A modulated layer silicate with a continuous Na and Mn octahedra sheet interstratified with a continuous tetrahedral sheet with pairs of 6-fold tetrahedral rings (Grice and Gault, 1995). Each pair of rings is linked to another pair with tetrahedra pointing in opposing direction so that adjacent sheets of octahedra are cross linked. Connecting pairs of tetrahedra are linked such that 4-fold tetrahedral rings and 10-fold rings are formed. The ideal chemical composition is Na₈Mn₂Si₁₀O₂₅(OH,Cl)₂ ^. 12H₂O. The mineral is extremely rare and occurs at the Demix-Varennes Quarry, Quebec, Canada in a peralkaline sill.
Cf., bementite, innsbruckite, pyrosmalite