Franklinphilite

Franklinphilite
General
Category	Silicate mineral
Formula (repeating unit)	K4(Mn)48(Si,Al)72(O,OH)216·n(H2O) (with n~6)
Strunz classification	9.EG.40
Crystal system	Triclinic pseudo-hexagonal
Unit cell	a = 5.52 Å, b = 9.56 Å, c = 36.57 Å, α = β = γ = essentially 90°
Identification
Color	Dark brown
Crystal habit	Fine radial aggregates
Crystal symmetry	Triclinic Space group: P1 or P1
Cleavage	Imperfect on {001}
Tenacity	Brittle
Mohs scale hardness	4
Luster	Vitreous to slightly resinous
Diaphaneity	Translucent to nearly opaque
Specific gravity	2.6 - 2.8
Optical properties	Biaxial (-)
Refractive index	nα = 1.545 nβ = 1.583 nγ = 1.583
Birefringence	δ = 0.038
Pleochroism	Distinct; X = pale yellow; Y = Z = deep brown
2V angle	10°
References	[1][2][3]

Franklinphilite is a phyllosilicate of the stilpnomelane group, found exclusively from the Franklin and Sterling Hill mines in Franklin, Sussex County, New Jersey.^[1]

Composition

Franklinphilite has an ideal chemical formula of K₄(Mn)₄₈(Si,Al)₇₂(O,OH)₂₁₆•n(H₂O) (with n~6).^[4] Reported formula is (K,Na)₄(Mn²⁺,Mg,Zn)₄₈(Si,Al)₇₂(O,OH)₂₁₆·6(H₂O).^[2][3]

Structure

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.^[5]

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.^[4] 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.^[4][6]

History

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.^[4] 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.^[4] Franklinphilite was named for the type locality (Franklin) and the Greek word φιλόϛ (philos) meaning friend.^[1] 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.^[4]

References

1. Anthony, J. W., BideauxR, A., Bladh, K. W., and Nichols, M. C., 2001, Franklinphilite, Handbook of Mineralogy, Mineralogical Society of America.
2. Franklinphilite on Mindat.org
3. Franklinphilite data on Webmineral
4. 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.
5. 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.
6. 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