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Minerals

Geology of the Hundred of Bradford, Wiltshire

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Wiltshire in general is not rich in minerals, because of the nature of its sedimentary clays, sandstones and limestones. The Bradford Hundred area is not an exception in this, but a few examples of minerals are found in the neighbourhood.

A piece of stalactitic and fibrous calcite from a vein in Limpley Stoke Quarry. Calcite crystals “grew” from the walls of a narrow fissure in limestone rock as the chemical was deposited from a lime-rich solution. A vein like this is not welcome to the quarriers because the stone around it would be wasted.

It was collected underground during a tour by members of the Bradford on Avon Museum Society.

A group of calcite crystals found just below the ridge at Budbury in Bradford. They may have developed inside a cavity in the Bradford Coral Bed. Pointed six-sided crystals of calcite were referred to by old-time miners as “dog-tooth spar”, but to mineralogists they are scalenohedra.

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Gypsum crystals were found when a pit was dug for a telegraph pole at Norrington Common, Broughton Gifford. The mineral is the result of the decomposition of iron pyrites in Lower Oxford Clay in which sulphuric acid reacts with calcite to form the gypsum (calcium sulphate).

An ironstone nodule, split in half to show the layers of different forms of iron oxide minerals, mostly yellowish limonite and brown goethite. It came from the embankment of an old tramway which carried stone from a big quarry next to Dundas Aqueduct to the River Avon, so it and others there were not in situ. It probably came from a ferruginous (iron-rich) band within the Twinhoe Beds in the quarry.

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Flint is a non-crystalline form of the mineral silica, so the same composition as quartz. It occurs as nodules in the Chalk Formation, which is absent from the area of the Bradford Hundred. Any flint found here has been brought in by human activity, such as these chipped flint blades, which were made by Neolithic people 5,000 years ago.

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