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α-PbO_2-type TiO_2(TiO_2-Ⅱ)is an important index mineral for ultrahigh-pressure metamorphism.After the discovery of a natural high-pressure phase of titanium oxide withα-PbO_2- structure in omphacite from coesite-bearing eclogite at Shima in the Dabie Mountains,China,a nano- scale(<2 nm)α-PbO_2-type TiO_2 has been identified through electron diffraction and high-resolution transmission electron microscopy in coesite-bearing jadeite quartzite at Shuanghe in the Dabie Mountains.The crystal structure is orthorhombic with lattice parameters a=4.58×10~(-1)nm,b= 5.42×10~(-1)nm,c=4.96×10~(-1)nm and space group Pbcn.The analysis results reveal that rutile {011}_R twin interface is a basic structural unit ofα-PbO_2-type TiO_2.Nucleation ofα-PbO_2-type TiO_2 lamellae is caused by the displacement of one half of the titanium cations within the {011}_R twin slab.This displacement reduces the Ti-O-Ti distance and is favored by high pressure.The identification ofα- PbO_2-type TiO_2 in coesite-bearing jadeite quartzite from Shuanghe,Dabie Mountains,provides a new and powerful evidence of ultrahigh-pressure metamorphism at 4-7 GPa,850℃-900℃,and implies a burial of continental crustal rocks to 130-200 kilometers depth or deeper.Theα-PbO_2-type TiO_2 may be a useful indicator of the pressure and temperature in the diamond stability field.
α-PbO 2 -type TiO 2 (an (TiO 2-Ⅱ) is an important index mineral for ultrahigh-pressure metamorphism. After the discovery of a natural high-pressure phase of titanium oxide with α-PbO 2-structure in omphacite from coesite-bearing eclogite at Shima in the Dabie Mountains, China, a nano-scale (<2 nm) α-PbO_2-type TiO_2 has been identified through electron diffraction and high-resolution transmission electron microscopy in coesite-bearing jadeite quartzite at Shuanghe in the Dabie Mountains.The crystal structure is orthorhombic with lattice parameters a = 4.58 × 10 -1 nm, b = 5.42 × 10 -1 nm, c = 4.96 × 10 -1 nm and space group Pbcn.The analysis results reveal that rutile {011} _R twin interface is a basic structural unit of α-PbO_2-type TiO_2.Nucleation of α-PbO_2-type TiO_2 lamellae is caused by the displacement of one half of the titanium cations within the {011} _R twin slab. This displacement reduces the Ti-O-Ti distance and favored by high pressure. The identification of α-PbO_2-type TiO_2 in coe site-bearing jadeite quartzite from Shuanghe, Dabie Mountains, provides a new and powerful evidence of ultrahigh-pressure metamorphism at 4-7 GPa, 850 ° -900 ° C, and implies a burial of continental crustal rocks to 130-200 km depth or deeper The α-PbO 2 -type TiO 2 may be a useful indicator of the pressure and temperature in the diamond stability field.