Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Barium aluminium silicate hydrate
Chemical Formula: BaAl2Si3O10.4H2O
Method(s) of Verification: Benallt Mine - XRD at the National Museum of Wales (XRD numbers NMW X-1608 & X-1611); Dolyhir Quarry - XRD at the National Museum of Wales (XRD no. NMW X-1861).

Chemical Group:

  • Silicates
Glassy, wedge-shaped edingtonite crystals with prismatic harmotome from Dolyhir Quarry, Old Radnor, Powys. Field of view is 4 mm across. © National Museum of Wales.
Introduction: edingtonite is a rare mineral which forms in cavities in mafic igneous rocks and nepheline syenites. It also occurs in hydrothermal veins.
Occurrence in Wales: edingtonite occurs as a minor component of sugary white veinlets cutting massive analcime at Benallt manganese mine at the southern end of Pen Llŷn (T.F. Cotterell, unpublished data). Distinct wedge-shaped microcrystals have more recently (July 2007) been discovered at Dolyhir Quarry, near Old Radnor in the Welsh Borderland (Cotterell et al., 2011).

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: semi-quantitative XRD analysis of sugary white veinlets cutting massive creamy analcime indicates that 3-6 % edingtonite is present associated with major analcime, banalsite and minor harmotome (T.F. Cotterell, unpublished data).
  • Dolyhir Quarry, Old Radnor, Powys: sub mm sized, glassy, characteristic wedge shaped crystals, have been identified in an altered dolerite dyke exposed briefly in 2007. Abundant core-bit twinned prismatic harmotome crystals occur in association (Cotterell et al., 2011).


  1. Cotterell, T.F., Green, D.I., Hubbard, N., Mason, J.S., Starkey, R.E. and Tindle, A.G., 2011. The Mineralogy of Dolyhir Quarry, Old Radnor, Powys, Wales. UK Journal of Mines and Minerals, 32, 5-61.