Mineral Database (Saesneg yn unig)

Mineral Database (Saesneg yn unig)
Mineralogy of Wales

Franklinphilite

Crystal System: Triclinic
Formula: (K,Na)4(Mn2+,Mg,Zn)48(Si,Al)72(O,OH)216.6H2O
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Hydrated potassium-sodium-manganese-magnesium aluminosilicate hydroxide
Method(s) of Verification: Llyn Du Bach Mine: XRD and SEM-EDS at NMW (XRD no. NMW X-2337). Full details are provided by Cotterell and Hubbard (2013).
Chemical Group:
  • Silicates
Geological Context:
  • Metamorphic : low-grade
Introduction: franklinphilite is the manganese analogue of stilpnomelane. Whilst stilpnomelane is quite common - occurring at many hundreds of localities worldwide - franklinphilite is very rare. It was was first described by Dunn et al. (1992) from the remarkably diverse mineral deposit at Franklin, New Jersey, U.S.A. where it occurs within a metamorphosed manganese- and zinc-rich mineral assemblage.
Occurrence in Wales: despite being a very rare mineral on a world scale franklinphilite is relatively common as a filling in veinlets, cross-cutting the Lower Cambrian age manganese ore bed, in the Harlech Dome particularly at Llyn Du Bach Mine near Moel Ysgyfarnogod (Cotterell and Hubbard, 2013).
Key Localities:
  • Llyn Du Bach Mine, Moel Ysgyfarnogod, Harlech, Gwynedd: franklinphilite is common in thin veinlets cutting through the hard, flinty, manganese ore bed at this remote site. See Cotterell and Hubbard (2013) for a detailed description of this occurrence.
References:
  • Cotterell, T.F. and Hubbard, N., 2013 Franklinphilite in veinlets in the Lower Cambrian manganese ore bed, Harlech, Merionethshire, Wales.  Journal of the Russell Society, 16, 51-59.
  • Dunn, P.J., Peacor, D.R. and Su, S.-C., 1992 Franklinphilite, the manganese analog of stilpnomelane, from Franklin, New Jersey.  Mineralogical Record, 23, 465-468.