Pyrite is a wonderful common mineral, it is composed of Iron Fe and Sulphur S with the chemical formula of FeS2.
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Pyrite forms in many different shaped crystals, this is due to twinning of the crystal forms of this mineral, as a result it exhibits a c
omplex array of crystal shapes. With so many variations of crystal shape it offers the mineral collector many new specimens to collect. Without doubt the most alluring feature of Pyrite is the lovely bright golden metallic colour often seen twinkling in the light, this sparkle catches the eye and is guaranteed to sets the finders pulse racing because it evokes feelings of hidden treasure. Pyrite often forms in an Oxygen poor and Sulphur rich sedimentary environment in fine grained deep sea Anaerobic deposits.
Crystal Forms
Amongst the many crystal forms in which Pyrite is found the most common is the perfect cube. Truly wonderful perfect cubes are frequently encountered, not only as single crystals but also as clusters of inter-grown bright shiny dice like crystals. The faces of the cubes can have either plain or striated surfaces, the parallel striations are mostly found in the crystal veins in mining areas and this is due to what is called twinning where a combination of two different crystal forms such as the cube and the pyritohedron combine. On careful examination you can see that the striations run perpendicular to each other on alternate faces. Plain cubic crystals by contrast are largely encountered in metamorphic environments, they can be found in intersecting dice like groups; this feature is due to penetration twining and results in the form often seen in the Crystals from Spain. In other examples, extremely small crystals are often found in massive clumps, these are sometimes given the name Chiapas, as you can see Pyrite is found in many different guises.
Other Crystal Forms
Apart from the common cubes mentioned there are also the octahedral crystals and the more complex Pyritohedral forms which often has slightly curved fac
es. Crystals can be found twinned with all three main crystal forms and display a mixture of crystal shapes such as the cubo-octohedra, which combines both the cubic and octahedral crystal habits. Other crystal forms give rise to totally different looking minerals, flat round discs are in this case the Orthorhombic crystal form of Marcasite that has the same chemical formula FeS2 but is Orthorhombic and different from the Cubic System, these discs occur when the crystal growth is restricted to a single plane in a shale. Also round Pyrite balls can form and these often originate from growth around an original fossil seed, this is then enclosed within the growing Pyrite in the mudstone.
Why Fools Gold
To the uninitiated mineral collector Pyrite can be mistaken for Gold, there is a well known story that describes how a shipping merchant went to South America and loaded his ship with what he thought was Gold, only to discover his grave mistake on his returned home, hence the name Fools Gold, a lovely story at least. Surprisingly Pyrite can actually contain a small amount of real gold, around 2% of Gold in fact, but this gold is difficult to extract and therefore it is un-economical to process. Pyrite often attracts other minerals to its growth such as Copper, resulting in both Chalcopyrite CuFeS2 and Bornite Cu5FeS4 and also even Arsenic in yet another Sulphide mineral called Arsenopyrite FeAsS with arsenic replacing some of the Sulphur.
Beautiful Cubes
Perfectly shaped cubes with mirror finished sides are famous from a mine in rocks from the Victoria Mine, Navajun, Spain, lots of examples of this can be found at Gem Fairs both in the original host rock and as separate crystals, the best examples display beautiful well formed cubes, often in stacked formation. There are many spectacular examples of the other crystal forms that come from the mining areas of Peru. China and Eastern Europe, these mining areas also produce a variety of complex crystal habits, often accompanied by needle Quartz, Sphalerite and Galena in small but very attractive groups of crystals.
Fossil replacements
Spectacular examples of Iron Pyrites replacing or coating fossils exist, this provides yet another example of this popular mineral. These replacements are quite common, often just as a very fine frosted coating on the top of a fossil or as a total replacement of the original fossil. These lovely fossils are found as beautiful casts of the well known Ammonites and they can preserve remarkable details, for example they often retain the minute details such as the convoluted suture lines, these fossils are truly beautiful examples. Good examples of these Ammonites can be found locally at Charmouth in Dorset, from the famous Jurassic Coast, beautiful Pyritised Crinoids are also found here. Another example of this type of preservation can be found in Ammonites with open body chambers, lined with sparkling micro-crystals and these come from Russia and reveal the crystal lined hollow chambers in exquisite detail, often carefully cut open to be made into beautiful jewellery pendants.
Marcasite
Care needs to be taken when collecting these pyritised fossils as there is a less stable form of Iron Pyrite that is called Marcasite and this can disintegrate to a grey powder much later on after the fossil has been collected, imagine the disappointment when you are confronted by just a pile of a grey powder where your lovely fossil once was, smelling strongly of Sulphur that also ruins any labels. Unfortunately it is often the case that both the stable form of Pyrite and the unstable form of Marcasite can be found within the same rocks, adding to the confusion.
Pyrite Suns
As mentioned earlier bright discs of pyrite exist and these can frequently be found at Gem Fairs, they are actually of a stable form of Marcasite and are found in a black shale from the coal mines of Sparta in the USA, these circular discs exhibit fine radiating striations on the flat surfaces that indicate the twinning of forms, the very best of these are perfectly circular. Similar discs also exist as very small bright versions and these are found seemingly floating inside clear quartz crystals, again these look quite spectacular, see previous article on Iron rich inclusions in Quartz published in issue 57 Autumn 2012 of Rock n gem.
Epitaxial coatings
Pyrite and Chalcopyrite can both exist as a very thin covering over other minerals and these can confuse the unwary collector as the crystal structure appears different from the usual Iron rich minerals and is in fact just a fine surface coating on top of the different crystal forms such as Sphalerite.
Barry Taylor
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