FRANKLINPHILITE

K₄(Mn,Mg,Zn)₄₈(Si,Al)₇₂(O,OH)₂₁₆.~10H₂O
Triclinic, P1 or P1; unit-cell parameters for the orthohexagonal unit-cell are a = 5.521, b = 9.560, c = 36.57 Å, Z = 3/8

	Figure 18-7. Drawing depicting the franklinphilite assemblage. The groundmass  is a serpentine-calcite-friedelite breccia. The dark band (1) is coarsely crystallized franklinphilite; the lighter, wider vein (2) is an impure mixture of franklinphilite  and friedelite. Drawing by Mary Parrish.

Franklinphilite was originally described as an unnamed Mn-dominant stilpnomelane from Franklin by Dunn et al. (1984b). Material which was more Mn-rich was subsequently described from Franklin by Dunn et al. (1992) and named franklinphilite. The crystal structure has not been determined; franklinphilite is assumed to be isostructural with ferrostilpnomelane. Franklinphilite is known only from Franklin; it has not been found at Sterling Hill.

Description

Franklinphilite occurs in two assemblages and has a markedly different appearance in each. Material from the original assemblage (Dunn et al., 1984b) is black with vitreous luster, good [001] cleavage, and a density of 2.75 g/cm³. This material is biaxial, sensibly uniaxial, negative, with a = 1.556, b = g = 1.600; pleochroism is strong: Y = Z = dark brown, X = faint brown to colorless; absorption is Y = Z > X. Some material fitting this description is lennilenapeite; chemical analysis is needed for verification.

The holotype franklinphilite is very dark brown; the streak is light brown; and the luster is vitreous to slightly resinous. It occurs as radial clusters of platy <1 mm crystals; euhedral crystals were not observed but franklinphilite is more coarsely- crystallized at vein margins. The hardness is approximately 4, and cleavage is imperfect on {001}. The observed density is 2.6 to 2.8 g/cm³, compared with 2.66 g/cm³ (calculated). Optically, the holotype franklinphilite is biaxial negative with 2V (measured) = 10^o (0^o calculated), and a = 1.545, b = 1.583, and g = 1.583. Pleochroism is distinct with Y = Z = deep brown, X = pale yellow; franklinphilite is not discernibly fluorescent in ultraviolet radiation. It is best distinguished from other stilpnomelanes by chemical analysis.

Composition

	Table 14. Chemical analyses of stilpnomelanes and related minerals.

Franklinphilite is a potassium manganese aluminum silicate hydroxide hydrate mineral of the stilpnomelane group. There is extensive solid-solution among the octahedral cations. The average of three closely agreeing microprobe analyses of the holotype yielded SiO₂ 44.0, Al₂O₃ 3.6, Fe₂O₃ 7.8, ZnO 5.9, MgO 6.4, MnO 22.3, K₂O 1.5, Na₂O 0.4, H₂O (by difference) 8.1, total = 100 wt. %. A microprobe analysis of the material of Dunn et al. (1984b) is given in Table 14.

Occurrence and paragenesis

The holotype franklinphilite (Dunn et al., 1992) occurs with the now-discredited baumite, first described by Frondel and Ito (1975), and discredited by Guggenheim and Bailey (1989, 1990). Franklinphilite occurred in an approximately 30-cm mass of a breccia-aggregate of calcite, willemite, aegirine, friedelite, and the lizardite-bearing, fine-grained mixture formerly known as baumite. Franklinphilite crosscuts this assemblage as a l-cm wide vein of impure light brown material with a coarser growth of franklinphilite at the grain margins (Figure 18-7).

The other previously known assemblage (Dunn et al., 1984b) consists of black 1-3 mm franklinphilite crystals associated with nelenite, rhodonite, and tirodite.

Name

Franklinphilite is named in allusion to the fact that its chemical composition contains most of the elements which make the Franklin deposit unique. The name is derived from the root Franklin (the deposit) and the Greek suffix philos, the word for friend. Franklinphilite means “friend of Franklin.” The name was designated to also honor the many geologists, mineralogists, and collectors who have been friends of Franklin and who have contributed to our understanding of the deposits. The name thus has a dual derivation.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 14, 2001