Wikipedia
Tistarite
Tistarite is an exceedingly rare mineral with the formula TiO, thus being the natural analogue of titanium(III) oxide. In terms of chemistry it is the titanium-analogue of hematite, corundum, eskolaite, and karelianite. Other minerals with the general formula AO are arsenolite, avicennite, claudetite, bismite, bixbyite, kangite, sphaerobismoite, yttriaite-(Y) and valentinite. Tistarite and grossmanite - both found in the famous Allende meteorite (so is kangite) - are the only currently known minerals with trivalent titanium. Titanium in minerals is almost exclusively tetravalent.