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Dawei Meng , China University of Geosciences (China)
Xiuling Wu, China University of Geosciences (China)
Xin Meng , Imperial College (United Kingdom)
Zhengjie Zhang, China University of Geosciences (China)
Hong Chen, China University of Geosciences (China)
Jianping Zheng, China University of Geosciences (China)
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The α-PbO2-type TiO2 (TiO2-II) and baddeleyite-type TiO2 in nature may are important index minerals for ultrahigh-pressure metamorphism. After the discovery of a natural high-pressure phase of titanium oxide with α-PbO2-structure in omphacite from coesite-bearing eclogite at Shima in the Dabie Mountains, China (Wu et al., 2005), a nano-scale lamellae (< 2 nm) of the α-PbO2-type polymorph of TiO2 sandwiched between twinned rutile inclusions in jadeite has been confirmed by electron diffraction and high-resolution transmission electron microscopy (HRTEM), backed up by image simulation techniques, from ultrahigh-pressure jadeite quartzite at Shuanghe in the Dabie Mountains, China. The crystal structure is orthorhombic with lattice parameters a = 0.458 nm, b = 0.542 nm, c = 0.496 nm and space group Pbcn. A three-dimensional structural model has been constructed for the rutile to α-PbO2-type TiO2 phase transformation based on high-resolution electron microscopic images. Computer image simulation and structural model analysis reveal that rutile {011} twin interface is a basic structural unit of α-PbO2-type TiO2. Nucleation of α-PbO2-type TiO2 lamellae is caused by the displacement of one half of the titanium cations within the {011} twin slab. This displacement reduces the Ti-O-Ti distance and is favored by high pressure. The identification of α-PbO2-type TiO2 in coesite-bearing eclogite (Shima) and jadeite quartzite (Shuanghe) from Dabie Mountains, China, provides a new and powerful evidence of unltra high-pressure metamorphism at 4-7 GPa, 850-900 °C and implies a burial of continental crustal rocks to 130-200 kilometers or deeper. The α-PbO2-type TiO2 may be a useful indicator of the pressure and temperature in the diamond stability field. The authors gratefully acknowledge financial support from NNSFC grants (Nos. 40672136, 40572114 and 40425002) and SRFDP (No. 20060491504).
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