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
Local location: Stancombe Quarry, Flax Bourton (Working limestone quarry)
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Cross section of Baryte vein -Stancombe Quarry
Picture credits Richard Kefford
Larger pictures can be seen here
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Paragenesis
#1 - Bilbao supergene type
During 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
Uses:
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.
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| Bladed rosettes of Baryte on associated red ochre
Sample found by Leon Sparrow at Winford
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Sources:
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.
Richard Kefford
References:
Chidlaw, N. (2012) Metamorphism and Mineralisation in the Bristol - Mendip area
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