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.

There are no key localities for this specimen.

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 Unpublished Ph.D. thesis, University of Bristol.

There are no references for this specimen.