Crystals and minerals of the London Clay
Bob Williams (UK)
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
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, 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.
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. At 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.
If you turn off the lights and shine an ultra-violet lamp on selenite crystals, they will shine a bright, pale green colour as they react to the light. This is a property called “fluorescence”. If, after a short time, you turn the lamp off again, some of the crystals (but not all) will continue to shine, literally glowing in the dark, and will fade only gradually. This property is known as “phosphorescence”.
The reason for this is as follows. Natural daylight or “white light” consists of beams of visible electromagnetic radiation. These beams are made up of electromagnetic waves that have different wavelengths, with each wavelength producing light of a different colour. The colours are the “colours of the rainbow”: red, orange, yellow, green, blue, indigo and violet (try “Richard Of York Gained Battles In Vain”). Red is electromagnetic radiation with the longest wavelengths and the shortest wavelengths produce blue. In natural daylight, these colours combine to produce white light or natural daylight. Electromagnetic radiation also exists in wavelengths we cannot see but can detect. Radiation of wavelengths longer than those of red light is known as “infra-red” radiation. It is invisible to us but we can feel it as heat. The warmth of the sun is infrared radiation. Radio waves consist of even longer wavelengths that we can’t see but can hear using radios.
At the other end of the spectrum, radiation with wavelengths shorter than that of blue light is known as “ultra-violet” or UV radiation. UV light is also invisible to us, although some species of bird and insects can detect it. However, we can detect its presence: a “suntan” is the product of UV radiation and “sunburn” is the result of over-exposure to it!
Radiation with wavelengths even shorter than that of UV light is extremely useful to humans in the form of ‘X-rays’, which are obviously important for medical purposes. Even shorter wavelengths produce radiation called “gamma rays” that is useful to astronomers. For our purposes, UV light exists in two forms: “UV-a” and “UV-b”, the first of which is the most harmful form to us. UV-a light has a slightly shorter wavelength than UV-b light.
Some minerals will react to UV light radiation. They absorb energy from light that stimulates an electron orbiting the nucleus of an atom (in the basic atomic structure of the mineral). This causes that electron temporarily to orbit the nucleus at a higher level. This higher level is not the natural level for the electron, so it immediately regains its natural orbit by releasing that energy again. That energy is released by the mineral in the form of electromagnetic radiation that we can see (that is, electromagnetic radiation in the visible wavelength spectrum). This is the property referred to as “fluorescence”. It is not the UV light that we see, but the mineral’s reaction to exposure to that light.
However, some electrons do not return to their original orbital positions quickly and are slow to do so. Therefore, the release of energy is also slow, resulting in the “glow-in-the-dark” property of some minerals that fades as the electrons gradually regain their original orbits. This is the property referred to as “phosphorescence”.
Some minerals will not react to UV-b light but will do so to UV-a light and vice versa. UV-a light may also stimulate a phosphorescent response.
UV lamps are commercially available pieces of scientific apparatus used for a variety of purposes. When I exposed my selenite crystals to UV-light for the first time, I was surprised to notice something else. By accident, I had arranged the selenite crystals in close proximity to a baryte specimen found in the London Clay on the Isle of Sheppey and the baryte crystals reacted to the light in exactly the same way. The adjacent florette-shaped crystalline formations fluoresced in a most appealing manner. However, imagine my surprise when I switched the lamp off to reveal three, pale-green flower-shapes glowing in the dark!
I soon ascertained that the longer the exposure to a UV-a light, the longer the period of consequent phosphorescence. To date, I have achieved a time period of just over 35 seconds of phosphorescence for those crystals. I can also now boast a collection of over 30 different types of mineral that react to UV light and I am more than aware that there are more to acquire. Ironically, I now find the variation in response to UV-light on its own provides a reason for me to collect them.
My personal interest in crystals started by collecting a few of them while fossil collecting. This has inexorably lead to a cascade of knowledge: in the desire to learn more about the crystals and their properties, I learnt about the physics of light and the structures and formation of crystals, and I learnt about other aspects of their geology. For me, this is the beauty of having an interest in geology.
In addition, one of the best things about assembling a collection of crystals is that I have been able to collect some of these crystals myself and not always far from home. It’s just a matter of knowing where to look and this may be on your own doorstep. In the main, no special permissions are needed, just the right attitude and some respect for nature.
The Practical Encyclopaedia of Rocks and Minerals by John Farndon. Anness Publishing Ltd. 2006. Andrew Parker. Seven Deadly Colours. Simon & Schuster UK Ltd. 2005.