Celsian

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: Barium aluminium silicate
Chemical Formula: BaAl2Si2O8
Method(s) of Verification: Benallt Mine - XRD & wet chemistry (Spencer, 1942).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
Manebach-Baveno fourling crystal of celsian (13 mm across) from Benallt Mine, near Rhiw, Ll?n. National Museum of Wales Collection (NMW 27.111.GR.381), ex G.J. Williams Collection. Photo M.P. Cooper, © National Museum of Wales.
Manebach twinned celsian crystals from Benallt Mine. National Museum of Wales Collection (NMW 27.111.GR.384), ex G.J. Williams Collection. Photo M.P. Cooper, © National Museum of Wales.
Introduction: celsian is a rare barium feldspar, it is dimorphous with paracelsian (it has the same chemical composition but a different arrangement of atoms in the crystal lattice) and forms a series with hyalophane [(K,Na,Ba)Si2(Al,Si)2O8] and orthoclase. It is commonly found in contact or regionally metamorphosed manganese deposits where it can form by the breakdown of cymrite [BaAl2Si2(O,OH)8.H2O]. Celsian is rare in igneous rocks but has been described from alkali rocks from the Kola Peninsula
Occurrence in Wales: the first reported occurrence of celsian in the British Isles was a short note by Russell (1911) describing ‘some finely crystallized mineral specimens from North Wales’ which had been sent to him by G.J. Williams, then H.M. Inspector of Mines for North Wales. At the time this represented just the second world-wide recording of this mineral. Although a more complete description was not provided until many years later (Spencer, 1942).

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: Spencer (1942) described a massive granular rock composed entirely of celsian. Two crystal forms were identified, one being long, slender to acicular prisms, up to 5 mm in length and rarely twinned on the Carlsbad Law; the other forming larger, stout, short, prismatic crystals averaging 1 cm across and 0.05-1.0 mm in thickness showing both Manebach and Baveno twin forms. Specimens in the National Museum of Wales Collection are from the G.J. Williams Collection and were recovered during working operations at the mine in the early 1900s. The largest single crystal in the G.J. Williams Collection reaches 19.5 mm in length (NMW 27.111.GR.383).

References:

  1. Russell, A., 1911. An occurrence of the barium feldspar celsian in North Wales. Nature, 86, 180.
  2. Spencer, L.T., 1942. Barium-feldspars (celsian and paracelsian) from Wales. Mineralogical Magazine, 26, 231-245.