Lennilenapeite is a potassium magnesium aluminosilicate hydroxide hydrate mineral of the stilpnomelane group. There is extensive substitution of Fe2+, Fe3+, and Zn, for Mg. 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 Unpublished chemical analytical work by [Dunn] indicates that most stilpnomelane from Franklin is Mg-rich and thus lennilenapeite.
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.
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. The type material is anhedral, with a widely varying but predominantly vitreous luster; metallic bronzy lusters and tarnishes are common. The density is 2.72 g/cm3. There is no discernible response to ultraviolet radiation. It is best identified by chemical analysis.
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). 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. (Dunn, 1995)

 Location Found: Franklin (Type Locality)
 Year Discovered: 1984
 Formula: K6-7(Mg,Mn,Fe,Zn)48(Si,Al)72(O,OH)216 · 16H2O
 Essential Elements: Hydrogen, Magnesium, Oxygen, Potassium, Silicon
 All Elements in Formula: Aluminum, Hydrogen, Iron, Magnesium, Manganese, Oxygen, Potassium, Silicon, Zinc
 IMA Status: Approved
 To find out more about this mineral at minDat's website, follow this link   Lennilenapeite

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.474

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 25 No. 2 - Fall 1984, pg. 5Mineral Notes New To Science, Lennilenapeite
View IssueV. 24 No. 2 - Fall 1983, pg. 8Mineral Notes New To Science, Lennilenapeite

Lennilenapeite on dolomite from Franklin, NJ
Lennilenapeite (metallic gray-green) on dolomite (white) from Franklin, NJ. 3 1/4" x 2 1/8". From the collection of, and photo by Robert A. Boymistruk.

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