Tephroite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: Manganese silicate
Chemical Formula: Mn2SiO4
Method(s) of Verification: Benallt Mine - XRD (NHM, x2979, x3017, x3019, x3020, x3021, x13061 & x19405 & National Museum of Wales, NMW X-1487, 1497 & 1498); Nant Mine - XRD (National Museum of Wales, NMW X-1528, 1604, 1607, 1612 & 1615)

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
  • Metamorphic: skarn mineralization
Brown tephroite associated with metallic grey jacobsite. Nant Mine, Rhiw, Ll?n, Gwynedd. Specimen 8.8 cm long. National Museum of Wales specimen. Photo T.F. Cotterell, © National Museum of Wales.
Introduction: tephroite belongs to the broad olivine group of minerals and forms a series with the iron-rich olivine fayalite (Fe2SiO4). It is a typical mineral of iron-manganese ore deposits or their associated skarns, and in metamorphic rocks derived from manganese-rich sediments (Deer at al., 1982). It is associated with minerals such as rhodonite, bustamite, manganocalcite, hausmannite and spessartine.
Occurrence in Wales: tephroite is known from just one locality in Wales, although this represents the first recording of the mineral in the British Isles (Campbell Smith et al., 1944b).

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: tephroite was identified from Benallt Mine where it is intimately associated with alleghanyite, and forms composite, dark, narrow, blade-like crystals up to 20 mm in length and varying in thickness from 2 mm down to microscopic sizes (Campbell Smith et al., 1944b). The crystals are olive-green in thin section and are elongated parallel to the c-axis. Tephroite was found ‘on the footwall side of no.1 ore-body by no. 1 Chute, 50-60 feet west of the main shaft, and some 10-29 feet above the 130-foot level’ (Campbell Smith et al., 1944b). A second locality within the mine was also recorded by these workers, ‘near the foot-wall of no.2 ore-body, located 40 to 80 feet west of the Court Shaft’.
  • Nant Mine, Nant-y-Gadwen, Rhiw, Llŷn, Gwynedd: tephroite is also a major component of massive siliceous manganese ore from the geologically similar Nant Mine (Cotterell, 2006). Typical examples are olive-green to chocolate-brown in colour and associated with jacobsite.

References:

  1. Campbell Smith, W., Bannister, F.A. & Hey, M.H., 1944b. Banalsite, a new barium-feldspar from Wales. Mineralogical Magazine 27, 33-47
  2. Cotterell, T.F., 2006. Caryopilite and pyroxmangite from Nant Mine, Nant-y-Gadwen, Llanfaelrhys, Pen Llyn, Gwynedd, Wales UK Journal of Mines and Minerals, 27, 51-53.
  3. Deer, W.A., Howie, R.A. & Zussman, J., 1986. Rock-Forming Minerals, Vol. 1A, Orthosilicates, 2nd Ed. Longman Group Ltd, 918pp.