Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Potassium barium aluminium silicate
Chemical Formula: (K,Ba)Al(Si,Al)3O8
Method(s) of Verification: Benallt Mine - EMPA.

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal
  • Metamorphic: low-grade
Stubby prismatic hyalophane crystals from Benallt Mine, Llyn, Gwynedd. Field of view 3.5 mm wide. National Museum of Wales Collection (NMW 2002.60G.M.1). Photo T.F. Cotterell, © National Museum of Wales.
Introduction: hyalophane is a member of the orthoclase-celsian feldspar series and typically contains 2% to 60% BaAl2Si2O8 (Deer et al., 2001). It is found in metamorphosed manganiferous rocks or in mineral deposits rich in manganese.
Occurrence in Wales: hyalophane has been confirmed from just one locality in Wales - Benallt Mine - where it occurs with the similar-looking barium feldspar, celsian.

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: National Museum of Wales specimens (e.g. NMW 2002.60G.M.1 - M.3) collected by R. Tayler, show small, white, short prismatic crystals of hyalophane associated with piemontite and titanite.


  1. Deer, W.A., Howie, R.A. & Zussman, J., 2001. Rock-forming minerals Volume 4A 2nd Ed. Framework silicates: Feldspars, The Geological Society, London.