A modulated 2:1 layer silicate with a continuous octahedral sheet containing Mn and Na and tetrahedral sheets having 5-, 6-, 7-, and 8-fold tetrahedral rings (Yakovenchuk et al., 2007). One symmetry-unique Si tetrahedron is inverted relative to the others in the sheet and this tetrahedron links two adjacent tetrahedral sheets. The other tetrahedra link to the octahedral sheets. The ideal chemical composition is K₅Na₆Mn³⁺Mn²⁺₁₄(Si₉O₂₂)₄(OH)₁₀ . 4H₂O. Armbrusterite is found in the Khibiny alkaline massif, Kola Peninsula, Russia.
Cf., bementite, parsettensite, pyrosmalite, innsbruckite, varennesite