Fluorite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium fluoride
Chemical Formula: CaF2
Method(s) of Verification: very few localities have been verified by analytical methods, but the distinctive visual appearance of fluorite - typical cubic crystals - allows confident identification in most cases; Vaynor Quarry - XRD (National Museum of Wales).

Chemical Group:

  • Halides

Geological Context:

  • Igneous
  • Hydrothermal: Mississippi Valley Type veins
Pale yellow fluorite cube from Vaynor Quarry, Merthyr Tydfil. I.E. Jones Collection. Photo M.P. Cooper, © National Museum of Wales.
Colour-zoned fluorite crystal (17 x 12 mm), from Vaynor Quarry, nr. Merthyr Tydfil. I.E. Jones Collection. Photo M.P. Cooper, © National Museum of Wales.
Colourless fluorite cubes (to several mm on edge), Sully Island, nr. Barry, South Wales. National Museum of Wales Collection (NMW 87.73G.M.280). © National Museum of Wales.
Purple fluorite from Halkyn Mountain, Flintshire. National Museum of Wales Collection (NMW 83.41G.M.4131), ex R.J. King Collection. Photo D.I. Green, © National Museum of Wales.
Introduction: fluorite is found dominantly within hydrothermal veins and stratabound deposits, but also as an accessory mineral in granite, granite pegmatites and syenites, around fumaroles and in carbonatites and alkaline intrusives.
Occurrence in Wales: fluorite is one of the earliest recorded minerals from Wales, although its occurrence remains restricted to a few small parts of Wales. The great Welsh naturalist, Thomas Pennant (1726-1798), described fluorite in his own collection from the Halkyn mining district in Flintshire (Campbell Smith, 1913). Samples included yellow cleavage fragments and small, pale-purple cubes from Moel-y-Cria and dark purple cubes with calamine from ‘A mine 70 yards deep’. By the mid-19th Century fluorite was well known from Flintshire, Greg and Lettsom (1858) having described fine cubes 2.5 inches square, of a dark amethyst colour from Moel-y-Cria. By the early 20th century, fluorite had been recorded from neighbouring Denbighshire to the south of Halkyn (Strahan, 1890), and in small quantities from South Wales (Storrie, 1895; Howard, 1899) and Central Snowdonia (Williams, 1927). Fluorite was even reported from the Central Wales Orefield. Hall (1971) described that fluorspar and barites were once or twice noticed in the lode at Brynambor to the east of Lampeter. While, Jones & Moreton (1977) noted that fluorite was reported as a gangue mineral in the deep adit at Allty Crib Mine near Tal-y-bont in northern Cardiganshire (Ceredigion). However, there are no specimens to support either claim, and both workings are no longer accessible. During the 1980s and 90s mineral collectors made a number of interesting new discoveries in South Wales, but on the whole fluorite remains uncommon in Wales, with the exception of the Halkyn District of Flintshire.

Key Localities:

  • Aberduna Mine, Gwernymynydd, Mold, Flintshire: fluorite is listed as common by Bevins & Mason (2000).
  • Beaufort Consols, Mumbles Head, Gower, South Wales: fluorite was collected from the headland in the 1980s, from a vein described as rich in fluorite (Bevins & Mason, 2000).
  • Benllech, Anglesey: fluorite is reported from near the cromlech at Benllech (SH 513 819) (R.S.W. Braithwaite, personal communication, cited in Bevins, 1994).
  • Cardiff, South Wales: minute pale blue cubes were found along with rounded gold grains by John Storrie in, a thin sandy bed in the Keuper Marl (Mercia Mudstone Group marls) encountered, at a depth of 334 feet in a boring for water at the Phoenix Brewery in Working Street, Cardiff (Storrie, 1895; North, 1916).
  • Cefn mawr Quarry, Mold, Flintshire: deep purple microcrystalline coatings occur on fracture planes in limestone (National Museum of Wales collections).
  • Eryrys, Denbighshire: minor fluorite occurs locally in surrounding mines and trial workings including Bog, Nant and Westminster (Bevins & Mason, 2000).
  • Graig Quarry, Llanarmon-yn-lal, Denbighshire: Strahan (1890) reported fluorspar in a north and south joint in the quarry about 400 yards ESE of Tan-y-graig Mine. The fluorite is typically colourless with dark purple outer crystal zones (Bevins & Mason, 2000).
  • Halkyn District, Flintshire: the richest examples of fluorite from Wales have been collected from the Halkyn District of Flintshire. Records suggest its occurrence is restricted to the south side of Halkyn Mountain, largely around Moel-y-crio and Bryn-gwiog. Thomas Pennant’s collection of Flintshire minerals contained a number of specimens from Moel-y-crio, including green calamine (smithsonite) epimorphing purple fluorite cubes, clear yellow cleavage masses and small, pale-purple fluorite cubes (Campbell Smith, 1913). Fine dark amethyst coloured cubes to 2.5 inches square are recorded by Greg & Lettsom (1858) while large (65 mm) colourless fluorite crystals with dark purple outer zones have been found in recent years (National Museum of Wales specimen no. NMW 99.37G.M.65) on mine dumps at Moel-y-crio. At Bryn-gwiog fluorite and barite was reportedly abundant in the old hillocks of the mine (Straham, 1890), supported by a large suite of specimens in the National Museum of Wales’ Mineral Collection donated by T.M. Thomas during the 1960s. Colour ranges from colourless, yellow to all shades of purple and even blue (Straham, 1890). Crystals are cubic and reach 60 mm on edge with occasional calcite and chalcopyrite overgrowth. National Museum of Wales specimen, no. NMW 83.41G.M.4131, formerly in the R.J. King Collection is one of the finest examples of Welsh fluorite, displaying gemmy dark purple cubes partially overgrown by small calcite crystals. Some of the largest crystals to have been collected from Wales are those from the Bryngwyog Lode in Halkyn Mine. In 1912, Arthur Russell collected a number of specimens displaying aggregates of dark coloured crystals to 8.5 cm on edge (e.g. NMW 85.70G.M.36) containing chalcopyrite and enargite inclusions (T.F. Cotterell, unpublished data). A little to the south, in Rhosesmor Mine, veins contain some fluorite, but Carruthers et al. (1916) described this as ‘not in paying quantity’. Small, but very deep purple cubes overgrown by galena, calcite and orange sphalerite occur in cavities within limestone on National Museum of Wales specimen no. NMW 27.111.GR.188, while aggregates of pale purple cubic crystals reaching 26 mm on edge occur on NMW 27.111.GR.194. At Rhes-y-cae white fluorite is present with attractive contrasting purple surfaces, on cubes to 48 mm on edge (NMW 60.250.GR.8). Although all of the mines are long since closed, quarrying operations at Pant Quarry and Pant Y Pwll limestone quarry on Halkyn Mountain have recently revealed thin veins of purple fluorite.
  • Hendre Gravel Quarry, Rhydymwyn, Mold, Flintshire: small purple crystals with calcite (National Museum of Wales collections).
  • Hendy Quarry, Miskin, South Wales: Bevins & Mason (2000) note one specimen of fluorite and pyrite (replaced by goethite).
  • Llanarmon-yn-lal, Ruthin, Denbighshire: National Museum of Wales specimen from this locality (e.g. NMW 87.43G.M.37 & NMW 87.43G.M.12) display deep purple cubes up to 14 mm and larger (56 mm) colourless-cored, weathered pale purple crystals with a coating of small chalcopyrite crystals. The precise locality where these specimens were collected is not recorded, but their appearance suggests that they were collected from a mine dump following many years of weathering.
  • Llandudno Mine, Llandudno, Gwynedd: fluorite has been reported from this locality (R.S.W. Braithwaite, personal communication, as cited in Bevins, 1994).
  • Llyn Cwellyn, Beddgelert, Gwynedd: massive, green, white, and purple banded fluorite occurs on the dumps of un-named trial workings within the aureole of the Mynydd Mawr Microgranite Intrusion. The fluorite is of particular scientific interest due to the high rare earth content (Bevins & Mason, 1998). Bevins (1994) recorded small inclusions of fluorite within riebeckite crystals from the Mynydd Mawr Intrusion.
  • Maes-y-safn Mine, Maeshafn, Denbighshire: pale fluorite containing chalcopyrite inclusions is listed by Bevins & Mason (2000).
  • Risca, Newport, South Wales: fluorite was recorded by Howard (1899) as occurring in lead-bearing veins cutting the Carboniferous Limestone at Risca. No specimens are known to exist and no mention was made of this account by North (1916) in his account on the minerals of Glamorgan.
  • Sully Island, South Wales: colourless fluorite occurs in thin mineralized veins in Carboniferous Limestone, immediately below the unconformity with overlying Mercian Mudstones of Triassic age. Cubes up to 3 mm in size have been collected, but all specimens show an etched appearance resulting in a somewhat dull lustre.
  • Vaynor Quarry, Merthyr Tydfil, South Wales: attractive, transparent, purple fluorite occurs as single crystals, or small aggregates of cubic crystals, associated with dolomite and calcite in cavities and along fractures in Carboniferous Limestone. Crystals, up to 20 mm, have been collected (e.g. National Museum of Wales specimen NMW 93.94G.M.34) but are rare and tend to be frosted with obvious fractures. Although perfect, undamaged, deep purple crystals are rare, smaller, paler examples up to 5 mm in size are not uncommon.
  • Y Llymllwyd, near Nant Ffrancon, Snowdonia: Williams (1927) reported fluorite in slates from the contact aureole of the Bwlch y Cywion Intrusion, on the shoulder of Y Llymllwyd. Almandine garnet is noted in association.

There are no key localities for this specimen.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
  3. Bevins, R.E. & Mason, J.S., 1999. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Clwyd. National Museums & Galleries of Wales, Cardiff.
  4. Campbell Smith, W., 1913. The mineral collection of Thomas Pennant (1726-1798). Mineralogical Magazine, 16, 331-342.
  5. Carruthers, R.G., Pocock, R.W. & Wray, D.A., 1916. Fluorspar. Memoirs of the Geological Survey. Special Reports on the Mineral Resources of Great Britain, 4.
  6. Greg, R.P. & Lettsom, W.G., 1858. Manual of the Mineralogy of Great Britain and Ireland. John van Voorst, London, 483pp.
  7. Howard, F.T., 1899. Note on the lead deposits of the eastern fringe of the South Wales Coalfield. Transactions of the Cardiff Naturalists' Society, 30, 46-47.
  8. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  9. North, F.J., 1916. The minerals of Glamorgan. Transactions of the Cardiff Naturalists' Society, 49, 16-51.
  10. Storrie, J., 1895. Notes on the occurrence of grains of native gold in the New Red Marl strata during the boring of an artesian well at the Phoenix Brewery, Working Street, Cardiff. Transactions of the Cardiff Naturalists’ Society, 26, 107-109.
  11. Strahan, A., 1890. The geology of the neighbourhoods of Flint, Mold, and Ruthin. Memoirs of the Geological Survey. Explanation of Quarter-sheet 79SE.
  12. Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.

There are no references for this specimen.