Kutnohorite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: Calcium manganese magnesium iron carbonate
Chemical Formula: Ca(Mn,Mg,Fe2+)(CO3)2
Method(s) of Verification: Harlech manganese ore-bed - EMPA (Bennett, 1987b).

Chemical Group:

  • Carbonates

Geological Context:

  • Hydrothermal: sedimentary exhalative deposits
Kutnohorite, predominantly in the red bands of the manganese ore horizon, Harlech Dome area. Cut and polished slab, 25 cm across. Llyn Dywarchen Mines, north Harlech Dome area. © National Museum of Wales.
Introduction: kutnohorite is found typically in manganese ore deposits occurring with other manganese-bearing carbonates and silicates.
Occurrence in Wales: Bennett (1987a & 1987b) identified kutnohorite in the manganese-rich horizons (including the 'ore-bed') in the Hafotty Formation of Cambrian age from the Harlech Dome (now known as the Harlech Horst) region of Gwynedd. Further occurrences, listed below, have been reported from the Llŷn. Kutnohorite is a member of the dolomite group and forms solid solutions with both dolomite and ankerite; when crystalline the pinkish colour aids identification but the Welsh material is too fine-grained to permit this.

Key Localities:

  • Harlech Dome area, Gwynedd: the distinctive, banded red and yellow manganese ore-bed that persists around the south and west parts of this uplifted area of Cambrian rocks carries significant kutnohorite in its red bands associated with spessartine, hematite, magnetite, pyrophanite, quartz and aluminosilicates (Bennett, 1987b). It has been mined throughout the extent of its outcrop, which is approximately 9 m above the base of the Hafotty Formation (Allen & Jackson, 1985). The thickness varies from less than 20 cm to almost 1 m with grades approaching 32% Mn in places (Woodland, 1939) and the ore was mined chiefly between 1892 and 1928, when about 44,000 tons were extracted (Thomas, 1961). Most of the workings were opencast in nature. The ore-bed is thought to be a sedimentary-exhalative hydrothermal deposit, precipitated on the sea-bed at a considerable distance from the fluid source.
  • St Tudwal's Peninsula, Llŷn, Gwynedd: identified as a component of metabentonites (Roberts & Merriman, 1990) and from a concentrically zoned 'object' in a fine-grained altered tuff from the Cared Formation at Porth Ceiriad (Bennett, 1987a).

There are no key localities for this specimen.

References:

  1. Allen, P.M. & Jackson, A.A., 1985. Geology of the country around Harlech. Memoirs of the British Geological Survey. Explanation of sheet 135, with part of 149, 112pp.
  2. Bennett, M.A., 1987a. The Cambrian manganese deposits of North Wales. Unpublished Ph.D. thesis, University of Leeds.
  3. Bennett, M.A., 1987b. Genesis and diagenesis of the Cambrian manganese deposits, Harlech, North Wales. Geological Journal, 22, 7-18.
  4. Roberts, B. & Merriman, R.J., 1990. Cambrian and Ordovician metabentonites and their relevance to the origins of associated mudrocks in the northern sector of the Lower Palaeozoic Welsh marginal basin. Geological Magazine, 127, 31-43.
  5. Thomas, T.M., 1961. The mineral wealth of Wales and its exploitation. Edinburgh & London: Oliver & Boyd, 248pp.
  6. Woodland, A.W., 1939a. The petrography and petrology of the Lower Cambrian manganese ore of western Merionethshire. Quarterly Journal of the Geological Society of London, 95, 1-35.

There are no references for this specimen.