AbstractThe crystal structure of kyzylkumite, ideally Ti₂V³⁺O₅ (OH), from the Sludyanka complex in South Baikal, Russia was solved and refined (including the hydrogen atom position) to an agreement index, R₁, of 2.34% using X-ray diffraction data collected on a twinned crystal. Kyzylkumite crystallizes in space group P2₁/c, with a = 8.4787(1), b = 4.5624(1), c = 10.0330(1) Å , β = 93.174(1)º, V = 387.51(1) ų and Z = 4. Tivanite, TiV³⁺O₃OH, and kyzylkumite have modular structures based on hexagonal close packing of oxygen, which are made up of rutile [TiO₂] and montroseite [V³⁺O(OH)] slices. In tivanite the rutile:montroseite ratio is 1:1, in kyzylkumite the ratio is 2:1. The montroseite module may be replaced by the isotypic paramontroseite V₄₊O₂ module, which produces a phase with the formula Ti₂V⁴⁺O₆. In the metamorphic rocks of the Sludyanka complex, vanadium can be present as V⁴⁺ and V³⁺ within the same mineral (e.g.in batisivite, schreyerite and berdesinskiite). Kyzylkumite has a flexible composition with respect to the M⁴⁺/M³⁺ ratio. The relationship between kyzylkumite and a closely related Be-bearing kyzylkumite-like mineral with an orthorhombic norbergite-type structure from Byrud mine, Norway is discussed. Both minerals have similar X-ray powder diffraction patterns.