A modulated 2:1 layer silicate with a continuous octahedral sheet and a modulated tetrahedral sheet (Eggleton, 1972). The tetrahedral sheet forms approximately hexagonal islands with 6-fold tetrahedral rings and island connectors are formed by an inverted single 6-fold ring with a near trigonal configuration. The islands are comprised of seven near- hexagonal tetrahedral 6-fold rings. The inverted rings also link adjacent layers across the interlayer. Trigonal rings also form by paired trigonal rings joined by the apical oxygen atoms along Z. The primarily ferrous form of stilpnomelane, often informally referred to as “ferrostilpnomelane”, has a structural formula of K₅Fe²⁺₄₈(Si₆₃Al₉)O₁₆₈(OH)₄₈ ^. 12H₂O, whereas the analogous “ferristilpnomelane” has a structural formula of K₅Fe³⁺₄₈(Si₆₃Al₉)O₂₁₆ ^. 36H₂O, although neither end-member forms have been found. The Mg-rich stilpnomelane is called lennilenapeite and the Mn-rich mineral is franklinphilite. Older literature incorrectly equates parsettensite as the Mn-rich version. Stilpnomelane occurs in many geological environments, including greenshist facies rocks, iron deposits, metal sulfide deposits, and in weathering profiles.