Abstract
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 Ti4V23+ 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) A, β = 93.174(1)°. The ideal formula of kyzylkumite Ti4V23+ 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+ + O2−. Smyslova et al. (1981)—the discovereres of kyzylkumite—assumed its composition to be the same as for schreyerite V23+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.
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