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Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Manganese silicate hydroxide
Chemical Formula: Mn5(SiO4)2(OH)2
Method(s) of Verification: Benallt Mine – X-ray diffraction and wet chemical analysis (Campbell Smith et al., 1944).

Chemical Group:

Geological Context:

Introduction: alleghanyite is dimorphous (having the same chemical composition but a different atomic structure) with ribbeite and forms a series with chondrodite [(Mg,Fe2+)5(SiO4)2(F,OH)2] – both minerals of the humite group. It forms as a hydrothermal mineral in manganese-rich veins and it is typically associated with a wide range of other manganese minerals, particularly silicates and oxides.
Occurrence in Wales: the occurrence of alleghanyite in Wales is restricted to the manganese ore deposits at Benallt Mine, Llŷn. The ore minerals are believed to have been derived from hydrothermal activity involving manganese-rich fluids exhaled onto the Ordovician sea floor (Brown & Evans, 1989).

Key Localities:


  1. Brown, M.J. & Evans, A.D., 1989. Geophysical and geochemical investigations of the manganese deposits of Rhiw, western Llyn, North Wales. British Geological Survey Technical Report WF/89/14 (BGS Mineral Reconnaissance Programme Report No.102).
  2. Campbell Smith, W., Bannister, F.A., & Hey, M.H., 1944b. Banalsite, a new barium-felspar from Wales. Mineralogical Magazine, 27, 33-46.