Ganophyllite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Potassium sodium manganese magnesium aluminium silicate hydroxide hydrate
Chemical Formula: (K,Na)2(Mn,Al,Mg)8(Si,Al)12O29(OH)7.8-9H2O
Method(s) of Verification: Benallt Mine - XRD (Campbell Smith, 1948); Nant Mine - XRD & EMPA at National Museum of Wales.

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal
  • Metamorphic
Micaceous ganophyllite in a veinlet cutting Mn ore. Nant Mine, Rhiw, Ll?n, Gwynedd. Specimen 7 cm across. National Museum of Wales Collection (NMW 27.111.GR.434), ex G.J. Williams Collection. Photo T.F. Cotterell, © National Museum of Wales.
Introduction: ganophyllite is a rare mineral found in metamorphosed manganese deposits, particularly in zinc-manganese (Zn-Mn) deposits. There is some slight variation in the quoted formula for this mineral, which forms a series with eggletonite (Na-analogue) and tamaite (Ca-analogue).
Occurrence in Wales: ganophyllite was first collected in Wales in 1911 at Nant Mine, on Llŷn, by Arthur Russell, although mention of this occurrence was not made until a very brief report by Russell (1946), which represented the first occurrence of the mineral in the British Isles. A subsequent description of this mineral was made from Benallt Mine by Campbell Smith (1948). Recent research looking at the composition of ganophyllite specimens from Benallt and Nant mines has shown the presence of both eggletonite and tamaite intergrowths (T.F. Cotterell, unpublished data).

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: Campbell Smith collected specimens in 1945, in the company of A.W. Groves, who was surveying the site for mineral resources. Campbell Smith (1948) describes ganophyllite as six-sided cinnamon-brown flakes up to 4 mm across, possessing perfect basal cleavage. Some of what had been identified as ganophyllite was later re-identified as the new mineral bannisterite (Smith & Frondel, 1968). Further data on ganophyllite were produced by Dunn et al. (1983). Analysis of recently collected material suggests that the Ca-analogue, tamaite is also present (T.F. Cotterell, unpublished data).
  • Nant Mine, Nant-y-Gadwen, Rhiw, Llŷn, Gwynedd: Arthur Russell first found ganophyllite at Nant Mine in 1911. Specimens in the G.J. Williams Collection housed at the National Museum of Wales (specimen numbers NMW 27.111.GR.433 & 434) were collected in 1917. Recent analysis has shown this material to be barium-rich ganophyllite (T.F. Cotterell, unpublished data).

There are no key localities for this specimen.

References:

  1. Campbell Smith, W., 1948. Ganophyllite from the Benallt Mine, Rhiw, Caernarvonshire. Mineralogical Magazine, 28, 343-352.
  2. Dunn, P.J., Peacor, D.R., Nelen, J.E. & Ramkin, R.A., 1983. Ganophyllite from Franklin, New Jersey; Pajsberg, Sweden; and Wales; new chemical data. Mineralogical Magazine, 47, 563-566.
  3. Russell, A., 1946. On rhodonite and tephroite from Treburland manganese mine, Altarnun, Cornwall; and rhodonite from other localities in Cornwall, and Devonshire. Mineralogical Magazine, 27, 221-235.
  4. Smith, M.L. & Frondel, C., 1968. The related layered minerals ganophyllite, bannisterite, and stilpnomelane. Mineralogical Magazine, 36, 893-913.

There are no references for this specimen.