The crystal structure of mangan-neptunite, a manganese analogue of neptunite, has been refined in two space groups (Cc and C2/c). The mineral is monoclinic, with the correct space group Cc; the unit-cell dimensions are: a = 16.4821(6), b = 12.5195(4), c = 10.0292(3) Å, β = 115.474(1)°, and V = 1868.31 ų. The crystal structure has been refined to R₁ = 0.0307 (wR₂ = 0.0901) on the basis of 4892 observed reflections with F_hkl ≥ 4σ F_hkl . The most plausible acentric model is caused by the Ti- and (Fe, Mn, Mg)-ordering in the structure. Ti-octahedrons are strongly distorted and consist of short bond Ti-O (1.7 Å), one long bond (2.2 ̊), and four equal bonds (2.0 ̊). Fe-octahedrons are regularly shaped, with all Fe-O bonds being approximately identical.