Kaitianite, Ti³⁺₂Ti⁴⁺O₅, a new titanium oxide mineral from Allende

Abstract

Kaitianite, Ti³⁺₂Ti⁴⁺O₅, is a new titanium oxide mineral discovered in the Allende CV3 carbonaceous chondrite. The type grain coexists with tistarite (Ti₂O₃) and rutile. Corundum, xifengite, mullite, osbornite, and a new Ti,Al,Zr‐oxide mineral are also present, although not in contact. The chemical composition of type kaitianite is (wt%) Ti₂O₃ 56.55, TiO₂ 39.29, Al₂O₃ 1.18, MgO 1.39, FeO 0.59, V₂O₃ 0.08 (sum 99.07), yielding an empirical formula of (Ti³⁺_(1.75)Al_(0.05)Ti⁴⁺_(0.10)Mg_(0.08)Fe_(0.02))(Ti⁴⁺_(1.00))O₅, with Ti³⁺ and Ti⁴⁺ partitioned, assuming a stoichiometry of three cations and five oxygen anions pfu. The end‐member formula is Ti³⁺₂Ti⁴⁺O₅. Kaitianite is the natural form of γ‐Ti₃O₅ with space group C2/c and cell parameters a = 10.115 Å, b = 5.074 Å, c = 7.182 Å, β = 112º, V = 341.77 ų, and Z = 4. Both the type kaitianite and associated rutile likely formed as oxidation products of tistarite at temperatures below 1200 K, but this oxidation event could have been in a very reducing environment, even more reducing than a gas of solar composition. Based on experimental data on the solubility of Ti³⁺ in equilibrium with corundum from the literature, the absence of tistarite in or on Ti³⁺‐rich corundum (0.27–1.45 mol% Ti₂O₃) suggests that these grains formed at higher temperatures than the kaitianite (>1579–1696 K, depending on the Ti concentration). The absence of rutile or kaitianite in or on corundum suggests that any exposure to the oxidizing environment producing kaitianite in tistarite was too short to cause the precipitation of Ti‐oxides in or on associated corundum.

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