Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Iron magnesium aluminium borosilicate hydrate
Chemical Formula: :XY3Z6(BO3)3Si6O18(V)3(W), where X = Na + Ca; Y = Mg + Fe + Mn + Li + Al; Z = Al + Mg + Fe3+ + Cr; V = O + OH and W = O + OH + F
Method(s) of Verification:
Classification of tourmaline into alkali, calcic and vacancy groups (Hawthorne & Henry, 1999)
Classification of tourmaline groups into varieties (Hawthorne & Henry, 1999).
Introduction: tourmaline is the group name for a number of borosilicate minerals. These have now been divided into three subgroups on the basis of the X site content (Na, Ca or a vacancy – see formula for details and diagram below). The subgroups are as follows: 1) the alkali group (elbaite, schorl, dravite, buergerite and povondraite); 2) the calcic group (feruvite, liddicoatite and uvite); 3) the vacancy group (foitite). Each of the minerals in these groups can be further qualified by the content of the W site into hydroxyl-, oxy- and fluor- varieties (see Hawthorne & Henry (1999) for details). Tourmaline group minerals are most frequently encountered in granitic intrusive igneous rocks and pegmatites. They may also develop in large amounts as a result of metasomatism – a bulk-alteration process similar to metamorphism in that the mineralogy changes, but differing in that the bulk chemical composition of the affected rock also changes. The formation of metasomatic tourmaline, known as tourmalinization, involves the introduction into the rock of boron: hence the process is also known as boron metasomatism. Tourmaline is also a common detrital mineral in sediments derived from the erosion of granites and similar rocks. Without detailed optical or analytical data, the various members of the tourmaline group cannot be identified with confidence. Thus, many occurrences worldwide are still simply referred to as 'tourmaline'.
Occurrence in Wales: many of the early recordings of tourmaline in Wales pre-date the subdivision explained above. Examples of generic references to tourmaline are thus included here, but specific entries where the actual member of the group has been defined by analytical work are listed under dravite, foitite and schorl. Many of the instance of tourmaline occurrences in Wales are from sediments as detrital or authigenic minerals. One of the earliest records is provided by Fearnsides (1908) who located “tourmalinized-grits” at Cwm Dwythwc, Snowdonia, from a study of detrital minerals in river gravels. A more detailed description was subsequently provided by Williams (1927) who reported well-formed, blue to brown zoned tourmaline in Arenig grits, tracing them from Bwlch Gwyn to Brithdir. Beavon (1963) located further outcrops of tourmalinized grits in the vicinity of Aberglaslyn, as well as in blocks in the slightly younger Llyn Dinas Breccias, exposed to the east of Beddgelert (see dravite and schorl entries). Bromley (1969) identified similar brecciated and tourmalinized rocks in the contact aureole of the Tan-y-Grissiau microgranite. In all these cases the tourmaline is zoned from blue-green to brown. Bromley (1969) considered the tourmalinization to be related to pneumatolytic (late stage magmatic) activity associated with contemporary acid-magmatism. On Anglesey, tourmaline has been identified as a minor component in various rock units in the Monian Supergroup, such as the Holyhead Formation (‘Holyhead Quartzite’), the South Stack Formation, Tyfry Beds and Gwna Group grits and quartzites (Greenly, 1919). It has also been described from the Coedana Complex gneisses and the hornfels (Greenly, 1919). Woodland (1938) reported minute, euhedral, pleochroic pale yellowish-green to greenish-brown, prismatic tourmaline crystals (0.20 -0.035 mm in length and 0.005-0.015 mm in width) in mudstone of the Cambrian Harlech Grits Group, exposed in the Harlech Dome region, as for example on the east side of “the Diphwys” (Diffwys, grid ref. SH 659 233) The occurrence of tourmaline in this area was subsequently confirmed by Béthune (1972).
There are no key localities for this specimen.
- Béthune S. de, 1972. Etude á la microsonde électronique des minéraux métamorphiques du Cambrien de Harlech (Pays de Galles). Bulletin de la Société Belge de Géologie de Paléontologie et d'Hydrologie, 81, 255-265.
- 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.
- Bromley, A.V., 1969. Acid plutonic igneous activity in the Ordovician of North Wales. In: Wood, A. (ed.) The Pre-Cambrian and Lower Palaeozoic rocks of Wales. University of Wales Press, 387-408.
- Fearnsides, W.G., 1908. The tourmaline rocks of Cwm Dwythwc, near Llanberis (North Wales). Geological Magazine, New Series, Decade 5, 5, 465-466.
- Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
- Hawthorne, F. C. & Henry, D. J., 1999. Classification of the minerals of the tourmaline group. European Journal of Mineralogy, 11, 201-215.