The discovery of TiO2-II, the α-PbO2-structured high-pressure polymorph of rutile, in the Suizhou L6 chondrite

We report the discovery of TiO2-II in the unmelted rock of the shocked Suizhou L6 chondrite. Natural TiO2-II was previously found in ultrahigh-pressure metamorphic and mantle-derived rocks, terrestrial impact structures, and tektite. Our microscopic, Raman spectroscopic, electron microprobe and transmission electron microscopic investigations have revealed: (1) All observed TiO2-II grains are related with ilmenite and pyrophanite; (2) TiO2-II occurs as needle- and leaf-shaped inclusions in ilmenite and patch-, tape-shaped body in pyrophanite; (3) The composition of TiO2-II is identical with that of its precursor rutile; (4) The Raman spectrum of TiO2-II is in good agreement with that of natural and synthesized alpha-PbO2-type TiO2; (5) TiO2-II occurs mainly in the form of well-ordered nano-domains and small mis-orientation among the domains can be observed. (6) All electron diffraction reflections from TiO2-II can be indexed to alpha-PbO2 structure in space group Pbcn with lattice parameters of a = 4.481 angstrom, b = 5.578 angstrom and c = 4.921 angstrom; (7) The exsolution inclusions of rutile from host ilmenite are mostly connected with an alternation process along the lamellar twinning plane of ilmenite induced by shock-induced high pressure and high temperature; (8) The P-T regime of 20-25 GPa and 1000 degrees C estimated for the Suizhou unmelted rock is suitable for phase transition of rutile into TiO2-II phase.