museum of Natural History, University of Florida "Fossil Preparation and Conservation" http://www.flmnh.ufl.edu/natsci/vertpaleo/resources/prep.htm#top