TiO2 II: The high‐pressure Zr‐free srilankite endmember in impact rocks

Abstract

AbstractTiO2II, a high‐pressure polymorph of titanium dioxide, is a diagnostic indicator of shock metamorphism in impact rocks. Due to its typical micro‐to‐nanometer scale, there are no ab initio structure solutions of natural TiO2II, thereby generating uncertainty about its crystal structure and its known similarity with srilankite (Ti0.67,Zr0.33)O2. Nanoscale electron diffraction investigation of TiO2II from the Australasian tektite strewn field provides the first ab initio structure solution revealing a primitive orthorhombic lattice with cell parameters a = 4.547 Å, b = 5.481 Å, c = 4.891 Å, and space group Pbcn, that is, the same as srilankite and scrutinyite α‐PbO2. The linear a and c decrease, and b increase with Ti content indicate TiO2II as Zr‐free srilankite endmember in the binary system ZrO2‐TiO2. Thereby the name srilankite should be used referring to TiO2II, according to the International Mineralogical Association recommendations. We provide the first evidence for a topotactic subsolidus rutile‐to‐TiO2II transition, founding their finely intermixing nanocrystals in the same TiO2 crystal, where TiO2II is within the crystal and surrounded by rutile in direct contact. They also show recurrent iso‐orientation, with TiO2II [100] parallel to rutile [100], TiO2II [010] parallel to rutile [011], and TiO2II [001] parallel to rutile (0–11). The rutile‐TiO2II iso‐orientation suggests the compression of rutile (0–11) planes as a possible transition mechanism from rutile to TiO2II, with a consequent shortening of ~0.5 Å per cell. The presence of TiO2II in the distal (~1200 km) impact ejecta from the Australasian tektite strewn field indicates shock pressures of ~12–15 GPa and post‐shock temperatures below 500°C followed by rapid quenching.