Franklinphilite

Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Hydrated potassium-sodium-manganese-magnesium aluminosilicate hydroxide
Chemical Formula: (K,Na)4(Mn2+,Mg,Zn)48(Si,Al)72(O,OH)216.6H2O
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
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.
Franklinphilite from Llyn Du Bach Mine.
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.
SEM image of franklinphilite, caryopilite and allanite.
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 2011.2G.M.1.
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.

There are no key localities for this specimen.

References:

  1. 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.
  2. 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.

There are no references for this specimen.