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 - wet chemistry and X-ray powder photography (Spencer, 1942).
Prismatic crystal of paracelcian from Benallt Mine. Specimen National Museum of Wales (NMW 83.41G.M6982), photo M.P. Cooper, © National Museum of Wales.
Introduction: paracelsian, a polymorph of celsian, is a rare barium feldspar mineral with only a few occurrences known World-wide. Well crystallised material is known from only one locality, that being the occurrence in Wales.
Occurrence in Wales: paracelsian was first described in 1905 from Candoglia, Valle d'Ossola, Piedmont, Italy. Six years later, Russell (1911), suggested that a specimen of celsian from North Wales, presented to him by G.J. Williams, former Assistant Inspector for Metalliferous Mines and Quarries for the North Wales and Ireland Division, contained a second, dimorphous form of the feldspar. A full description of this material from Benallt Mine, Pen Llŷn, was provided much later by Spencer (1942). He described paracelsian associated with celsian in a band in Ordovician shales and sandstones associated with lavas, tuffs, and beds of black manganese ore. This is the only occurrence of paracelsian known from Wales, and is also the first, and only known UK occurrence.
- Benallt Mine, Llŷn, Gwynedd: paracelsian from this locality forms translucent white to milky-white prismatic crystals, with a vitreous lustre and chisel-shaped terminations. Specimens typically consist of an aggregate of prismatic crystals, up to 5 cm long and 1 cm wide (e.g. NMW 27.111.GR.387, ex G.J. Williams Collection), on less well-formed crystalline material. One specimen in the National Museum of Wales Collection (NMW 90.24G.M.1) shows a pale pink fluorescence.
- Russell, A., 1911. An occurrence of the barium feldspar celsian in North Wales. Nature, 86, 180.
- Spencer, L.T., 1942. Barium-feldspars (celsian and paracelsian) from Wales. Mineralogical Magazine, 26, 231-245.
- Tacconi, E., 1905. Di un silicato di alluminio e bario [paracelsian] dei calcefiri di Candoglia in valle del Toce. Rendiconti R. Istituto, Lombardo, Science. Letters., Milano, 38, 636-643.