CategorySilicate mineral
(repeating unit)
K4(Mn)48(Si,Al)72(O,OH)216·n(H2O) (with n~6)
IMA symbolFkp[1]
Strunz classification9.EG.40
Crystal systemTriclinic
Unknown space group
Unit cella = 5.52, b = 9.56
c = 36.57 [Å]; Z = 6
α = β = γ = essentially 90°
ColorDark brown
Crystal habitFine radial aggregates; pseudo-hexagonal
CleavageImperfect on {001}
Mohs scale hardness4
LusterVitreous to slightly resinous
DiaphaneityTranslucent to nearly opaque
Specific gravity2.6 - 2.8
Optical propertiesBiaxial (-)
Refractive indexnα = 1.545 nβ = 1.583 nγ = 1.583
Birefringenceδ = 0.038
PleochroismDistinct; X = pale yellow; Y = Z = deep brown
2V angle10°

Franklinphilite is a phyllosilicate of the stilpnomelane group. Known from only two localities (with a third unconfirmed locality in Switzerland)[5] It was found exclusively from the Franklin and Sterling Hill mines in Franklin, Sussex County, New Jersey.[2] until 2013, when a locality in Wales was confirmed [6]


Franklinphilite has an ideal chemical formula of K4(Mn)48(Si,Al)72(O,OH)216·n(H2O) (with n~6).[7] Reported formula is (K,Na)4(Mn2+,Mg,Zn)48(Si,Al)72(O,OH)216·6(H2O).[3][4]


Consistent with phyllosilicates, franklinphilite is composed of long flat sheets of linked silicon-oxygen and aluminium-oxygen tetrahedra bounded by an octahedral layer containing either potassium or manganese and is isostructural with lennilienapeite.[8]

Physical properties

Franklinphilite is dark brown to black and possesses a vitreous to slightly resinous luster. It is brittle with a hardness of about 4 (Mohs) and cleaves imperfectly along the {001} plane. The density varies due to impurities, but ranges from 2.6 to 2.8 g/cm3 compared to the calculated value of 2.66 g/cm3.[7] It is translucent to nearly opaque, translucent in thin section and has a light brown streak. Pleochroism is distinctive with X = pale yellow and Y,Z = deep brown. Dispersion was not detected and no evident fluorescence under ultra-violet radiation was observed.[7][9]


In 1938, C. Osborne Hutton described a manganoan member of stilpnomelane and incorrectly assumed it to be parsettensite. It was later described in 1984 by Pete J. Dunn, Donald R. Peacor and William B. Simmons as a Mn dominant stilpnomelane similar to parsettensite. Naming franklinphilite was deferred then due to findings of only 34 mol % of the theoretical manganese end-member, making it manganese dominant by plurality. In 1992, franklinphilite was revisited by Dunn, Peacor and Shu-Chu Su and given an International Mineral Association recognized name.[7] Although all franklinphilite specimens originated exclusively from the Buckwheat dump, large amounts of manganese silicates in the area suggest a high probability of other assemblages.[7] Franklinphilite was named for the type locality (Franklin) and the Greek word φιλόϛ (philos) meaning friend.[2] The name alludes to the unique elements of its chemical composition that contribute to the uniqueness of Franklin and Sterling Hill and the geologists, mineralogists and collectors, friends of Franklin, who helped contribute towards the understanding of the region.[7]


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c Anthony, J. W., BideauxR, A., Bladh, K. W., and Nichols, M. C., 2001, Franklinphilite, Handbook of Mineralogy, Mineralogical Society of America.
  3. ^ a b Franklinphilite on
  4. ^ a b Franklinphilite data on Webmineral
  5. ^ "Tanatz Alp, Splügen, Rheinwald, Viamala Region, Grisons, Switzerland".
  6. ^ "Mineral Database".
  7. ^ a b c d e f Dunn, P.J., Peacor, D.R., and Su S.-C., 1992, Franklinphilite, the manganese analog of stilpnomelane, from Franklin, New Jersey. Mineral Record v. 23 p. 465-468.
  8. ^ Dunn, P. J., Peacor, R. D., and Simmons, W. B., 1984, Lennilenapeite, The Mg-Analogue Of Stilpnomelane, And Chemical Data On Other Stilpnomelane Species From Franklin, New Jersey, Canadian Mineralogist v. 22 p. 259-63.
  9. ^ Dana, J. D., Dana, E.S., Gaines, R.V., and Dana, J.D., 1997, Dana’s New Mineralogy: The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana. 8th ed., p. 1553-1554