Minerals

Metal-rich minerals of Clevedon Pier

The area around Clevedon Pier is notified as a Site of Special Scientific Interest (SSSI) for the unusual minerals exposed in the cliffs and on the shore. More than thirty different species have been identified.

The mineralisation at Clevedon is associated with a geological fault. A fault is a fracture in the Earth’s crust between two blocks of rock. The rocks on either side of the fault plane slide past one another and such movements may generate earthquakes. More importantly for the narrative at Clevedon, faults can act as pathways for fluids and leave spaces in which minerals can be deposited. 

The diagram below illustrates the fault at Clevedon. The small cliff at the north end of the beach and below the Toll House marks its position. The rocks which make up the cliff are quite different to those along the beach. The cliff exposes rocks which formed as screes next to large mountains during the Triassic period (about 220 million years ago) and some much older sandstones formed by ancient rivers during the Devonian period (about 359 million years ago). On the beach there are much paler, cream-coloured, rocks also of Triassic age, which formed in ancient lakes. 

A map showing the extent of the Clevedon beach SSSI. Rocks and minerals should not be collected within the site without permission of Natural England and North Somerset Council.
The diagram shows a view of the rocks within the SSSI and around the Pier. To understand the view imagine that you are out to sea and looking back towards the town. The pier toll house is built on top of the Triassic breccias and Devonian sandstones. The cliff is the scarp marking the fault. Triassic sedimentary rocks formed in a lake occur on the beach next to the cliff and Carboniferous rocks are found to the south towards the jetty.
The mineralized fault is located at the base of the cliff. The pale Triassic lake sediments form the flat pale rocky surfaces on the beach while boulders of the Triassic scree breccia have fallen onto the beach from the cliffs.
The photograph shows broken up sandstone near the fault with pink baryte (barium sulphate) on between broken fragments of sandstone. The camera lens is about 5 centimetres wide.

Hot metal-rich brines percolating through fractures in the rock deposited the primary minerals. In the absence of oxygen, metals such as lead, zinc and copper combined with sulphur to form sulphide minerals. Geologists describe occurrences of metal-rich minerals as ores. The table below lists some of the names and chemical compositions of primary minerals identified at Clevedon Beach.

Table of primary minerals at Clevedon Beach. The chemical symbols are copper (Cu), iron (Fe), lead (Pb), zinc (Zn), barium (Ba), sulphur (S), arsenic (As) and oxygen (O).
Grey crystals of galena (lead sulphide) are surrounded by pink to white baryte (barium suphate). The shininess of the galena at the top is described as metallic lustre. (Image Roy Starkey)
Galena forms almost perfect cubes due to internal arrangement of atoms. White baryte surrounds the cubes. (Image by Richard Bell)
When minerals grow into cavities (open spaces) they can have beautiful forms. The image shows delicate feathery white baryte crystals from Clevedon Beach. (Image by Richard Bell)

At Clevedon Beach the only primary ore that is easily visible is galena (lead sulphide). Look out for thin grey streaks and patches in the rocks. You may be able to see a silvery metallic glint on a sunny day. It is commonly accompanied by baryte (barium sulphate) which forms pale pink to cream veins and veinlets.

The precise age and origin of the fluids which produced the primary mineralisation is yet to be determined, but we can be certain that it formed in the remote geological past when the rocks were deeply buried. 

In more recent times erosion and uplift raised the rocks back towards the surface and the primary minerals began to be altered by ground water rich in oxygen.  Sulphide minerals are not chemically stable in the presence of dissolved oxygen. Chemical reactions liberated the metals which reacted with other dissolved chemicals to form secondary minerals. These secondary minerals formed in the recent geological past.

Some of the secondary minerals found at Clevedon are quite rare and this is one of the main reasons that the site is notified as an SSSI. They are occasionally visible as yellow and green patches in the rocks, but patience and a magnifying glass are needed to find them! The table here lists some of their names and chemical compositions.

Table of the main secondary minerals discovered at Clevedon Beach. The chemical symbols are copper (Cu), lead (Pb), zinc (Zn), iron (Fe), carbon (C), arsenic (As), oxygen (O) and hydrogen (H).
Intense blue azurite crystal (hydrated copper carbonate) from Clevedon Beach. Azurite was used a pigment in antiquity. (Image by Richard Bell)
Green malachite is another hydrated copper carbonate from Clevedon Beach. It is used as a pigment and is a popular semi-precious stone. (Image Richard Bell)
Phosgenite is a rare mineral of lead which contains carbonate and chlorine. This mineral likely forms by reaction of galena with sea water. (Image Richard Bell)
Crystal Gallery from Clevedon Pier.

The scientific importance of the site was recognised as a result of the efforts of amateur mineralogists, particularly members of the Russell Society. Fine specimens are now preserved in museum collections. 

Almost every facet of modern life requires rare elements such as those that are concentrated in the small deposit at Clevedon. There is no evidence that any mining ever went on here, but the exposure provides an example of a type of mineral deposit that is of considerable commercial interest. 

Today’s focus on climate change and ‘green’ technologies means that rare elements are becoming increasingly important. Huge amounts of copper, lithium and cobalt will need to be mined if we are to reach net zero carbon and drive around in electric cars. Mining will also need to become cleaner and more mindful of minimising adverse effects on the environment.

Resources

Gough, J.W. (1967) The Mines of Mendip. David and Charles, Newton Abbot. 269pp.

Burr, P.S. (2015) Mines and Minerals of the Mendip Hills (2 vols). Mendip Cave Registry and Archive. 1000pp 

Wells Museum has a useful display on Mendip lead mining  https://www.wellsmuseum.org.uk/

Bristol Museum and Art Gallery has a display of local minerals and some general geological exhibits https://www.bristolmuseums.org.uk/bristol-museum-and-art-gallery/

Much background information and many excellent photographs of minerals are to be found at https://www.mindat.org/

To find out more about geology, mining and minerals contact a specialist group such as The Russell Society https://russellsoc.org/ ; The Geologists’ Association https://geologistsassociation.org.uk/ or The Bristol Naturalists’ Society https://bristolnats.org.uk/

For information about sustainable mining look at a new virtual exhibition: 

https://www.miningasustainablefuture.org/