Dolomite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Calcium magnesium carbonate
Chemical Formula: CaMg(CO3)2
Method(s) of Verification: full characterization of minerals in the dolomite group from Wales awaits attention. Machen Quarry - EPMA (T.F. Cotterell, unpublished data).

Chemical Group:

  • Carbonates

Geological Context:

  • Hydrothermal: Mississippi Valley Type veins
  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: volcanogenic massive sulphides
  • Sedimentary
  • Hydrothermal: limestone hosted hematite deposits
  • Hydrothermal: copper-dolomite
Salmon-pink curved rhombohedral dolomite crystals on massive cream coloured dolomite from Machen Quarry, Caerphilly, South Wales. Field of view 5 cm across. National Museum of Wales specimen (NMW 2005.16G.M.1). © National Museum of Wales.
Curved rhombohedral dolomite crystals from Blaenrhondda, South Wales Coalfield. National Museum of Wales Collection (NMW 38.515.GR.4). © National Museum of Wales.
Introduction: dolomite occurs widely as a diagenetic mineral in a variety of sedimentary rocks and as a secondary replacement of original calcite in dolomitized limestones. It is also found in many types of hydrothermal ore deposits as a gangue mineral accompanying quartz and sulphides. Dolomite forms a compositional series with both kutnohorite and ankerite and many examples of dolomite contain appreciable iron and/or sometimes manganese. Such compositions cause the mineral to weather to shades of brown.
Occurrence in Wales: dolomite occurs in many areas of Wales, particularly those in which limestones outcrop. It is particularly abundant along the Carboniferous Limestone outcrop between the South Wales Coalfield and the Bristol Channel coast. Elsewhere it is present in a number of areas in which hydrothermal vein mineralization occurs. Many examples from the orefields of Wales are, however, simply rusty-weathering carbonates which have yet to be fully characterized and for this reason the term 'ferroan dolomite', indicating a dolomite-group mineral with some iron content, is widely met with in the literature. Consultation of the ankerite mineral entry is recommended.

Key Localities:

  • Dolgellau Gold-belt, Gwynedd: pinkish-buff dolomite is widespread in this area as a relatively late part of the mesothermal lode assemblage. Localities include Friog Undercliff, Cefn Coch, Ffridd-goch and Prince Edward mines (J.S. Mason, unpublished data).
  • Great Orme Copper Mines, Llandudno, Gwynedd: dolomite is of great abundance at this locality, where the Carboniferous Limestone host is extensively dolomitized and the hypogene mineralization is dominated by chalcopyrite associated with beds of curved dolomite crystals (so-called saddle-dolomite). This assemblage - the copper dolomite association - is an internationally recognised class of ore deposits (Ixer & Davies, 1996).
  • Llangynog Orefield, NW Powys: ferroan dolomite is abundant at some of the old mineworkings in this area, particularly at Llangynog and Craig Rhiwarth mines (Bevins & Mason, 1997).
  • Llanrwst Orefield, Gwynedd: dolomite occurs at a number of the mines in this area (e.g. Parc, Gorlan, Aberllyn) as a relatively minor constituent of the gangue, subordinate to the abundant calcite (Haggerty, 1995; Bevins & Mason, 1998).
  • Pant-y-Gaseg Mine, Anglesey: dolomite is a gangue mineral at this small copper-mine and forms well-developed rhombohedral crystals (National Museum of Wales specimens).
  • Parys Mountain, Anglesey: some well-formed crystals of ferroan dolomite were reported by Southwood & Bevins (1995) from dumps derived from recent underground workings in the Morfa-Ddu area of the site.
  • South Wales (limestone outcrop): dolomite is widespread as a secondary replacement of calcite in Carboniferous Limestone. The process involves a reduction in the volume of the rock, so that the result is a cavernous matrix with vugs lined with well-formed dolomite crystals (National Museum of Wales specimens; Bevins & Mason, 2000), such as are seen in abundance at Ton Mawr, Machen and other quarries in the district.
  • South Wales Coalfield: dolomite occurs within the ironstone nodule assemblage of the South Wales Coalfield with a well-crystallized example in the National Museum of Wales Collection being from the Fforchaman Colliery, Aberdare.

There are no key localities for this specimen.

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., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
  3. Bevins, R.E. & Mason, J.S., 2000. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Glamorgan and Gwent. National Museums & Galleries of Wales, Cardiff
  4. Haggerty, R., 1995. The Mineralization of the Llanrwst Orefield, North Wales. UK Journal of Mines and Minerals, 15, 5-10.
  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. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  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. Southwood, M. & Bevins, R.E., 1995. Parys Mountain -The type locality for Anglesite. UK Journal of Mines & Minerals 15, 11-17.

There are no references for this specimen.