Ankerite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Calcium iron magnesium manganese carbonate
Chemical Formula: Ca(Fe,Mg,Mn)(CO3)2
Method(s) of Verification: Nant Helen Opencast - EDX (T.F. Cotterell, unpublished data); Loveden & Eaglebrook mines, Central Wales Orefield - EDX (T.F. Cotterell, unpublished data).

Chemical Group:

  • Carbonates

Geological Context:

  • Hydrothermal: epithermal polymetallic veins & pipes
Pale buff, brown-weathering, rhombohedral ankerite crystals, occurred as an extensive crust on joint surface in sandstone. Nant Helen Opencast. Specimen National Museum of Wales (NMW 99.37G.M.19), photo by D.I. Green, © National Museum of Wales.
Coarsely crystalline ankerite from Esgairfraith Mine, in the Central Wales Orefield. Specimen 16.5 cm across. National Museum of Wales Collection (NMW 87.47G.M.366), ex J.S. Mason Collection. Photo T.F. Cotterell, © National Museum of Wales.
Introduction: ankerite is widespread as a diagenetic mineral in sedimentary rocks. It is also frequently encountered in veins and other hydrothermal mineral deposits, in which it may be associated with quartz, calcite, siderite and a wide variety of sulphide minerals. Ankerite forms two mineral series, one with dolomite (magnesium-rich end-member) and one with kutnohorite (manganese-rich end-member). Somewhat confusingly iron-deficient (or magnesium-rich) ankerite is termed ferroan dolomite. This has led to reports of dolomite from localities where only ankerite (variety ferroan dolomite) occurs.
Occurrence in Wales: ankerite occurs widely in Wales but records of notable occurrences are few. It has been reported from a number of mining districts: for example, Gilbey (1968) noted its occurrence within the Dolgellau Gold-belt; Russell (1944) noted its presence at the Bwlch Antimony Mine near Deganwy; Southwood & Bevins (1995) listed it as occurring at Parys Mountain and Raybould (1974) cited it as a major gangue mineral in the Central Wales Orefield. Ankerite is also an important mineral in the South Wales Coalfield, where it occurs both in the clay-ironstone septarian void infill assemblage and more particularly on joints in massive sandstones, from which some excellent specimens have been recovered in recent years.

Key Localities:

  • 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 microprobe analysis (T.F. Cotterell, unpublished data) of carbonates from Loveden and Eaglebrook mines has given compositions consistent with magnesium-rich (or iron-deficient) ankerite, also known as ferroan dolomite.
  • Nant Helen Opencast, Abercraf, South Wales: excellent ankerite 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). Pale buff, brown-weathering, rhombohedral ankerite crystals up to 3 cm (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 were collected from this, the finest Welsh occurrence in specimen terms.
  • 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. Recent X-ray diffraction studies at the National Museum of Wales have confirmed the presence of ankerite. Additionally, sandstone-joint associated ankerite, similar to (but not of such high quality as) that of Nant Helen occurs on many coal-tips.

References:

  1. 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
  2. Firth, J.N.M., 1971. The Mineralogy of the South Wales Coalfield. Unpublished Ph.D. thesis, University of Bristol.
  3. 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.
  4. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  5. 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.
  6. 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.
  7. 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.
  8. Russell, A., 1944. Notes on some minerals either new or rare in Britain. Mineralogical Magazine, 27, 1-10.
  9. Southwood, M. & Bevins, R.E., 1995. Parys Mountain -The type locality for Anglesite. UK Journal of Mines & Minerals 15, 11-17.