Crystal System: Monoclinic,Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Iron magnesium manganese aluminium silicate hydroxide
Chemical Formula: (Fe2+,Mg,Mn)2Al4Si2O10(OH)4
Method(s) of Verification: all occurrences cited - polarizing microscope identification; Rhyd-ddu - EMPA (Brearley, 1988); Cwm Pennant - EMPA (Roberts et al., 1989).
- Metamorphic: low-grade
Minute crystal (maximum length 0.1 mm) of chloritoid from Rhyd-ddu, viewed under the microscope in crossed polarised light. Specimen National Museum of Wales (NMW 90.17G.R.26), © National Museum of Wales.
Introduction: chloritoid is found in low to medium grade regionally metamorphosed pelitic rocks, particularly those rich in aluminium and ferric iron (Fe2+) and poor in calcium, magnesium, potassium and sodium. Although resembling a mica, chloritoid is distinguished from members of this mineral group by the inflexibility of its mineral flakes.
Occurrence in Wales: chloritoid occurs widely in Snowdonia, although it has only been reported from a few localities as identifications can only be made by thin section examination. It was first reported in Wales, from Ordovician rhyolitic lavas in Snowdonia, by Harker (1889, p.17), with specific mention of occurrences in the Diganwy (Deganwy) Hills, near Conway and north of Beddgelert. These occurrences have not been verified subsequently and it is possible that they may be of a chlorite group mineral. More recently Evans (1968) noted chloritoid from Ordovician slates in the contact aureole of the Aber-Drosgl Intrusion, near, Bethesda; Brearley (1988) recorded it from exposures of black slates near Rhyd-ddu and Roberts et al. (1989) noted it from Ordovician slates at Cwm Pennant.
- Cwm Pennant, Snowdonia, Gwynedd: small porphyroblasts of chloritoid occur in slate at Cwm Pennant. The crystals are of a similar size to those at Rhyd-ddu, but Roberts et al. (1989), describe textures indicating that the chloritoid grew both during and after deformation and slate formation.
- Rhyd-ddu, Snowdonia, Gwynedd: chloritoid forms microscopic tabular crystals within the black slates of the Nant Ffrancon Formation. It occurs in a matrix of quartz, chlorite, and muscovite/paragonite as crystals 30-100 μm (0.03-0.1 mm) long and 2-20 μm wide, some of which show classic fan tail forms. Brearely (1988) considered the chloritoid to have grown prior to the cleavage formation, however Roberts et al. (1989) disagreed with this interpretation. The National Museum of Wales collections contain chloritoid-bearing mudstones from Drws-y-coed Uchaf [SH 5640 5268], near Rhyd-ddu (e.g. NMW 90.17G.R.26).
- Brearley, A.J., 1988. Chloritoid from low-grade pelitic rocks in North Wales. Mineralogical Magazine, 52, 394-396.
- Evans, C.D.R., 1968. Geological succession and structure of the area east of Bethesda. Unpublished Ph.D. thesis, University of Wales, Aberystwyth.
- Harker, A., 1889. The Bala Volcanic Series of Caernarvonshire. The Cambridge University Press, 130pp
- Roberts, B., Evans, J.A., Merriman, R.J. and Smith M., 1989. Discussion of ’Low grade metamorphism of the Welsh Basin Lower Palaeozoic succession: an example of diastathermal metamorphism'. Journal of the Geological Society, 146, 885-890.