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Cymraeg

Quartz

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Ubiquitous
Chemical Composition: Silicon dioxide
Chemical Formula: SiO2
Method(s) of Verification: all cited examples have all been identified visually with confidence.

Chemical Group:

Geological Context:

Prismatic quartz crystal (10.5 cm tall) with inclusion rich base and clear termination, Fotty & Bowydd Quarries. National Museum of Wales Collection (NMW 27.111.GR.140), ex G.J. Williams Collection. photo D.I. Green, © National Museum of Wales.
Colourless quartz crystal on prehnite, St. David's Head. National Museum of Wales Collection (NMW 98.71G.M.1). photo D.I. Green, © National Museum of Wales.
Prismatic quartz crystal 6 cm tall from Mynydd Drws-y-coed, Central Snowdonia, Gwynedd. National Museum of Wales Collection (NMW 83.41G.M.2043), ex R.J. King Collection. Photo M.P. Cooper, © National Museum of Wales.
Colourless, doubly-terminated, so-called 'Merthyr diamond' quartz crystal, 2 cm long, in a cavity within clay ironstone, from International Colliery, Blaengarw, Mid Glamorgan. I.E. Jones Collection. Photo M.P. Cooper, © National Museum of Wales.
Introduction: perhaps the most familiar mineral to many (because it is so frequently met with), quartz brings to mind images of rock crystal, amethyst, smoky and rose-coloured varieties and the cryptocrystalline varieties typified by jasper, agate and chalcedony. Yet much quartz goes unseen, occurring in countless millions of small grains, cemented together in sedimentary rocks. Quartz is hard and highly resistant to both physical and chemical weathering and thus it survives in abundance in sedimentary environments: in some sedimentary rocks such as sandstones the grains may be large enough to see while in others, such as the Triassic red marls of South Wales (composed primarily of quartz and iron oxides), they are microscopic in size. Quartz is also an important rock-forming mineral in igneous and metamorphic rocks. It is additionally an extremely frequent component of mineral veins of all types, with or without carbonates and sulphides. The hardness and crystal morphology of quartz make it an easy mineral to identify in hand specimens: its resistance to acids is also notable (with the exception of hydrofluoric acid, which dissolves it). Quartz is particularly useful when investigating mineral deposits as the textures in thin section yield information about subsequent deformation (see images below) and it is also particularly adept at trapping and preserving inclusions of the original mineralizing fluids (so-called fluid inclusions) which, when analysed, can tell us a great deal about the temperature and composition of the fluid and the pressure at which the mineral crystallized. This in turn helps us to work out how a particular mineral deposit was formed.
Occurrence in Wales: quartz is present in one form or another everywhere in Wales: it is true to say that virtually every square metre of the surface of the Welsh landscape has some quartz present, and in some places (e.g. parts of Snowdonia) it is so abundant in veins as to give rock outcrops a white-streaked appearance. This abundance contrasts strongly with the distribution of some of the minerals categorized as 'rare' in this database, some of which are only known from one or maybe two specimens! Outside of the myriad rockforming occurrences, some fine specimens of quartz are also known from Wales. Rock crystal from North Wales used to be sold, as long ago as the early 19th Century, as 'Snowdon Diamonds', which were bought as curios by tourists. Since then, many fine specimens have surfaced, with many of the best, in terms of form and clarity, coming from the alpine fissure-type veins of Snowdonia. The metalliferous hydrothermal lodes of the various Welsh orefields have likewise produced large numbers of quartz specimens which make up in size what they lack in clarity. In Central Wales, the later (A2) stage of vein mineralization includes an assemblage in which coarsely-crystalline quartz is a major feature (Mason, 1997). Coloured varieties of crystalline quartz are less of a feature but hydrothermal lodes in South, Central and North Wales have all locally produced smoky or amethystine crystal groups. The cryptocrystalline varieties are less of a feature, with the exception of jasper which is particularly abundant on the Llŷn Peninsula (Matley, 1928); agate only occurs sporadically as in the 'Potato Stones' occurring in Triassic rocks in South Wales and as rare, small vesicle fillings in Lower Palaeozoic lavas in some areas. Because it is so widespread, only sites that have produced fine specimens of quartz are listed.

Key Localities:

References:

  1. De La Beche, H.T., 1846. On the formation of the Rocks of South Wales and South Western England. Memoirs of the Geological Survey, 1, 296pp.
  2. Firth, J.N.M., 1971. The Mineralogy of the South Wales Coalfield. Unpublished Ph.D. thesis, University of Bristol.
  3. Green, D.I. & Middleton, D., 1996. Alpine-type vein minerals from Tanygrisiau, Gwynedd. U.K. U.K. Journal of Mines and Minerals, 16, 30-33.
  4. Green, D.I., Rust, S.A. & Mason, J.S., 1996. Classic British mineral localities: Frongoch Mine, Dyfed. UK Journal of Mines & Minerals, 17, 29-38.
  5. Greg, R.P. & Lettsom, W.G., 1858. Manual of the Mineralogy of Great Britain and Ireland. John van Voorst, London, 483pp.
  6. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  7. Mason, J.S., 1997. Regional polyphase and polymetallic vein mineralisation in the Caledonides of the Central Wales Orefield. Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Science), 106, B135-B144.
  8. Matley, C.A., 1928. The Pre-Cambrian complex and associated rocks of south-western Lleyn (Carnarvonshire) Quarterly Journal of the Geological Society of London, 84, 440-504.
  9. Starkey, R.E. & Robinson, G.W., 1992. Famous mineral localities, Prenteg, Tremadog, Gwynedd, Wales. Mineralogical Record, 23, 391-399.
  10. Starkey, R.E., Hubbard, N. & Bayley, M.P., 1991. Mineralization at Hendre Quarry, Glyn Ceiriog, Clwyd, Wales. U.K. Journal of Mines and Minerals, No. 10, 48-51.
  11. Watson, J.J.W., 1859. The haematitic deposits of Glamorgan. Geologist, 2, 241-256.