Dravite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Sodium magnesium aluminium hydroxy borosilicate.
Chemical Formula: NaMg3Al6Si6O18(BO3)3(OH)3(OH)
Method(s) of Verification: Cwm Bychan & Hanter Hill - EMPA (National Museum of Wales/A.J. Tindle, unpublished data).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: skarn mineralization
  • Metamorphic
  • Igneous

SEM backscatter image of zoned tourmaline from Cym Bychan (largest crystal 0.2 mm across); core composed of schorl and rim of dravite. Other crystals ar composed of either schorl alone or schorl and foitite. National Museum of Wales specimen.

Introduction: dravite is a magnesium-rich, alkali tourmaline (see tourmaline entry for details of the various tourmaline groups) that forms a series with schorl (iron-rich alkali tourmaline). It is found typically in metamorphic or skarn-type mineral assemblages and is often associated with axinite and datolite. The boron is introduced into the rock by fluids (metasomatism) and in most cases, granite magma is thought to be the source of the boron.
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 New data (National Museum of Wales/A.J. Tindle, unpublished data) confirms the presence of dravite in at least two of these occurrences. As dravite 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 dravites are dominated by hydroxyl anions and therefore can be termed hydroxyl-dravite (although this classification, proposed by Hawthorne & Henry (1999), has not been formally accepted by the IMA). As more microprobe data becomes available it is anticipated that a more widespread distribution of dravite in Wales will be demonstrated.

Key Localities:

  • Cwm Bychan, Beddgelert, Gwynedd: samples of Caradocian sandstone studied by Beavon (1963) (e.g. NMW 80.30G), held in the National Museum of Wales, contain aggregates of subhedral basal sections and prisms, showing concentric optical zoining. Microprobe analyses reveal these to be crystals 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 (Horák, NMW 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). Microprobe analyses (NMW unpublished data) give a range in dravite compositions. Tourmaline at Hanter Hill is linked to the extensive alteration of the gabbro, and it is likely that the boron was derived from either the country rock sediments (now unexposed) or from the associated granite component of the intrusion. A sample of dravite from this locality is present in the National Museum of Wales Collection (NMW 2005.45G.R.1).

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.