The Group's aim is to identify, survey, protect and promote geological and geomorphological sites in the former County of Avon - the modern unitary authorities of Bath and North East Somerset, Bristol, North Somerset and South Gloucestershire. RIGS are selected for their educational, research, historical and aesthetic value.

Saturday, 9 June 2012

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:-

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’.

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.

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.

- 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

1 comment:

  1. Fascinating document at http://www.irsociety.co.uk/Archives/22/Paradise.htm regarding quarrying at Leigh Woods.