Minerals of the Avon region
Gypsum (CaSO4 – 2H2O)
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.
Selenite ( Serenity )
Alabaster (Saccharoidal )
May be tabular, prismatic, acicular, fibrous, granular or massive.
2.31 – 2.33
1.5 – 2 on the Moh scale
Gypsum is a sulphate group evaporite mineral
Please follow the Geologist's Code here.
Gypsum - Satin Spar
Gypsum - Selenite
Gypsum - Desert Rose
Gypsum - Alabaster
The four pictures above are from the Geology Collection, University of Bristol. Larger pictures are here.
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.
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.
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.
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.
*Aust Cliff is a SSSI and removal of specimens from the cliff face is both hazardous and illegal.
Geology Collection, University of Bristol.