Mineral Database

Mineral Database
Mineralogy of Wales

Jacobsite

Crystal System: Cubic
Formula: (Mn,Fe,Mg)(Fe,Mn)2O4
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: Manganese iron magnesium oxide
Method(s) of Verification: Benallt Mine - X-ray powder diffraction (Campbell Smith et al., 1944b).

Massive, metallic grey jacobsite from Nant Mine, Rhiw, Llŷn, Gwynedd. Specimen 8.8 cm long. National Museum of Wales specimen. Photo T.F. Cotterell, © National Museum of Wales.

Chemical Group:
  • Oxides & Hydroxides
Geological Context:
  • Metamorphic
Introduction:

jacobsite belongs to the spinel group of minerals and forms a series with magnetite (Fe2+Fe3+2O4) and is dimorphous (having the same chemical composition but a different arrangement of the atoms) with iwakiite. Jacobsite is magnetic and is a constituent mineral in many manganese deposits.

Occurrence in Wales:

the first recorded British occurrence of jacobsite was from Benallt Manganese Mine, on Llŷn by Campbell Smith et al. (1944a). Further details of this occurrence are given by Campbell Smith et al. (1944b).

Key Localities:
  • Benallt Mine, Llŷn, Gwyneddŷ: Campbell Smith et al. (1944b) described black, angular, magnetic patches, up to 1 cm in length, associated with banalsite, calcite, baryte, tephroite and alleghanyite within thin veins cutting dark, spotted manganese ore. It 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 10-20 feet above the 130-feet level’.
References:
  • Campbell Smith, W., Bannister, F.A. & Hey, M.H., 1944a A new barium-feldspar from Wales.  Nature, 154, 336-337.
  • Campbell Smith, W., Bannister, F.A. & Hey, M.H., 1944b Banalsite, a new barium-feldspar from Wales.  Mineralogical Magazine 27, 33-47