Fluorite, variety "Blue John"

from Castleton, Derbyshire,England


[157 kb]

Above: a polished triangular slice of the famous, beautiful "Blue John" variety of fluorspar (fluorite, CaF2). This variant is banded, with predominant purple-blue bands in a clear to yellowish matrix. This is sample 921.5 (as a young idealist I chose a mineral catalogue format that used the chemical classification of Dana's "The System of Mineralogy", despite the fact that, by the time I owned a copy of this great historical work, I had gone on (regressed?) to a chronological accession system for all samples, relying on brute computing power to retrieve the desired samples from a database!). The sample, weight 88.43 grams, is 6x4.5x2 cm in size, is not appreciably magnetic (magnetic susceptibility <0.001x10-3 SI units) and does not seem to respond to either LW or SW ultraviolet light. The purple layers lie normal to the evident direction of crystal growth into an open space (such as the interior of an open vein-fracture). The dark layers vary from roughly 1 mm to 10 mm in thickness, and the margins can be either sharp or diffuse, presumably reflecting changes in vein-fluid composition with time.

"Rock of the Month #140, posted for February 2013" ---

"Blue John" is a gorgeous variety of fluorite, a common vein mineral frequently employed for lapidary purposes. The ornamental uses have to allow for the fact that fluorite is soft (Mohs hardness 4) and cleaves readily on cubic and octahedral partings.

Local expert Trevor Ford has written extensively on the geology and ore deposits of the Pennine hills. He soon found (Ford, 1955, 1962) that the traditional attribution of the origin of Blue John mining to Roman times is probably folklore, or at the least lacks firm archaeological support. Medieval workings are known, and mines were worked at the time of the Norman conquest of 1066. Mining reached its zenith with the Industrial Revolution in the 18th century (Ford, 1962).

The local region, known as the Peak District, includes most of the southern Pennine Hills, and lies almost entirely in the quadrilateral defined by Manchester, Sheffield, Derby and Stoke-on-Trent. It lies mostly in Derbyshire, with parts in Cheshire and Staffordshire. The local scenery, beloved of hill walkers and rock climbers, is underlain by Carboniferous limestone and Millstone Grit (Cope et al., 1980; Ford, 1996). The limestones contain a varied fauna of fossils. There are also limited expressions of volcanic rocks ("toadstones"). Local mines and caverns, some of them partially flooded, have long been a tourist attraction. These include Speedwell Cavern (Ford, 1962), Treak Cliff and the Blue John mine (Ollerenshaw et al., 1964). The host rock to the veins is Carboniferous limestone. Local veins may be lined by fluorite (colourless, yellow or blue), calcite, and other minerals, includes barite and the lead and zinc sulphides, galena and sphalerite. The ornamental Blue John occurs in veins averaging 3 inches (circa 8 cm) thick, or in nodular masses lining Carboniferous limestone host rock (Ollerenshaw et al., 1964).

The veins are referred to in local parlance as:

  • a) rakes - major veins that can be followed for a mile or more, generally near-vertical and often 10 feet or more in width;
  • b) scrins - thinner, impersistent minor veins that splay off a rake; and
  • c) pipes - which are irregular patches, often near-horizontal, that may occupy space between rakes (Ford, 1962).

Trace elements in fluorite (e.g., REE and Y, Sr, Pb and Nd) can provide a useful signature of the vein fluids (Bau et al., 2003).

Blue John pieces have featured in high-end collections for centuries (e.g., Ollerenshaw et al., 1964; Cooper, 2005). There is probably a little more Blue John hidden within the Pennine hills. However (Anon, 2005) the current artisanal production of this gem of a rock is very small, maybe half a tonne a year.

The colour of Blue John

Fluorite is an allochromatic mineral, showing many different colours. In China, Hunan province in particular is yielding a remarkable range of colours of fluorite in fine specimens with shades of green, blue, pink and purple (Liu et al., 2013, pp.51-68).

As early as 1802, mineralogist John Mawe speculated that the characteristic bluish-purple bands of Blue John were caused by bitumen (Ollerenshaw et al., 1964). The role of carbonaceous inclusions has been cited since (e.g., Ford, 1955). Rare earth elements (REE) have also been suggested as colouring agents. Mackenzie and Green (1971) dismissed both bitumen and REE, suggesting a role for colloidal calcium within the fluorite. Mackenzie (1972) also discounted sulphur as a likely chromophore. Galwey et al. (1979) also found no evidence for the role of species such as S, Fe or K. They suggested that the blue bands represent zones of radiation damage formed during temporary deposition of radioactive species during crystal growth. This "microstructural" explanation is an alternative to the older ideas concerning trace elements or bitumen, as noted by Ford (1980). Some artisans apparently have lightened the darkest pieces by heat treatment (Ollerenshaw et al., 1964).

In summary, Blue John colour has been variously ascribed to organic impurities, inorganic impurities, and colour centres. Braithwaite et al. (1973) made a detailed comparison of chemical and physical properties of colourless and purple zones in fluorite from Blue John Cavern and many other locations in the Pennines and elsewhere. They found no hydrocarbons, nor any significant differences in the trace-element contents of colourless and purple zones. A microstructural reason is set forth ("colloidal calcium"), with trapping of Ca in lattice defects. The excess calcium could have been liberated from the crystal structure by irradiation of the fluorite host by uranium-bearing phosphates in nearby strata. Furthermore, a strong blue fluorescence in ultraviolet light, seen in samples from Weardale and elsewhere in the northern Pennines, may be due to the presence of REE. The intense Blue John "antozonite" purple colour tends to be more intense than these REE-induced colours, and is actually visible in petrographic thin sections. A sample from Treak Cliff was the first Derbyshire material to show UV fluorescence, and is pale reddish purple, with bright red fluorescence at a wavelength of 365 nm (Braithwaite et al., 1973).

Acknowledgements: Like many of my earliest samples, acquired in the 1960s, I believe that this particular slice came from my late Uncle John. This winter, Doug Astill made a timely loan of the two locally-published booklets cited in the text.

References

Anon (2005) Tucson show guide. Colored Stone 18 no.1, 498pp., page 310.

Braithwaite,RSW, Flowers,WT, Haszeldine,RN and Russell,M (1973) The cause of the colour of Blue John and other purple fluorites. Mineral.Mag. 39, 401-411.

Bau,M, Romer,RL, Luders,V and Dulski,P (2003) Tracing element sources of hydrothermal mineral deposits: REE and Y distribution and Sr-Nd-Pb isotopes in fluorite from MVT deposits in the Pennine orefield, England. Mineral.Deposita 38, 992-1008.

Cooper,MP (2005) The Devonshire mineral collection of Chatsworth House: an 18th century survivor and its restoration. Mineral.Record 36, 239-272.

Cope,FW, Broadhurst,FM, Downie,C and Jackson,JW (1972) The Peak District. Geologists' Association Guide 26, 2nd edition, 38pp.

Ford,TD (1955) Blue John fluorspar. Proc.Yorks.Geol.Soc. 30, 35-60.

Ford,TD (1962) The Story of the Speedwell Cavern, Castleton, Derbyshire. Lilywhite Ltd, Brighouse, Yorkshire, for R.J. and D. Harrison, Castleton, Derbyshire, U.K., 30pp., undated.

Ford,TD (1980) Field meeting: the economic geology of the Peak District. Proc.Geol.Assoc. 91, 229-234.

Ford,TD (1996) The Castleton Area, Derbyshire. Geologists' Association Guide 56, 94pp.

Galwey,AK, Janes,KA and Reed,R (1979) The blue colouration in banded fluorite (Blue John) from Castleton, Derbyshire, England. Mineral.Mag. 43, 243-250.

Liu,G, Lavinsky,RM, Meieran,ES, Schmitt,HH, Moore,TP and Wilson,WE (2013) Crystalline Treasures: the Mineral Heritage of China. Mineral.Record 44, supplement, 104pp., January.

MacKenzie,KJD (1972) The possible role of sulphur in the colouration of Blue John fluorite. Mineral.Mag. 38, 979-981.

MacKenzie,KJD and Green,JM (1971) The cause of colouration in Derbyshire Blue John banded fluorite and other blue banded fluorites. Mineral.Mag. 38, 459-470.

Ollerenshaw,AE, Harrison,RJ and Harrison,D (1964) The History of Blue John Stone: Methods of Mining and Working Ancient and Modern. Lilywhite Ltd, Brighouse, Yorkshire, 26pp., undated.


MUSEUM MOMENT #3

Natural History Museum, London, England, 13 November 2012.

The systematic collection did not feature Blue John, though I suspect a tonne of the material is on display elsewhere in the building. The relevant systematic-mineral cabinetry did however feature 20 pieces of fluorite, the largest a magnificent green piece with white barite from the Madoc district, in the Grenville province of southeast Ontario (scarcely 40 km from Turnstone's home base!). The finest blue piece, with calcite and dolomite, is from Ullcoats mine, Egremont, Cumberland. Four pieces were from a single district of Cornwall. These were: a-b) almost colourless, very coarse zoned cubes, and pale green cubes with galena and quartz, from Menheniot Mines, Liskeard; c) yellow cubes with galena from Wheal Mary Ann, Menheniot; and d) a nice blue drusy mass with white, hexagonally terminated quartz from Carndon Mines, Liskeard.



Graham Wilson, with Museum Moment, 17-20 December 2012, typographic edit 01 January 2013,
additional note 06 February 2013 / format 15 April 2013

Visit the Turnstone "Rock of the Month" Archives!