Celestine
By Alex Lowe
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What is Celestine?
Sometimes called 'spar', a loose term used by miners and quarrymen for any light-coloured crystalline rock, celestine is strontium sulphate, chemical symbol SrOSO3 (Sr=Strontium, O=Oxygen, S=Sulphur) officially and usually given in all textbooks as SrSO4; SrOSO3 is the way the elements are arranged, and Sr substitutes for Ba, forming a more or less continuous series between celestine and baryte. It has a melting point of 1,600°C and is insoluble in water.
The element Strontium is named after Strontian in Argyllshire, where the mineral was first identified, in 1787. It is a crystalline mineral whose name 'celestine' comes from the Latin 'caelestis', meaning, roughly, 'heavenly', from the celestial blue colour of the first specimen identified. The best crystals are a delicate blue; massive and granular celestine is generally white to nearly colourless. Well formed blue crystals are very collectable as mineral specimens, quite apart from their value as a strontium source. They are advertised widely for use in crystal therapy! (Although this may be stretching the meaning of 'use'.) Try putting 'celestine' into a search engine to find some beautiful photos.
Where to find it
Celestine is found in sedimentary rocks, including the Mesozoic formations in the South Gloucestershire area.
Worldwide, Madagascar, Germany, and North America (Lake Erie, Ohio) are given as principal sources for celestine, but others include Australia, Mexico, Sicily and Spain. However, the Yate area had a high-quality concentration of the mineral.
Celestine occurs in sedimentary rocks as granular and nodular masses, with beautifully developed crystals in places, and is thought to be a replacement for the evaporite minerals, gypsum and/or anhydrite, in the local Triassic strata. These minerals formed when trapped lakes of seawater, rich in mineral salts, dried out, rather like Death Valley today. They became part of the sedimentary sequence and gypsum can be seen within the sequence of red Mercian Mudstones at Aust Cliff. Large specimens of gypsum can be seen on the foreshore, and an occasional chunk of celestine can also be found (Note that Aust Cliff is a SSSI and any removal of material from the cliff face is both very dangerous and also illegal). The source of the strontium is not known, but is thought to lie within the Palaeozoic rocks underlying the Triassic mudstones. In other localities celestine can be found in hydrothermal veins, usually within limestones, associated with intrusive igneous rocks. Similar minerals are gypsum (calcium sulphate), anhydrite, (also calcium sulphate), baryte, (barium sulphate) as well as the carbonates, calcite and strontianite. The specific gravity is a real giveaway: baryte is very heavy, and the crystals or formation can usually be recognised; celestine is pale and could be mistaken for calcite or gypsum, but a piece is much heavier than either of the other two. They are all fairly soft; gypsum can be scratched with a sturdy fingernail, and the others with a penknife blade.
The history of local celestine mining
The Board of Trade records celestine exports in the 1880s; commercial quarrying for the mineral ended in 1994, and in 1995, the processing plant operated off Tanhouse Lane by the Bristol Mineral and Land Company Limited closed.
Local deposits had been worked out, consequently foreign celestine, despite its inferior quality, became competitive. So great was the value of local celestine that even the chance of finding it was sufficient reason for the Planning Committee to block Sodbury Rural Council's proposal to build housing on the former Yate aerodrome, in 1949!
Some of the many and varied uses of celestine
In the 1890s, it was bought in bulk by German sugar refineries for use in extracting sugar from sugar beet. Later, strontium compounds found a huge variety of applications, in pyrotechnics (e.g. fireworks and flares for a bright crimson flame), television screens and tubes [as a phosphor], electrical capacitors and valves, paint filler and whitener, anti-corrosive primers, chrome-plating and metal-purifying, medicines, toothpaste and depilatories, plastic and rubber fillers and PVC plastic stabilisers, caustic soda production, artificial gemstones and fertilizer.
Mining Techniques
At first picks and shovels were used, and the possibility of mechanisation was discounted because labour was cheap and plentiful (celestine was worked part-time, by agricultural labourers paid according to the amount they dug). Buyers also believed that mechanical mining would produce poorer quality material (lacking the workman's discriminating eye), and farmers feared that it would damage soil fertility.
Celestine working in Yate area - Bristol Mineral & Land Co -circa 1950
However, in response to labour shortages after The Second World War, the Bristol Mineral and Land Company introduced mechanical excavators, bulldozers and pneumatic drills (to break very large nodules). Topsoil was replaced after a deposit was worked out, so fertility was unaffected.
Preparation for Use
Before mechanization, celestine ore was transported by horse and cart to railway stations, such as the one at Wickwar, and sent for processing. Following investment by the Bristol Mineral and Land Company, celestine was mechanically crushed, washed, ground, graded and bagged at Yate.
Local Business
Some landowners in the late 19th Century quarried celestine piecemeal, such as the Rector of Yate, the Reverend Mr. Pontifex, (he mined the Church's land until it found out!), or Robert Drew at Wickwar. Soon after, various celestine brokers set up in business, buying from landowners and selling material on, or even buying land to quarry themselves. The Bristol Mineral and Land Company Limited was established in 1910 as a brokerage. One of its directors, Mr.Pauli, was connected with the German sugar beet refineries, and his family remained in the firm for three generations.
You can read more about Celestine in our book.
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