A natural high-pressure phase of titanium oxide with α-PbO2-structure has been found in omphacite from coesite-bearing eclogite at Shima in the Dabie Mountains, China. High-resolution transmission electron microscope observations have revealed an orthorhombic lattice, corresponding to α-PbO2-type TiO2 with cell parameters a = 0.461 nm, b = 0.540 nm, c = 0.497 nm and space group Pbcn. It occurs as nanometer-thick (<2 nm) lamellae between multiple twinned rutile crystals, providing additional evidence of very high-pressure, metamorphism at 7 GPa, 900 °C. This implies subduction of continental material to a depth of more than 200 kilometers. α-PbO2-type TiO2 could be an extremely useful index mineral for ultrahigh-pressure.

You do not currently have access to this article.