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

作者：Xiande Xie^*; Xiangping Gu; Ming Chen

来源：Acta Geochimica, 2023, 42(01): 1-8.

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 llmenite 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 α-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 α-PbO2 structure in space group Pbcn with lattice parameters of a=4.481 ?,b=5.578 A and c=4.921 A;(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 shockinduced high pressure and high temperature;(8) The P-T regime of 20-25 GPa and 1000 ℃ estimated for the Suizhou unmelted rock is suitable for phase transition of rutile into TiO2-Ⅱ phase.

• 单位
  同位素地球化学国家重点实验室; 中南大学