Celadonite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Potassium magnesium iron aluminium silicate hydroxide
Chemical Formula: K(Mg,Fe2+)(Fe3+,Al)Si4O10(OH)2
Method(s) of Verification: Llangynog, Carmarthenshire - polarizing microscope (Cope & Bevins, 1993); Llanelwedd Quarry - polarizing microscope (Metcalfe, 1990), XRD (R.J. Merriman, BGS, specimen NMW 86.23G.M.1); Mew Stone - XRD (R.J. Merriman, BGS, specimen NMW 86.24G.M.1).

Chemical Group:

  • Silicates
Introduction: celadonite is a member of the mica group and typically develops in vesicles or replacing ferromagnesian minerals in altered intermediate to basic igneous rocks. It has a characteristic blue-green colour.
Occurrence in Wales: several occurrences of celadonite have been reported from Wales in recent years, all from rocks of basic to intermediate composition which have undergone low-grade metamorphism. Representative specimens are held in the collections of the National Museum of Wales.

Key Localities:

  • Llanelwedd Quarry, Builth Wells, Powys: celadonite has been identified as a thin coating on altered tuffs of the Builth Volcanic Group exposed at this quarry. Specimen NMW 90.38G.T.164 (National Museum of Wales collections) shows celadonite associated with prehnite and pumpellyite.
  • Llangynog, Carmarthenshire: Cope & Bevins (1993) record sparse celadonite from the altered volcaniclastic sandstones of the late Precambrian Coomb Volcanic Formation.
  • Skomer Island, Pembrokeshire: basic lavas of Silurian age, in the vicinity of Mew Stone, Skomer Island show thin green coatings of celadonite similar to those described from Llanelwedd Quarry.

References:

  1. Cope J.C.W. & Bevins, R.E., 1993. The stratigraphy and setting of the Precambrian rocks of the Llangynog Inlier, Dyfed, South Wales. Geological Magazine, 130, 101-111.
  2. Metcalfe, R., 1990. Fluid-rock interaction and metadomain formation during low-grade metamorphism in the Welsh marginal basin. Unpublished Ph.D. thesis, University of Bristol.