High-pressure polymorph of TiO2-II from the Xiuyan crater of China

Abundant TiO₂-II, a high-pressure polymorph of titanium dioxide, was found in the gneiss fragments of impact-produced breccias from the Xiuyan crater. Rutile in the gneiss was severely fragmented and fine-grained clasts less than 2 μm in size had been transformed to TiO₂-II. Irregular thin layered TiO₂-II is also observed in coarse-grained rutile fragments, where the TiO₂-II layers distributes along fractures and cracks in rutile. About 30 percent of rutile in the gneiss had been transformed to TiO₂-II. Fine grains of TiO₂-II display light bluish grey to light yellow brown in plane-polarized reflected light. Crystallographic investigation shows that TiO₂-II has an orthorhombic structure with space group Pbcn. The cell parameters are a=4.543(1) Å, b=5.491(9) Å and c=4.895(2) Å. Its empirical formula calculated on the basis of two oxygen atoms can be written as (Ti_0.985Fe_0.008Nb_0.006-Si_0.003Zr_0.001)_1.003O₂, or simply formula TiO₂. According to the shock effects of quartz and feldspars, the peak shock pressure and post-shock temperature in the TiO₂-II-bearing gneiss are estimated to be between 35 and 43 GPa and 300–900°C, respectively. The finding of TiO₂-II in the shock-metamorphosed gneiss provides another mineral physics evidence for shock origin of the Xiuyan crater.