Abundant TiO 2 -II, a high-pressure polymorph of titanium dioxide, was found in the gneiss fragments of impact-produced breccias from the Xiuyan crater. Rutile in the gneiss was severely fragmented and fine-grained clasts less than 2 m in size had been transformed to TiO 2 -II. Irregular thin layered TiO 2 -II is also observed in coarse-grained rutile fragments, where the TiO 2 -II layers distributes along fractures and cracks in rutile. About 30 percent of rutile in the gneiss had been transformed to TiO 2 -II. Fine grains of TiO 2 -II display light bluish grey to light yellow brown in plane-polarized reflected light. Crystallographic investigation shows that TiO 2 -II has an orthorhombic structure with space group Pbcn. The cell parameters are a=4.543(1) Å, b=5.491(9) Å and c=4.895(2) Å. Its empirical formula calculated on the basis of two oxygen atoms can be written as (Ti 0.985 Fe 0.008 Nb 0.006 -Si 0.003 Zr 0.001 ) 1.003 O 2 , or simply formula TiO 2 . According to the shock effects of quartz and feldspars, the peak shock pressure and post-shock temperature in the TiO 2 -II-bearing gneiss are estimated to be between 35 and 43 GPa and 300-900°C, respectively. The finding of TiO 2 -II in the shock-metamorphosed gneiss provides another mineral physics evidence for shock origin of the Xiuyan crater. Rutile, a titanium dioxide, is a common accessory mineral in various types of rocks. At high pressure, rutile can be transformed into high-pressure polymorphs of TiO 2 -II (-PbO 2 ), baddeleyite, orthorhombic I and cotunnite phases with increasing pressure [1]. High-pressure polymorphs of baddeleyite, orthorhombic-I and cotunnite-type TiO 2 reverse to TiO 2 -II on decompression. TiO 2 -II is the only highpressure polymorph that can be recovered experimentally at ambient conditions [2]. i - had been firstly synthesized by static compression experiment [3]. A number of static compression experiments succeed in the transition of rutile to TiO 2 -II at pressure range between 4 and 12 GPa and temperature between 400 and 1500°C [3][4][5][6][7]. Shock-loading experiments revealed that rutile transforms to i - at peak shock pressure from 20 to 100 GPa [8][9][10]. The formation of natural TiO 2 -II could be related to highpressure endogenic geological processes or bolide impact events. Occurrence of natural TiO 2 -II had been previously reported in ultrahigh-pressure metamorphic rocks [11,12], mantle-derived rocks [13], terrestrial impact structures [14][15][16], and tektite [17]. However, the mineralogical features and the formation conditions of natural TiO 2 -II still needs to be further investigated because of the very limited material that had been found.The Xiuyan crater is a recently confirmed impact structure in China. The target rocks of the crater were shock-metamorphosed, from which impact-produced breccia, coesite, planar deformation features in quartz have been identified [18][19][20]. In this paper, we report the occurrence and mineralogical features of TiO 2 -IIin the impact-produced breccias
Tuite is a high‐pressure γ‐form of Ca3(PO4)2. An occurrence of tuite partly transformed from merrillite and chlorapatite was observed in the chondritic area adjacent to the shock veins in the Suizhou meteorite. Tuite grains are found in contact with both merrillite and chlorapatite, indicating two different transformation pathways. Tuite isochemically transformed from merrillite contains much higher contents of Na2O and MgO than those transformed from chlorapatite. Tuite transformed from merrillite does not contain Cl, but tuite transformed from chlorapatite contains 1.90–3.91 wt% of Cl, hence indicating an incomplete phase transformation from chlorapatite to tuite. P‐T conditions of above 12 GPa and 1100 °C are probably required for the transformation from merrillite and chlorapatite to tuite. A temperature gradient from the hot vein at 2000 °C to the surrounding chondritic area at 1000 °C corresponds to the partial phase transitions in the Suizhou phosphates. Fast cooling of the thin shock veins plays a key role in the preservation of phosphates that suffered partial high‐pressure phase transformation.
Wangdaodeite, the shock-induced lithium niobate-structured polymorph of ilmenite, was found in the Suizhou L6 chondrite. It occurs as small irregular particles (2-20 lm in size) inside or adjacent to the shock melt veins. Wangdaodeite coexists in veins with ringwoodite, majorite, and xieite. The chemical formula of wangdaodeite is FeTiO 3. The empirical formula is: (Fe 0.85 Mg 0.10 Mn 0.05) Σ1.00 Ti 0.99 O 3 , which is similar to that of its host ilmenite. The Raman spectra of wangdaodeite display the bands at 174-179, 273-277, 560-567, 738-743 cm À1 , which are different to those for ilmenite. TEM images show that ilmenite is composed of polysynthetic-twinned crystals while wangdaodeite is composed of random-oriented nanometric domains sized 20-50 nm. Electron diffraction established wangdaodeite to be trigonal with the lithium niobate structure. Cell parameters are: a = 5.13(1) A, c = 13.78(1) A; c/a = 2.69; space group R3c; calculated density = 4.72 g cm À3. The P-T conditions for formation of wangdaodeite were estimated to be 20-24 GPa and >1200°C.
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