Minerals of the Avon region
Baryte (Barium Sulphate – BaSO4)
Colour: White but is often coloured by other minerals to a pink or pale brown
Polymorphism: Forms a series with Selestine
Name: From the Greek for weight due to it’s high specific gravity
Crystal system: Orthorhombic with up to 70 forms
Specific gravity: 4.5 measured (4.47 calculated)
Hardness: 2.5 – 3.5
Group: Barite Group
Association: Fluorite, calcite, dolomite, rhodochrosite, gypsum, sphalerite, galena, stibnite.
Occurrence: A gangue mineral in low-temperature hydrothermal veins; in residual deposits from weathered barite-bearing limestones.
Local location: Stancombe Quarry, Flax Bourton (Working limestone quarry)
Cross section of Baryte vein -Stancombe Quarry
Picture credits Richard Kefford
Larger pictures can be seen here
Paragenesis #1 - Bilbao supergene typeDuring the Late Triassic, iron rich saline oxidising water leached through the rift basins formed during the early Permian (~290 Ma) to the late Jurassic (~150 Ma).
Paragenesis #2 - Mississippi Valley Type ( MVT )
In the Mendip – Bristol vein field, baryte has been deposited by hydrothermal fluids in tension cracks and fissures in the Carboniferous Limestone. The primary minerals in these fissures are Galena and Sphalerite. Gangue minerals such as Baryte and Calcite occur in banded formations where the veins pinch out. This is known as the Mississippi Valley Type (MVT) and took place during the Middle Jurassic (~170Ma).
Baryte with associated galena and calcite - Stancombe Quarry
By far the greatest use of this mineral ~80%, is for the production of drilling mud for use in oil exploration. The main reason for this is that it is very heavy and so helps to prevent blow outs in the drilling stage of the exploitation of an oil reservoir. It is also chemically inert. The specification for drilling mud includes a requirement that the specific gravity should be 4.2 or greater.
Non drilling applications of barytes are comparatively small, although still important because of their higher value. High purity grades of barytes with fine and controlled particles sizes are used as fillers in marine and industrial paints, in brake lining/ friction materials and in plastics. A specialised use of barytes based on its high density and ability to absorb radiation, is as an aggregate in dense concrete for shielding applications in the nuclear industry and hospital radiation departments.
|Bladed rosettes of Baryte on associated red ochre|
Sample found by Leon Sparrow at Winford
Barytes is produced in England and Scotland. In England it is now only produced as a by product of fluorspar mining and processing. In Scotland, barytes is extracted as the sole mineral from the Foss Mine near Aberfeldy.
Chidlaw, N. (2012) Metamorphism and Mineralisation in the Bristol - Mendip area