Mineral Database

Mineral Database
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).

X-ray powder diffractogram for franklinphilite from Llyn Du Bach Mine (red line) overlain by the published peak positions for franklinphilite (Powder Diffraction File reference code 00-046-1455) in dark blue.

Dark brown franklinphilite-caryopilite veinlets cross-cutting an 8 cm thick section of the Lower Cambrian manganese ore bed from Llyn Du Bach Mine. Specimen no. NMW 2012.2G.M.1.

Scanning electron micrograph backscatter electron image of intergrown franklinphilite (dark grey) and caryopilite (medium grey) with a larger crystal of zoned allanite-(Ce)/ferriallanite-(Ce) in the centre (bright grey). Specimen no. NMW 2012.2G.M.1.

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.