Anatase and brookite have been converted in opposed-anvil high pressure apparatus to a new phase, TiO₂-II, which has the αPbO₂ structure. Starting with anatase and II, or brookite and II, p–T reaction boundaries were obtained which yield apparent triple points among the polymorphs anatase-II-rutile and brookite-II-rutile in the region of 480°C and 9 kbars. Inasmuch as no reversal transitions were observed along these boundaries, the boundaries approximate equilibria on the high temperature side, in a stable or metastable relation. Further, because brookite may be a crystalline solution, it would fall out of the system TiO₂.

Other results using the polymorphs and a gel as starting materials in opposed-anvils and hydrothermal apparatuses suggest the critical importance of fluid composition in the various transitions to rutile.