LENNILENAPEITE

K_6-7(Mg,Mn,Fe,Zn)₄₈(Si,Al)₇₂(O,OH)₂₁₆.16H₂O
Triclinic, a = 21.9, d₀₀₁ = 12.18 Å, Z =1

	Figure 18-8. Platy crystals of lennilenapeite (lustrous gray) on calcite (white) from Franklin. Specimen is 8 cm in maximum dimension. Smithsonian Institution, #163875. Photo by the author.

Lennilenapeite was originally described by Dunn et al. (1984b) from Franklin; it has not been found at Sterling Hill. The material was first noted as a “chlorite” by Palache (1935) and was described as stilpnomelane by Frondel and Ito (1965b). The crystal structure of lennilenapeite has not been studied; it is assumed to be identical to that of ferrostilpnomelane.

Description

Lennilenapeite occurs in two markedly different habits: as dark brown 1-3 mm grains (the type material) and as light green druses and crystalline aggregates, which are much more abundant (Figure  18-8). The type material is anhedral, with a widely varying but predominantly vitreous luster; metallic bronzy lusters and tarnishes are common. Cleavages are perfect on {001} and imperfect on (hk0). The density is 2.72 g/cm³. Optically, lennilenapeite is biaxial, sensibly uniaxial, negative, 2V = 0^o, with indices of refraction a = 1.553, b = g = 1.594; pleochroism is strong: X = faint brown to colorless, Y = Z = dark brown; absorption = Y = Z > X. There is no discernible response to ultraviolet radiation. It is best identified by chemical analysis.

Composition

Lennilenapeite is a potassium magnesium aluminosilicate hydroxide hydrate mineral of the stilpnomelane group. There is extensive substitution of Fe²⁺, Fe³⁺, and Zn, for Mg; a representative analysis is presented in Table 14.  Additional analyses are given by Dunn et al. (1984b) and show the range of solid solution to franklinphilite and other species. Unpublished chemical analytical work by the writer indicates that most stilpnomelane from Franklin is Mg-rich and thus lennilenapeite.

Occurrence and paragenesis

The type assemblage for lennilenapeite, apparently of limited extent, consists of dark brown lennilenapeite, willemite, tirodite, and nelenite (Dunn et al., 1984b). Lennilenapeite occurs within all three associated species, but is concentrated at the contacts between them. Some of this dark brown material is franklinphilite; it can be differentiated in this assemblage only by chemical analysis.

Another occurrence was noted, but not analyzed, by Frondel and Ito (1965b) who described light green to light brownish green druses on sphalerite and dolomite; this material was restudied by Dunn et al. (1984b) (Figures 18-8, 21-22, and 21-24). This lennilenapeite was referred to as “chlorite” by Palache, and the occurrence was reported to be in the 1100

stope north at the 300 and 450 foot levels in the Franklin Mine. Here, lennilenapeite is associated with calcite, dolomite, sphalerite, magnesioriebeckite, and willemite in a breccia-vein assemblage. Much of this material is in the collections at Harvard University.  

Name

Lennilenapeite was named for the Lenni Lenape Indians, who lived in the Franklin area and were presumably the original people to see these deposits. The words “Lenni Lenape” mean “the original people.”

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