Stilpnomelane

Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: Potassium iron magnesium aluminium silicate hydroxide
Chemical Formula: (K,Ca,Na)(Fe,Mg,Al)8(Si,Al)12(O,OH)36•nH2O
Method(s) of Verification: Pen-yr-allt Mine - polarizing microscope; St. David’s Head - optical microscope (Roach & Bevins, 1982); Tal y fan - EMPA (Bevins & Merriman, 1988)

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
Bladed crystals of stilpnomelane, from Tyllau-Mwn Mine, Aberangell. Specimen National Museum of Wales (NMW 83.43G.M.1), photo taken by D.I. Green, © National Museum of Wales.
Introduction: stilpnomelane can show quite a wide range in composition. It is typically a low-grade metamorphic mineral, being found in greenschist facies rocks associated with albite, epidote, actinolite and chlorite but may also be found in blueschists. In iron-formations such as banded ironstones, it is accompanied by other iron and iron-silicate minerals. Where stilpnomelane is associated with sulphide deposits it is a product of surface weathering (Deer et al., 1992).
Occurrence in Wales: stlipnomelane has a sporadic occurrence across Wales in rocks of suitable composition and metamorphic grade. Bevins & Rowbotham (1983) documented its occurrence in relatively fractionated igneous rocks, principally of Ordovician age, which have been affected by low-grade metamorphism and provide analysis from a selection of locations across Wales.

Key Localities:

  • Pen-yr-allt Mine, nr. Penrhydeudreath, Gwynedd: Hallimond (1924) identified the presence of a brittle dark-brown mineral in Ordovician pisolitic ironstones exposed at Pen-yr-allt Iron Mine. This strongly resembled biotite but he identified this as stilpnomelane and this represents the first recording of this mineral in Great Britain. Hallimond noted that the mineral had already been observed by Mr G.J. Williams (former Inspector of Mines for North Wales) and by Sir Arthur Russell. Russell also passed on to Hallimond a stilpnomelane specimen from the oolitic ironstones exposed at Cross Foxes, Dolgellau, Gwynedd. Stilpnomelane specimens from the G.J. Williams Collection, now housed in the National Museum of Wales, are labelled as being from Bryn Mawr Mine, Llanbedr, Gwynedd. In the oolitic iron ore of the same age, Pulfrey (1933) reported blades and veins of stilpnomelane in ore from mines at Pen-yr-allt and Betws Garmon in Gwynedd.
  • St. David's Head, Pembrokeshire: Roach (1969) and Roach & Bevins (1982) identified abundant stilpnomelane in the more fractionated parts of the St. David’s Head Intrusion; in the granophyric gabbros it forms up to 13% of the rock by volume.
  • Tal y Fan, Conway: a demonstration of the restriction of stilpnomelane to the more fractionated basic igneous rocks is provided by its occurrence in only the ferrodolerite horizons of the metabasites of the Tal y Fan Intrusion (Merriman et al. 1986; Bevins & Merriman,1988).

References:

  1. Bevins, R.E. & Roach, R.A., 1982. Ordovician igneous activity in south-west Dyfed. In Bassett, M.G. (ed.) Geological excursions in Dyfed, south-west Wales. National Museum of Wales, Cardiff, 65-80.
  2. Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
  3. Bevins,R.E. & Merriman, R. J., 1988. Compositional controls on co-existing prehnite-actinolite and prehnite-pumpellyite assemblages in the Tal y Fan metabasite intrusion, North Wales: implications for Caledonian metamorphism field gradients. Journal of Metamorphic Geology, 6, 17-39.
  4. Bloxham, T.W. & Price, N.B., 1961. Stilpnomelane in North Wales. Nature, 52, 563-526.
  5. Cox, A.H. & Wells, A.K., 1920. The Lower Palaeozoic rocks of the Arthog-Dolgelly district. Quarterly Journal of the Geological Society of London, 76, 254-324.
  6. Deer, W.A., Howie, R.A. & Zussman, J., 1986. Rock-Forming Minerals, Vol. 1A, Orthosilicates, 2nd Ed. Longman Group Ltd, 918pp.
  7. Hallimond, A.F., 1924. On stilpnomelane from North Wales. Mineralogical Magazine, 20, 193-197.
  8. Matthews, D.W. & Scoon, J.H., 1964. Notes on a new occurrence of stilpnomelane from North Wales. Mineralogical Magazine, 33, 1032-1037.
  9. Merriman, R. J., Bevins, R.E. & Ball, T.K., 1986. Petrology and geochemical variation within the Tal y Fan Intrusion: a study of element mobility during low-grade metamorphism with implications for petro-tectonic modelling. Journal of Petrology, 27, 1409-1436.
  10. Pulfrey, W., 1933. The iron-ore oolites and pistolites of North Wales. Quarterly Journal of the Geological Society, 89, 401-430.
  11. Roach, R.A., 1969. The composite nature of the St. David's Head and Carn Llidi intrusions of North Pembrokeshire. In: Wood, A. (ed.) The Pre-Cambrian and Lower Palaeozoic rocks of Wales, University of Wales Press, Cardiff, 409-433.
  12. Williams, H., 1930. The Snowdon District: Report of the Easter field meeting of the Geologists’ Association. Proceedings of the Geologists’ Association, 41,190-205.