Mineral Database (Saesneg yn unig)

Mineral Database (Saesneg yn unig)
Mineralogy of Wales

Ankerite

Crystal System: Hexagonal
Formula: Ca(Fe,Mg,Mn)(CO3)2
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Calcium iron magnesium manganese carbonate
Method(s) of Verification: Pant-y-Gaseg Mine: EPMA (A.G. Tindle pers. comm., June 2023).
Chemical Group:
  • Carbonates
Geological Context:
  • Hydrothermal : epithermal polymetallic veins & pipes
  • Sedimentary
  • Hydrothermal : mesothermal polymetallic veins
Introduction:

ankerite is a carbonate mineral which forms two series, one with dolomite (magnesium-rich end-member) and one with kutnohorite (manganese-rich end-member). Formerly considered to include all compositions where >10% FeCO3 is present (eg. Hey, 1955) a redefinition in the early 21st century by the International Mineralogical Association (IMA), whereby Fe>Mg, has made ankerite compositions quite rare. This redefinition has also affected previous X-ray Diffraction (XRD) data including the interpretation of ankerite in XRD data by Bevins (1994) which is now considered invalid.

Once thought to be widespread as a diagenetic mineral in sedimentary rocks and encountered in veins and other hydrothermal mineral deposits, most former ankerite occurrences are now interpreted as dolomite. 

Occurrence in Wales:

ankerite was thought to occur widely in Wales – often under the name “ferroan dolomite” but the revised definition of ankerite by the IMA whereby Fe>Mg has placed nearly all the Welsh occurrences into the field of dolomite. Consequently, records in the literature are confused and should all be treated as dolomite until detailed analytical data proves otherwise. Consultation with the dolomite mineral entry is recommended. The only proven ankerite in Wales is from a small occurrence at Pant-y-Gaseg Mine, Anglesey where it occurs alongside dolomite in complex compositionally zoned rhombic crystals (A.G. Tindle pers. comm., June 2023).

Key Localities:
  • Central Wales Orefield: Raybould (1974) cited ankerite as a major gangue mineral in the Central Wales Orefield. Ferroan dolomite or ferroan carbonate is widespread in this area and occurs as a component of four discrete mineral assemblages. Mostly it fills quartz cavities, but in one assemblage it is abundant and where this occurs it is present on mine-tips in large blocks, often cementing brecciated quartz-chalcopyrite-galena veinstone (Mason, 1994; 1997). Well-crystallized examples are not particularly common but small crystal-lined vugs carrying rhombs 1-2 cm in size are known from Hafan, Henfwlch, Eaglebrook and Esgairfraith mines. Recent SEM EDS analysis (T.F. Cotterell, unpublished data) of carbonates from Loveden and Eaglebrook mines has given compositions consistent with iron-rich dolomite, or ferroan dolomite – see the dolomite mineral entry.
  • Dolgellau Gold-belt, Gwynedd: Gilbey (1968) noted the occurrence of ankerite within the Dolgellau Gold-belt, but this is almost certainly an iron-rich dolomite.
  • Nant Helen Opencast, Abercraf, South Wales: excellent ferroan dolomite specimens were recovered from joint surfaces in thick (1-1.5 m) sandstone units intercalated with shales and coal seams during the late 1990s (Bevins & Mason, 2000: referred to as ankerite) through to about 2018. Pale buff, brown-weathering, rhombohedral ferroan dolomite crystals up to 30 mm (more usually 2-15 mm) occurred as extensive crusts, associated with clear quartz crystals, chalcopyrite, millerite, siegenite, galena and sphalerite. Some large plates of crystals collected from this occurrence are preserved in the collections at Amgueddfa Cymru.
  • Pant-y-Gaseg Mine, Anglesey: dolomite is a gangue mineral at this small copper-mine and forms well-developed rhombohedral crystals (Amgueddfa Cymru specimens). Data from electron microprobe analyses reveals that the crystals are compositionally zoned with some areas slightly within the compositional field of ankerite (A.G. Tindle pers. comm., June 2023).
  • South Wales Coalfield: North & Howarth (1928) described the presence of ankerite from the clay-ironstone septarian nodules that are abundant throughout the area, though Firth (1971) considered the carbonate mineralogy to be siderite-dominated. Based on modern terminology North & Howarth’s account describes dolomite, albeit ferroan dolomite. Similar mineralization occurs on joints in massive sandstones from which some excellent specimens were recovered during the period of opencast coal mining (see separate entry for Nant Helen Opencast). Limited chemical analysis of specimens at Amgueddfa Cymru suggests that siderite and dolomite are present within both the clay ironstone nodules and sandstone joints (T.F. Cotterell unpublished data). Some of the well-formed rhombic ferroan dolomite crystals in sandstone joints at Nant Helen Opencast produce compositions very close to the boundary with ankerite and it seems likely that true ankerite might exist within this material.
References:
  • Bevins, R.E., 1994 A Mineralogy of Wales  National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  • 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
  • Firth, J.N.M., 1971 The Mineralogy of the South Wales Coalfield.  Unpublished Ph.D. thesis, University of Bristol.
  • Gilbey, J.W., 1968 The mineralogy, paragenesis and structure of the ores of the Dolgellau Gold Belt, Merionethshire, and associated wall rock alteration.  Unpublished Ph.D thesis, University of London, UK.
  • Mason, J.S., 1994 A Regional Paragenesis for the Central Wales Orefield.  Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  • 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.
  • North, F.J. & Howarth, W.E., 1928 On the occurrence of millerite and associated minerals in the Coal Measures of South Wales.  Proceedings of the South Wales Institute of Engineers, 44, 325-348.
  • Raybould, J.G., 1974 Ore textures, paragenesis and zoning in the lead-zinc veins of mid-Wales.  Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), 83, B112-B119.
  • Russell, A., 1944 Notes on some minerals either new or rare in Britain.  Mineralogical Magazine, 27, 1-10.
  • Southwood, M. & Bevins, R.E., 1995 Parys Mountain -The type locality for Anglesite.  UK Journal of Mines & Minerals 15, 11-17.