Schorl

Crystal System:
Status of Occurrence:
Distribution:
Chemical Composition: Sodium iron aluminium borosilicate hydroxide
Chemical Formula: NaFe3Al6Si6O18(BO3)3(OH)3(OH)
Method(s) of Verification:

Hanter Hill – EMPA (NMW & A.G. Tindle, unpublished data); Cwm Bychan - EMPA (NMW & A.G. Tindle, unpublished data)

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
  • Sedimentary: allogenic (detrital)

SEM backscatter image of zoned tourmaline crystals from Cym Bychan. The largest crystal, 0.2 mm across, has an inner core of schorl and an outer rim of dravite. The prismatic crystal is composed of schorl alone. National Museum of Wales Collection.

Introduction:

schorl is an iron-rich alkali tourmaline (see tourmaline entry for details of the various tourmaline groups) that forms a solid solution with the magnesium-rich, alkali tourmaline, dravite.

Occurrence in Wales:

although several occurrences of tourmaline have been reported from Wales (see tourmaline entry), a lack of detailed optical or analytical data has precluded most of these from being classified to mineral species level. New data (NMW & A.G. Tindle unpublished data) confirms the presence of schorl in at least two of these occurrences. As schorl can be further divided into fluor- hydroxyl- or oxy varieties, depending on the filling of the W site in the crystals lattice, microprobe analyses have also established that the Welsh schorls are dominated by hydroxyl anions and therefore can be termed hydroxyl-schorl (although this classification, proposed by Hawthorne & Henry (1999), has not been formally accepted by the IMA).

Key Localities:

  • Cwm Bychan, Beddgelert, Gwynedd: samples of Caradocian sandstone studied by Beavon (1963) (NMW 80.30G), held in the National Museum of Wales, contain aggregates of subhedral basal sections (up to 0.2 mm across) and prisms (up to 0.28 mm long). These show concentric optical zoning and microprobe analyses reveal the crystals to have cores of schorl and foitite, with outer rims of dravite.
  • Hanter Hill, nr. Old Radnor, Powys: Holgate & Hallowes (1941) described tourmaline clots within late Neoproterozoic, altered gabbro from Hanter Hill in the Welsh Borderland. Although Holgate (1977) described clots that are ‘up to 15 mm across and consist of a single crystal or group of crystals in near parallel orientation’, considerably larger clots (over 40 mm across) have been observed (J.M. Horák, unpublished data). In thin section the tourmaline shows pleochroism from pale slate-grey to olive-grey and slate blue-grey to deep slate-green. Holgate (1977) published wet chemical analyses and XRD data to classify the tourmaline towards the dravite end of the schorl-dravite series and microprobe analyses show that the crystals range from dravite to schorl in composition, but do not support the high calcium content recorded by Holgate (1977). Tourmaline at Hanter Hill is linked to the extensive alteration of the gabbro, and it likely that the boron was derived from either the country rock sediments (now unexposed) or from the associated granite component of the intrusion.

References:

  1. Beavon, R.V., 1963. The succession and structure east of the Glaslyn River, North Wales. Quarterly Journal of the Geological Society of London, 119, 479-512.
  2. Hawthorne, F. C. & Henry, D. J., 1999. Classification of the minerals of the tourmaline group. European Journal of Mineralogy, 11, 201-215.
  3. Holgate, N., 1977. Tourmaline from amphibolized gabbro at Hanter Hill, Radnorshire. Mineralogical Magazine, 41, 124-127.
  4. Holgate, N. & Hallowes, K.A.K., 1941. The igneous rocks of the Stanner-Hanter District, Radnorshire. Geological Magazine, 78, 241-267.