I developed a passion for crystals while collecting fossils. To me, crystals don’t have to be fancy, rare or expensive to be of immense interest. Even a good specimen of the commonly encountered “fools gold” (iron pyrite, more technically referred to as iron sulphide) will be of great interest to me.
I live in south-east England, which is perhaps not the best place in the country for collecting interesting crystal specimens. However, I have a special interest in a geological deposit known as “London Clay” that is highly fossiliferous and includes fossils of crabs and lobsters. Many people will not associate this deposit with interesting minerals, but this would be to underestimate its potential.
Crystal groups display the geometry of the crystal structure that is associated with a particular mineral and their forms can vary a great deal. The atoms, from which a substance is built, combine into structures known as “unit cells”. The atomic structure of a unit cell is then identically repeated, forming assemblies that give rise to the final crystalline form (that is, the mineral itself). Some compounds produce small, crystalline structures while others can produce individual crystals that are massive in size and striking in overall appearance. Amethyst is a good example of this and is, perhaps, the most familiar and most commercially available mineral of this type.
A closer look at the crystal structure of any mineral will reveal objects of such incredible, geometric accuracy that they appear to be man-made. Unless somehow damaged or distorted (as many specimens can be without detracting from their overall beauty), each edge is perfectly straight and each angle is geometrically perfect. This is no matter how many times it is repeated: each form will be an exact repetition of the form next to it.
And finally, there are some surprising other qualities to some crystals, including those from the London Clay that I will describe at the end of this article.
Crystals from the London Clay
Phosphatic nodules. London Clay does not have any obviously distinct layering. However, when conditions allow (perhaps in a quarry), harder structures of varying sizes can be seen to occur in separate, individual beds. The chemical composition of the clay includes quantities of calcium phosphate and this may harden considerably into a dark substance, known as a “phosphatic nodule”. Size is usually restricted to a few centimetres and fossils are frequently associated with these nodules. It is believed that this is because the phosphorous element of the nodules is obtained from associated organic remains.
Septarian nodules. Larger objects called “septarian nodules” are the most obvious individual structures found in the London Clay. They can be of considerable size, as you can see from the picture, and are also very hard such that cracks can often form internally. The cracks take the form of sheet-like, internal separations of the nodule structure called “septa” (singular, “septum”) giving the name “septarian”. On weathering, particularly on an exposed beach such as the Isle of Sheppey in Kent, the nodules will disintegrate along the paths of the cracks. The cracks occasionally get lined with the mineral calcite and, although usually small, crystalline structures may be visible.
Iron pyrite. Another mineral associated with these cracks is “fool’s gold” (iron pyrite or iron sulphide). Again, the cubic, crystalline structures of this mineral may be large enough to be visible to the naked eye. (The largest single crystals of this mineral I have seen, albeit from abroad, measured almost 10mm2 and were perfect cubes.) Various crystal forms of this mineral are found in different sediments around the country, many of which are easily accessible. I have found golf-ball sized spheres of small, cubic crystals of fool’s gold at various locations around the chalk cliffs of the south coast of England and also, from the same areas, flat assemblies of radiating, needle-like structures, known as “pyrite suns”
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