Analcime

Crystal System: Tetragonal,Orthorhombic,Monoclinic,Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Sodium aluminium silicate hydrate
Chemical Formula: Na(Si2Al)O6.H20
Method(s) of Verification: Plas Newydd - X-ray diffraction (NMW X-737, NHM No.2500-1975 P75), chemical analysis (Henslow, 1822); Gimlet Quarry - X-ray diffraction (NMW X-57, 27.111.GR.377); all other localities identification is by optical means.

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal
  • Metamorphic: low-grade
  • Igneous
Scanning electron micrograph of analcime crystals from Gimlet Rock, Pwllheli, Gwynedd. National Museum of Wales Collection. Image T.F. Cotterell, © National Museum of Wales.
Introduction: analcime is the sodium equivalent of wairakite [CaAl2(SiO3)4.2H2O] and is a member of the zeolite group of minerals, forming a series with pollucite [(Cs,Na)2Al2Si4O12.H2O]. Analcime typically occurs in both the groundmass of silica-poor basic and intermediate igneous rocks and in vesicles, where it forms by the action of late stage hydrothermal solutions or low temperature alteration.
Occurrence in Wales: analcime was first described in Wales by Henslow (1822) as a contact metamorphic mineral associated with the Plas Newydd Palaeogene dyke on Anglesey. Subsequently at least four further localites have been identified where analcime has been generated by hydrothermal alteration or low grade metamorphism of basic igneous rocks (Williams, 1924; Campbell Smith, 1946; Metcalfe, 1990). Only one reported occurrence of primary igneous analcime is known from Wales (Boulton, 1911; Haslett, 1992).

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: Campbell Smith (1946) described analcime and pennantite from thin (5 mm) veins cutting massive manganese ore in the Ty Canol incline of Benallt Mine.
  • Gimlet Quarry, Pwllheli, Gwynedd: although specimens can no longer be collected from this site, those in the collection of the National Museum of Wales show analcime crystals up to 25 mm across. These vary from glassy to milky in appearance but are always found coating quartz (e.g. NMW 27.111.GR.375-377).
  • Glen Court & Golden Hill, Usk, Monmouthshire: just one occurrence of primary igneous analcime has been recorded from Wales, from near Usk. Colourless, euhedral to anhedral crystals are described from the groundmass of the monchiquite intrusion at Golden Hill (Boulton, 1911) and near by at Glen Court (Haslett, 1992).
  • Llanelwedd Quarry, Builth Wells, Powys: Ordovician lava in Llanelwedd Quarry, just outside Builth Wells, were subjected to hydrothermal alteration associated with volcanic activity (Bevins, 1985). This alteration is recorded by analcime, quartz, calcite and laumontite veins which cut the lavas (Metcalfe, 1990).
  • Plas Newydd, Anglesey: Henslow (1822) described perfectly formed, faceted crystals of analcime with a trapezoid form. He provided an analysis, which suggested ‘analcime with an excess of iron’. Harker (1887) remarked that this analysis implied that the analcime is half-converted to prehnite, however more recent X-ray diffraction work confirms the presence of analcime.
  • Snowdonia, Gwynedd: glassy, colourless analcime is present as vesicle infillings within the suite of Palaeogene (‘Tertiary’) olivine-dolerite dykes exposed between Marchlyn Mawr Reservoir and Blaen-y-nant to the west of Nant Ffrancon (Williams, 1924; 1930).

References:

  1. Anthony J., Bideaux R., Bladh K. & Nichols M., 1995. Handbook of Mineralogy, vol. 2 (parts 2/1 and 2/2), Silica, Silicates
  2. Bevins, R.E., 1985. Pumpellyite-dominated metadomain alteration at Builth Wells, Wales – evidence for a fossil submarine hydrothermal system? Mineralogical Magazine, 49, 451-456.
  3. Boulton, W.S., 1911. On a monchiquite intrusion in the Old Red Sand stone of Monmouthshire. Quarterly Journal of the Geological Society, London, 67, 460-476.
  4. Campbell Smith, W., Bannister, F.A. & Hey, M.H., 1946. Pennantite, a new manganese-rich chlorite from Benallt Mine, Rhiw, Carnarvonshire. Mineralogical Magazine, 27, 217-220.
  5. Harker, A., 1887. On some Anglesey dykes. Geological Magazine, New Series, Decade 3, 4, 409-416.
  6. Haslett, S.K., 1992. Petrology of a monchiquite from the Welsh Borderlands. Mercian Geologists,13,43-46.
  7. Henslow, J.S., 1822. Geological description of Anglesea. Transactions of the Cambridge Philisophical Society, 1, 155-156.
  8. 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.
  9. Williams, D., 1930. The geology of the country between Nant Peris and Nant Ffrancon (Snowdonia). Quarterly Journal of the Geological Society of London, 86, 191-232.
  10. Williams, D., 1924. On two olivine-dolerite dykes in Snowdonia. Proceedings of the Liverpool Geological Society, 14, 38-47.