A new type of composite structure is described consisting of the todorokite-type [Mn6O12] framework with large square tunnels accommodating columnar fragments of the rock-salt structure. The crystal structure of the new todorokite-type [SrF0.8(OH)0.2] 2.526[Mn6O12] compound is solved from transmission electron microscopy, and the structure of its anhydrous form [SrF0.8O0.1]2.526[Mn6O 12] is refined from X-ray powder diffraction data. The [Mn 6O12] framework consists of mutually perpendicular walls built of three edge-sharing rutile-type strings of MnO6 octahedra delimiting large square tunnels with the size of 3 × 3 octahedra. The interior space in the tunnels is filled with rock-salt type [Sr(F,OH)] 4 columns. The structure can be interpreted as being an incommensurate composite structure with the modulation vector q1 = γc1* (γ = 0.63157(3)) parallel to the direction of tunnel propagation. The octahedral tunnel walls compose subsystem I with a [Mn6O12] composition and a periodicity c1 = 2.84 Å, whereas the [Sr(F,OH)]4 columns belong to subsystem II with a periodicity c1/γ = 4.49 Å, resulting in a [Sr(F,-OH)]4γ[Mn6O12] composition. [SrF0.8(OH)0.2]2.526[Mn6O 12] demonstrates a much larger number of cations inside the tunnels in comparison with the known synthetic and natural marine todorokites. The [SrF0.8-(OH)0.2]2.526[Mn6O 12] compound shows a spin-glass behavior below T* ≈ 26 K with a dominant antiferromagnetic correlation.