Kyzylkumite: a finding in the southern Baikal region, Russia and refinement of its crystal chemical formula

Kyzylkumite has been found in Cr-V-bearing metamorphic rocks of the Sludyanka Complex, Southern Baikal region; it has been identified by X-ray powder diffraction method. This is a late secondary mineral developed after Ti-V-oxides (schreyerite, berdesinskiite) and V-bearing rutile and titanite. Kyzylkumite represents a new structural type with composition Ti₄V ₂ ³⁺ O₁₀(OH)₂ corresponding to octahedral coordination of Ti⁴⁺ and V³⁺. Its unit-cell dimensions are: a = 8.4787(1), b = 4.5624(1), c = 10.0330(1) Å, β = 93.174(1)°. The ideal formula of kyzylkumite Ti₄V ₂ ³⁺ O₁₀(OH)₂ corresponds to composition, wt %: 65.56 TiO₂, 30.75 V₂O₃, 3.69 H₂O. Indeed, the contents (wt %) of these constituents range from 62 to 70 TiO₂ and from 23 to 33 V₂O₃. Variations in contents and the Ti/V value are caused by partial substitution V³⁺ for V⁴⁺, isovalent substitutions Ti⁴⁺ and V³⁺ for V⁴⁺ and Cr³⁺, respectively, and coupled substitution V³⁺ + OH^? ? Ti⁴⁺ + O^2?. Smyslova et al. (1981)—the discovereres of kyzylkumite—assumed its composition to be the same as for schreyerite V ₂ ³⁺ Ti₃O₉ that principally different from kyzylkumite from the Sludyanka Complex. Therefore, re-examination of the kyzylkumite holotype or cotype from its type locality is needed.