Spessartine

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Manganese aluminium silicate
Chemical Formula: (Mn2+)3Al2(SiO4)3
Method(s) of Verification: Harlech area – EMPA (de Béthune, 1972; Bennett, 1987a); Llanfairpwllgwyngyll - EMPA (Gyopari, 1984)

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
  • Metamorphic
Banded sedimentary manganese ore from the Middle Cambrian of the Harlech area. The brick-red bands are rich in spessartine. The cut and polished sample, 25 cm across, is from the Llyn Dywarchen mines in the N Harlech Dome area. © NMW.
Spessartine in thin section, Cwm Mynach Mine, Harlech Dome, Gwynedd. National Museum of Wales Collection (NMW 58.397.GR.3b). © National Museum of Wales.
Introduction: spessartine is a member of the garnet group which forms a series with almandine. Spessartine forms during metamorphism of manganiferous mudrocks.
Occurrence in Wales: spessartine is widely developed in Wales in low-grade metamorphic rocks.

Key Localities:

  • Harlech area, Gwynedd: spessartine is widespread in the area to the east of Harlech, where it forms a major component in bedded manganese ores of Cambrian age. Originally, the silicate component of these ores was considered to be rhodonite (see Dewey & Bromhead, 1915; Cox & Wells, 1927) but subsequently it has been shown to be spessartine (Woodland, 1935; Matley & Wilson, 1946). Bennett (1987b) provides a detailed modern study of these ores.
  • Llanfairpwllgwyngyll, Anglesey: a more unusual occurrence of spessartine is as small pin-head garnets (0.02 mm diameter) at the junction between basic rocks and quartz-phengite (muscovite) schists metamorphosed to blueschist facies conditions. These were recorded from temporary exposures made during the construction of the A55 in the 1980’s. Although the garnet is not visible in hand specimen a profusion of euhedral crystals are observed in thin section (Gyopari, 1984).

References:

  1. 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.
  2. Bennett, M.A., 1987a. The Cambrian manganese deposits of North Wales. Unpublished Ph.D. thesis, University of Leeds.
  3. Bennett, M.A., 1987b. Genesis and diagenesis of the Cambrian manganese deposits, Harlech, North Wales. Geological Journal, 22, 7-18.
  4. 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.
  5. Gyopari, M., 1984. A study of blueschist mineral chemistry and a new look at the Penmynydd-Gwna Boundaey in SE Anglesey. Unpublished M.Phil thesis, University of Wales, 123pp.
  6. Matley, C.A. & Wilson, T.S., 1946. The Harlech Dome, north of the Barmouth Estuary. Quarterly Journal of the Geological Society of London, 102, 1-40.
  7. Woodland, A.W., 1935. Spessartine in the Cambrian manganese ores of Merionethshire. Geological Magazine, 72, 384.