Lennilenapeite, a new mineral species, is triclinic, with a = 21.9(1) Aa and d (sub (001)) = 12.18(5) Aa The formula in accordance with the Mg-dominant member of the stilpnomelane group is K (sub 6-7) (Mg,Mn,Fe (super 2+) ,Fe (super 3+) ,Zn) 48 (Si,Al) 72 (O,OH) 216 .16H 2 O, Z = 1. Lennilenapeite occurs in two colors; the holotype is dark brown and the cotype is light green. Physical properties of the holotype include hardness (Mohs) approximately 3; lustre, vitreous; cleavage, perfect {001} and imperfect nearly perpendicular to (001); density is 2.72 g/cm 3 (meas.). Optically lennilenapeite is pseudo-uniaxial, 2V = 0, with indices of refraction alpha = 1.553(2), beta = gamma = 1.594(4); pleochroism is strong; X faint brown to colorless, Y = Z dark brown; absorption Y = Z > X. Analytical, X-ray, and optical data are given for a Mn-dominant stilpnomelane from Franklin, tentatively assumed to be parsettensite. The composition of a ferristilpnomelane from Franklin is also presented.--Modified journal abstract.

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