Abstract: The superstructure of ganophyllite has been analysed using subcell single crystal X-ray data and electron-optical observations. The full cell (supercell), space group A2/a, a = 16.6, b = 26.6, c = 50 Å, β = 94° has the approximate formula (K,Na,Ca)6+7.5(Mg,Fe,Mn)24[Si32.5Al7.5]O96(OH)1621H2O, Z = 8. A structure is proposed in which triple chains of silica tetrahedra parallel to X form narrow 2 : 1 layers with sinusoidal Mn-octahedral sheets. The triple chains are connected laterally, and across the interlayer, by pairs of inverted tetrahedra, linking to each other into four-member rings parallel to (010). Polytypes are generated by various displacements of the inverted tetrahedra.