Dave, an undergraduate Geology and Geography student, describes his choice of the mineral pyrite as Object of the Month.
Hi, my name’s Dave and I’m a third year undergraduate at the University of Birmingham studying Joint Honours Geology and Geography.
For this month’s Object of the Month I have chosen the mineral pyrite, more commonly named as Fool’s Gold - so called because gold and pyrite look very similar. They can, however, be distinguished by a simple hardness test. If I scratch the surface of this pyrite with my nail you can see it leaves no visible mark. However if I was to do the same to a piece of gold it would leave a very obvious scratch on the surface.
Pyrite has the chemical formula FeS2, meaning it is made up of one iron molecule, Fe, and two sulphur molecules, S. These then combine to form the cubic structure. This is a single pyrite crystal which you can see forms a perfect cube. A cube is not the only crystal shape that pyrite can form, however, and it can form many other crystal shapes including pyritohedrons, a twelve-sided shape named after the mineral.
The cubic structure, while not unique to pyrite, is rarely this perfect. Stacked crystals can form weird and wonderfully-shaped agglomerations like you can see on this sample here. The perfect cube structure, along with the metallic sheen of the mineral, is why I have chosen this to be my Object of the Month.
One common way of forming pyrite is in a deep marine setting after the deposition of organic-rich sediments. Bacteria breaks down the organics forming bisulphide, the S2 component of pyrite. This then reacts with an iron compound forming pyrite. This can result in fossils being preserved as pyrite as the mineral replaces the decaying organic material preserving the original fossil structure. This is a process called pyritisation. You can see this very clearly in this fossilised fish where the scales are perfectly preserved in pyrite. It can also be seen in this goniatite, where the shell has been completely replaced by pyrite.
Today pyrite is used commercially in the production of sulphuric acid and sulphur dioxide. Thanks to its abundance and its metallic sheen, small pieces are also commonly sold as souvenirs in seaside towns.
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