Calcite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Ubiquitous
Chemical Composition: Calcium carbonate
Chemical Formula: CaCO3
Method(s) of Verification: the physical and optical properties of macroscopic calcite allow confident non-analytical identification in the vast majority of cases.

Chemical Group:

  • Carbonates

Geological Context:

  • Metamorphic: low-grade
  • Igneous
  • Metamorphic: medium-grade
  • Hydrothermal: alpine type veins
  • Hydrothermal: Mississippi Valley Type veins
  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: epithermal polymetallic veins & pipes
  • Sedimentary
  • Hydrothermal: sedimentary exhalative deposits
  • Hydrothermal: limestone hosted hematite deposits
  • Hydrothermal: copper-dolomite
Doubly-terminated, orange-tipped calcite crystal 6 cm in length from Taff's Well Quarry, Mid Glamorgan. I.E. Jones Collection (no. 1174). © National Museum of Wales.
Introduction: calcite occurs in a great diversity of parageneses and geological settings. It is an important rock-forming mineral (e.g. as the major constituent of limestone); it is a widespread product of low to medium-grade metamorphism and hydrothermal alteration in many rock-types and it is a common hydrothermal vein-mineral. The range of associated minerals is too extensive to quote. When well-crystallized the distinctive crystal habit and perfect rhombohedral cleavage of calcite make identification relatively straightforward.
Occurrence in Wales: calcite occurs throughout Wales and is known in fine specimens from many areas. In the older rocks of Wales it is chiefly found in hydrothermal vein-systems. It occurs throughout the Dolgellau Gold-belt, for example, both as a constituent of the mesothermal gold-bearing lodes and as a major product of the pervasive regional hydrothermal alteration that has affected the igneous intrusions of the area. It is a major vein mineral across Snowdonia in post-Caledonian Pb-Zn bearing veins, such as those mined in the Llanrwst area. Further south, in the Central Wales Orefield, calcite is a major constituent of certain assemblages. In the Upper Palaeozoic and Mesozoic rocks of Wales, calcite is an important rock forming mineral, given the abundance of limestones, marls and calcareous sandstones. It is also a major component of both the Fe-oxide and the Pb-Zn veins that are hosted by Carboniferous limestone-dominated strata in north-east and South Wales. Calcite is also an important supergene mineral in some weathered ore deposits, particularly at the Great Orme copper mines. There are in fact few localities in Wales that lack calcite in some form or other. Because it is so widespread, only the most notable specimen localities are listed here.

Key Localities:

  • Central Wales Orefield: calcite is widespread in Central Wales but only occurs in major quantity in the relatively late 'A2-c' assemblage within the regional paragenesis (Mason, 1994, 1997). Well-formed but often weathered crystals, often aggregated together into stalactite-like masses, are common at Nantiago Mine. Greg & Lettsom (1858) only mention one locality - Logaulas Mine, where well-formed crystals (now in the NMW Collection) were recovered underground in Level Fawr in 1992 by J.S. Mason. Platy crystals associated with octahedral galena were collected around the same time from the Pandy Mine near Tre'r-ddol; some of these are overgrown with small pyrite crystals. Further south, good flat rhombohedral calcite was reported by Jones & Moreton (1977) from Penrhiw Mine. An unusual specimen, in the National Museum of Wales Collection, collected from the Cwmbrwyno Mine by J.S. Mason in the 1980s, consisted of upright pseudohexagonal tabular plates 12 mm on edge. Finally some attractive groups of stacked rhombic crystals found by in the mid 1980s in a rockfall on the sea-cliffs between Clarach and Borth are again now held in the NMW Collection (ex J.S. Mason Collection).
  • Great Orme Copper Mines, Llandudno, Gwynedd: three generations of supergene calcite have been recognised and were distinguished by their differing luminescent properties. They commonly enclose malachite and other supergene copper minerals (Ixer & Davies, 1996).
  • Halkyn-Minera district, NE Wales: calcite is a major constituent of the wide and frequently vuggy mineral veins of this area and well-formed crystals were formerly common. The Minera district produced excellent nailhead crystals and optically clear 'Iceland-spar' also occurs. Some fine specimens have been collected underground in recent years (P. Appleton, pers. comm.). Old mines around Halkyn Mountain produced fine calcite crystals; Mountain (1924) described and illustrated examples from Bryn-y-gwiog Mine while old specimens from Halkyn and Rhosesmor mines are among those in the NMW Collection. Today, with many dumps heavily overgrown, the best material tends to come from working quarries which occasionally break into mineralized ground. During a survey of Clwyd's mineral localities in 1998, fine calcite crystals were found at Cefn Mawr Quarry near Mold, while weathered chalcopyrite bearing calcite coated in azurite was collected at Pant Quarry near Halkyn (Bevins & Mason, 1999).
  • Llanrwst Orefield, Gwynedd: calcite is the chief gangue mineral in the Lower Palaeozoic-hosted Pb-Zn rich veins of this area and many notable specimens have been recovered, particularly during mining operations. Habits include rhombs and platy 'schiefer-spar', for which the Parc Mine is particularly celebrated. Specimens are frequently coated in small pyrite crystals. Very little good material now remains in this area following landscaping operations.
  • Snowdonia, Gwynedd: well-crystallized calcite is only known from a few localities in the large area of southern Snowdonia. Of particular note are platy aggregates lining cavities in breccia from the slate mines of the Corris area (and notably Braich Goch Mine) and purplish nailhead crystals to 3 cm on joint-faces in microgabbro at Tonfannau Quarry, near Tywyn, collected in 1997 (Bevins & Mason, 1998). Specimens from these finds are held by the National Museum of Wales.
  • South Wales: calcite is occasionally found as euhedral crystals in cavities in clay-ironstone nodules on coal-tips, but the most spectacular calcites from South Wales are the huge (>30 cm) scalenohedral crystals, coated with stepped rhomb faces, from the area NW, N and NE of Cardiff. They line large cavities in dolomitized limestone and are associated with baryte, quartz and iron oxides. Crystal groups in excess of 1 m across have been found and removed (with difficulty) in the past, but the finest specimens are often much smaller. The smaller calcites include clean scalenohedra, nailheads and rhombic aggregates, often finely dusted with hematite to give a striking reddish coloration. The most noteworthy locations today are the working quarries, such as Taff's Well and Machen, but formerly some very fine specimens were recovered from quarries at Blaengwynlais, Creigiau and Ton Mawr and former iron mines in the same area such as Llanharry, Mwyndy and Little Garth (e.g. Gayer & Criddle, 1970). Further to the east, fine scalenohedral crystals have been collected from the Caerwent Quarry in Gwent. On the South Wales coast, calcite is common as vein and void fillings in the Mesozoic sedimentary rocks: of particular note are the large, calcite lined voids occurring in Triassic sandstones on Sully Island, the formation of which is somewhat enigmatic.
  • Welsh Borderland: fine calcite crystals are known from the Weston Mine near Middletown, Powys. Small but well-formed crystals are known from Dolyhir Quarry near Kington, Powys (National Museum of Wales Collection); some of these appear to be supergene in origin as they enclose grains of azurite and native copper.

References:

  1. 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.
  2. 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.
  3. Gayer, R.A. & Criddle, A.J., 1970. Mineralogy and genesis of the Llanharry iron ore deposits, Glamorgan. Proceedings of the Ninth Commonwealth Mining and Metallurgy Congress, London, 1969, 2, 605-626.
  4. Greg, R.P. & Lettsom, W.G., 1858. Manual of the Mineralogy of Great Britain and Ireland. John van Voorst, London, 483pp.
  5. Ixer, R.A. & Davies, J., 1996. Mineralisation at the Great Orme Copper Mines, Llandudno, North Wales. U.K. Journal of Mines and Minerals, 17, 7-14.
  6. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  7. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  8. 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.
  9. Mountain, E.D., 1924. Calcite crystals from Holywell, North Wales. Mineralogical Magazine, 20, 212-216.