Alleghanyite

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:

  • Silicates

Geological Context:

  • Hydrothermal
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:

  • Benallt Mine, Llŷn, Gwynedd: clove-brown, alleghanyite was identified from this locality by Dr Brian Mason and reported by Campbell Smith et al. (1944). Both individual crystals and composite crystals occur with tephroite (Mn2+2SiO4) as criss-crossing veins cutting banalsite, calcite and baryte in dark spotted manganese ore from the footwall side of the No. 1 ore-body (More specifically – ‘by the No. 1 chute 50-60 feet west of the main shaft and some 10-20 feet above the 130 foot level’, Campbell Smith et al. (1944)). The composite alleghanyite crystals are up to 20 mm long and vary in thickness from microscopic to 2 mm and have a blade-like form. Compared to alleghanyite from the type locality (Bald Knob, Alleghany County, North Carolina, USA), the Benallt occurrence is Ti-rich, containing up to 4.8 wt% TiO2 (Campbell Smith et al., 1944).

References:

  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.