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