Natrolite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Sodium aluminium silicate hydrate
Chemical Formula: Na2Al2Si3O10.2H2O
Method(s) of Verification: material from Benallt Mine has been confirmed by XRD (Hey, 1932); Glen Court (Haslett, 1992), Great House (Boulton, 1911) and Snowdonia (Williams, 1930) have all been identified by polarizing microscope.

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal: amygdale infill & veins in volcanic rocks
  • Metamorphic: low-grade
Blocky prismatic natrolite crystals from Benallt Mine, Rhiw, Llyn, Gwynedd. National Museum of Wales Collection (NMW 27.111.GR.370) ex G.J. Williams Collection. Photo D.I. Green, © National Museum of Wales.
Introduction: a zeolite mineral dimorphous with tetranatrolite. Natrolite occurs in vugs and cavities in basic igneous rocks, typically as one of the last minerals to form.
Occurrence in Wales: one of the earliest recorded occurrences of natrolite from Wales was made by Harrison (1894, 1897) from Gimlet Quarry. A further four localities have been documented; from altered basic rocks ranging from late Precambrian to Palaeogene in age (Boulton, 1911; Greenly, 1919; Williams, 1930; Dewey & Bromehead, 1915; Hey, 1932). Probably the most spectacular of these is natrolite collected from Benallt Mine, Llŷn by Sir Arthur Russell.

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: specimens collected by Sir Arthur Russell in 1911, show discrete crystals up to 3.5 cm in length and 0.4 cm in cross section, as well as non-terminating crystal fragments up to 5 cm long and 1 cm square. It is reported that the crystals were clear when first collected, but then rapidly became white to opaque unless kept in a closed tube. The specimens were subsequently described in detail by Hey (1932) in his study of zeolite minerals. Dewey & Bromehead (1915) remarked that brecciated manganese ore at Benallt was sometimes cemented by massive crystalline natrolite. A further account of natrolite from this locality is provided by Campbell Smith et al. (1944b), who described it present in small interstices and minute veins cutting manganese ore from the footwall side of the no. 1 ore body by no. 1 chute, ’50-60 feet west of the main shaft and some 10-20 feet above the 130-foot level’.
  • Central Anglesey: a zeolite mineral was reported by Greenly (1919) and identified by H.H. Thomas as natrolite. This was found within Precambrian basic gneisses belonging to the Coedana Complex, exposed west of the farmhouse at Henblâs in central Anglesey.
  • Gimlet Quarry, Pwllheli, Gwynedd: Harrison (1894) described natrolite as radiating spherical groups ‘from ¼ to ½ inch in diameter’, associated with calcite and quartz in cavities within altered dolerite. A brief mention of this occurrence was also made in a report on the occurrence of prehnite at Gimlet Quarry (Harrison, 1897).
  • Glen Court & Golden Hill, Usk, Monmouthshire: Boulton (1911) considered that a fibrous, colourless, faintly birefringent zeolite mineral forming radiating aggregates in the groundmass of a monchiquitic basanite (alkali basalt) in the Great House diatreme (volcanic vent) was probably natrolite. Work by Haslett (1992) at nearby exposures of the diatreme at Glen Court confirmed the presence of natrolite.
  • Snowdonia, Gwynedd: Williams (1930) identified natrolite associated with thomsonite in amygdales, up to 2 cm across, within Palaeogene olivine-dolerite dykes in the tract of country between Nant Peris and Nant Ffrancon in central Snowdonia. These identifications cannot be made from hand specimens but required the use of microscope thin sections.

References:

  1. 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.
  2. Campbell Smith, W., Bannister, F.A. & Hey, M.H., 1944b. Banalsite, a new barium-feldspar from Wales. Mineralogical Magazine 27, 33-47
  3. Dewey, H. & Bromehead, C.E.N., 1915. Tungsten and manganese ores. Memoirs of the Geological Survey. Special Reports on the Mineral Resources of Great Britain, 1.
  4. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  5. Harrison, W.J., 1894. New localities for the minerals brookite, natrolite and barytes. Geological Magazine, New Series, Decade 4, 1, 567.
  6. Harrison, W.J., 1897. An occurrence of prehnite in Wales. Mineralogical Magazine, 11, 198
  7. Haslett, S.K., 1992. Petrology of a monchiquite from the Welsh Borderlands. Mercian Geologists,13,43-46.
  8. Hey, M.H., 1932. Studies on the zeolites. Part III. Natrolite and metanatrolite. Mineralogical Magazine, 23, 243-289.
  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.