London Clay Minerals - (Back)
By Bob Williams

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 formation 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 repetetively 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.

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.

Larger objects called “septarian nodules” are the most obvious individual structures found in the London Clay. They can be of considerable size, 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 at 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.

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”.

A third mineral that can be found adjacent to the fissures in the septarian nodules is the mineral baryte (barium sulphate). This appears as grey/white rosettes that decorate the nodule radiating out from a central point located in the fissure.

A fourth mineral, which is capable of forming striking, visible crystalline structures in the London Clay, is selenite - a form of gypsum (hydrated calcium sulphate). Gypsum results from the evaporation of salt water from the clay, which creates crystals in a variety of forms that can occur in beds of varying thickness. Selenite is the most common form of gypsum, and in London Clay, exists as transparent or translucent, white or yellowish crystals. The crystals are usually tabular in shape and may form spearhead or swallowtail twins. Occasionally, they can also be found as impressive, prismatic swords that may measure inches long. My largest individual specimen is over 12cm in length and I am sure that many will find it surprising that a crystal of such size and clarity can be found in the south-east of England.

When collecting from a riverside exposure of London Clay in Essex, just to the west of Burnham on the north bank of the River Crouch, I found a rather interesting and perfectly clear single crystal of selenite, perhaps 5cm long. Further trips to this site have yielded the best, self-collected, crystal specimens in my collection. A second site at Maylandsea on the River Blackwater, near to Althorne, has also yielded many interesting selenite crystals. They appear to take two forms:

one is a cluster of small, tabular, almost prismatic crystals, often of a yellowish colour. These are known locally as “desert roses”. The other forms long, tapering, prismatic, clear, individual crystals. (Both of these sites are also excellent for collecting fossil crabs and lobsters from the Eocene.) Once again, it seems strange that you can find crystals like that in south-east England.

Of course, other minerals exist in the clay, but they do not produce any readily collectable and striking crystal formations. They will be of interest to people studying the general geology of the London Clay, but perhaps not to the “crystal collectors” amongst us.

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