Baryte (BaSO4)

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 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.

Bladed rosettes of Baryte on associated red ochre

Sample found by Leon Sparrow at Winford

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:

www.bgs.ac.uk/downloads/start.cfm?id=1349 

http://www.aditnow.co.uk/documents/Bridford-Baryte-Mine/BARYTES-MINE.pdf

Chidlaw, N. (2012) Metamorphism and Mineralisation in the Bristol - Mendip area

Gypsum (CaSO4 - 2H2O)

Minerals of the Avon region 

Gypsum (CaSO₄ – 2H₂O)

Colour:    

               White but may also be clear, or stained pink or orange                       depending on the percentage of included iron minerals. One polymorph – Desert Rose – may be brown because of included sand.

Polymorphs:

                Selenite ( Serenity )

                Satin Spar

                Desert Rose

                Alabaster (Saccharoidal )

Crystal system: 

                May be tabular, prismatic, acicular, fibrous, granular or             massive.

Specific gravity: 

                2.31 – 2.33

Hardness: 

                1.5 – 2 on the Moh scale

Group:

                Gypsum is a sulphate group evaporite mineral

Location: 

                Aust Cliff*.

                Please follow the Geologist's Code here.
                       

Gypsum - Satin Spar

BRSUG B2317

Gypsum - Selenite

BRSUG B6236 

Gypsum - Desert Rose

BRSUG B3472

Gypsum - Alabaster

BRSUG B3932

The four pictures above are from the Geology Collection, University of Bristol. Larger pictures are here.

Paragenesis

There are very large deposits in the UK, the biggest is in East Sussex, with several seams in the Jurassic Purbeck beds. There are others in Staffordshire, Cumbria and Yorkshire. Gypsum is soluble in water but is unusual in that it becomes less soluble as the temperature rises. The deposits formed as seas or saline lakes dried out. It normally occurs as a massive rock or as crystals but can also form on the surface, as sand, where it is exposed to strong winds, such as the White Sands Monument in New Mexico.

General

        The Fauld gypsum mine in Staffordshire was the location of the biggest conventional explosion in either of the two world wars. in 1944, 3,500 tonnes of explosive blew up , killing 77 people and forming a crater 300ft deep.

         Because of its solubility in water and the fact that there are many shallow gypsum seams under Ripon, the city is known for an average of one subsidence event per year where solution cavities in the gypsum migrate to the surface and cause holes to open up. Details here.

         

Local exposures

        The best known local exposure of gypsum is at Aust Cliff*. The mineral occurs as both irregular masses and geodic nodules in the Mercia Mudstone. Here, in its alabaster massive form, it has the sugar lump or saccharoidal appearance, white or pink in colour.

It also occurs in secondary concentrations at the contact between the mudstones and underlying beds, as the ‘Satin Spar’ fibrous form which is invariably pure white.  

Uses

        Gypsum ( Alabaster ) was used in Somerset for ecclesiastical carvings but its main use now is for the production of dry lining boards for the building industry.

        It is also used as a soil conditioner for heavy, poorly draining, soils where the included sulfur (c. 15%) also aids plant growth by reducing the alkalinity. 

        It is a small constituent of Portland cement where the proportion controls the set time. The gypsum for this use in the UK invariably comes from the Sussex mine. It is said, by the Sussex miners, that every house built in the UK since about 1900 contains some gypsum from their mine.

        It also, of course, has a medical use as ‘plaster of Paris.’

        A side effect from the work to reduce the quantity of sulfur dioxide emitted from power stations is that a great deal of gypsum is produced by the desulfurisation process and so reduces the quantity required to be mined. This has, conversely, resulted in an increase in demand for calcium carbonate in the form of Limestone. The chemical reaction is

CaSO₃ (solid) + H₂O (liquid) + ½O₂ (gas) → CaSO₄ (solid) + H₂O.

Further details here. 

Richard Kefford

*Aust Cliff is a SSSI and removal of specimens from the cliff face is both hazardous and illegal.

References:

Geology Collection, University of Bristol.

Celestine (SrSO4)

Minerals of the Avon region 

Celestine (SrSO4)

Colour: Light blue (but may also be colourless, yellow or brown)

Crystal system: Orthorhombic

Specific gravity: ~3.96

Hardness: 3-3.5

Location: Yate, Aust, Clifton

Please read the Geologist's Code here:-
http://www.brerc.org.uk/rigs_site/geologists_code.htm

Celestine (SrSO4). Specimen on left shows distinctive light blue colouration and prismatic form (source: Mexico). Example on right is from Yate, nr. Bristol. http://www.bristol.ac.uk/centenary/look/cabinet/celestine.html

Celestine (also referred to as 'celestite' or 'spar') is a principal strontium (Sr) ore and was heavily quarried in South Gloucestershire in the 20th century. It is characteristically light blue in colour and its name is derived from the Latin caelestis meaning ‘heavenly’ or ‘from the sky’.

Paragenesis

Although celestine can form in hydrothermal veins, it is most commonly found in sedimentary deposits. In the South Gloucestershire, it is found in the Mercia Mudstone (a unit of mudstone and siltstone of Triassic age, sometimes known as the Keuper Marl). The Mercia Mudstone was deposited in an arid environment where saline lakes formed in desert basins. The evaporation of water from these playa lakes produced evaporite deposits of gypsum (CaSO4.2H2O), halite (rock salt) and celestine (both primary and as a replacement mineral). The extreme enrichment of Sr in the waters, and resulting prolific deposition of celestine in the Bristol area, is postulated have come from the dolomitisation of aragonite in the Carboniferous sediments; aragonite may contain up to 8,000 ppm Sr in comparison to 400 ppm Sr in calcite. Celestine is differentiated from other evapourite minerals by its high specific gravity and relative hardness.

Local exposures

The most abundant deposits of celestine in the Avon region are in the Yate area (Charfield, Leechpool and Goosegreen), Leigh Court in Bristol, and at Aust Cliff*. The mineral occurs as both irregular masses and geodic nodules in the Mercia Mudstone, and in secondary concentrations at the contact between the mudstones and underlying beds, such as Coal Measures or Silurian sediments.

Uses

Celestine is the world’s primary strontium ore. Quarrying of celestine took place around Yate from the 1820s to 1990s and it was exported to Germany for use in the refining of sugar beet. At one time, 90% of the world’s celestine yield was extracted in Yate; unfortunately, the main quarry site is now buried under Yate shopping centre.

Celestine quarrying in the Yate area - Bristol Mineral & Land Co.  c.1950. Photo credit: http://www.sgmrg.co.uk

In spite of former glories, extraction of the dwindling Yate celestine deposits is no longer economically viable and the majority of the world’s supply of Sr metal is now sourced from celestine and strontianite (SrCO3) from Mexico and Germany. Sr is used in pyrotechnics (it produces the crimson-coloured flame in fireworks) and in the manufacture of display screens (dense Sr-doped glass shields the viewer from x-rays).

Charly Stamper

*Aust Cliff is a SSSI and removal of specimens from the cliff face is both hazardous and illegal.

References:

- Deer WA, Howie RA, Zussman J. An introduction to the rock forming minerals, 2nd. ed. (1996)
- Green GW. British Regional Geology: Bristol and Gloucester District, 3rd ed (1992).

- Wood MW & Shaw HF (1976) Chem. Geol. 17: 179-193.

- South Gloucestershire Mines Research Group http://www.sgmrg.co.uk/celestine.php

- Yate Heritage Centre http://www.yateheritage.co.uk/history-of-yate/beneath-our-feet.htm