The_discovery..._L6_chondrite_Xiande Xie.pdf

ORIGINAL ARTICLEThe discovery of TiO2-II,the a-PbO2-structured high-pressurepolymorph of rutile,in the Suizhou L6 chondriteXiande Xie1Xiangping Gu2Ming Chen1,3Received:27 September 2022/Revised:23 October 2022/Accepted:3 November 2022/Published online:11 December 2022 The Author(s),under exclusive licence to Science Press and Institute of Geochemistry,CAS and Springer-Verlag GmbH Germany,part ofSpringer Nature 2022Abstract We report the discovery of TiO2-II in theunmelted rock of the shocked Suizhou L6 chondrite.Nat-ural TiO2-II was previously found in ultrahigh-pressuremetamorphic and mantle-derived rocks,terrestrial impactstructures,and tektite.Our microscopic,Raman spectro-scopic,electron microprobe and transmission electronmicroscopic investigations have revealed:(1)All observedTiO2-II grains are related with ilmenite and pyrophanite;(2)TiO2-II occurs as needle-and leaf-shaped inclusions inilmenite and patch-,tape-shaped body in pyrophanite;(3)The composition of TiO2-II is identical with that of itsprecursor rutile;(4)The Raman spectrum of TiO2-II is ingood agreement with that of natural and synthesized a-PbO2-type TiO2;(5)TiO2-II occurs mainly in the form ofwell-orderednano-domainsandsmallmis-orientationamong the domains can be observed.(6)All electrondiffraction reflections from TiO2-II can be indexed to a-PbO2structure in space group Pbcn with lattice parametersof a=4.481 A?,b=5.578 A?and c=4.921 A?;(7)Theexsolution inclusions of rutile from host ilmenite aremostly connected with an alternation process along thelamellar twinning plane of ilmenite induced by shock-induced high pressure and high temperature;(8)The PTregime of 2025 GPa and 1000 C estimated for theSuizhou unmelted rock is suitable for phase transition ofrutile into TiO2-II phase.Keywords Rutile TiO2-II High-pressure polymorph Shock metamorphism Suizhou meteorite1 IntroductionRutile is a common accessory mineral in various types ofterrestrial and extraterrestrial rocks.TiO2-II is a high-pressure polymorph of rutile that was first synthesized bystatic compression experiment(Bendeliany et al.1966)andby shock wave techniques(McQueen et al.1967).Thepowder X-ray diffraction data show that TiO2-II isisostructural with a-PbO2-type TiO2,space group Pbcn,synthesized by shock experiments(McQueen et al.1967)and in-situ high-pressure experiments using a diamond-anvil cell(Gerward and Olsen 1997).It was also revealedthat TiO2-II is the only high-pressure polymorph of rutilethat can be recovered experimentally at ambient conditions(Wu et al.2010).The occurrence of natural TiO2-II was previouslyreported in ultrahigh-pressure metamorphic rocks(Huanget al.2000;Wu et al.2005),mantle-derived rocks(Dobrzhinetskaya et al.2009),terrestrial impact structures(El Goresy et al.2001;Jackson et al.2006;McHone et al.2008;Chen et al.2013),and tektite(Glass and Fries 2008),as well as in Neoarchean spherule layers(Smith et al.2016),but no finding of TiO2-II in meteorites was reportedin the literature.In this paper,we report the discovery of Xiande X1Key Laboratory of Mineralogy and Metallogeny/GuangdongProvincial Laboratory of Mineral Physics and Materials,Guangzhou Institute of Geochemistry,Chinese Academy ofSciences,Guangzhou 510640,China2School of Geosciences and Info-Physics,Central SouthUniversity,Changsha 410083,China3State Key Laboratory of Isotope Geochemistry,GuangzhouInstitute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China Acta Geochim(2023)42(1):18https:/doi.org/10.1007/s11631-022-00585-4TiO2-II in a meteorite,namely,in the shocked Suizhou L6chondrite.2 Samples and analytical methodsThe Suizhou L6 chondrite fell on April 15,1986,at Day-anpo,Suizhou City,Hubei Province,China.This meteoriteconsists of olivine,low-Ca pyroxene,plagioclase,FeNi-metal,troilite,merrillite,chlorapatite,chromite,ilmenite,pyrophanite,native copper,and shenzhuangite(Xie et al.2001a,2011;Xie and Chen 2016;Bindi and Xie 2017).Rutile is a very rare phase observed only inside someilmenite and pyrophanite grains.The Suizhou meteoritewas classified as a strongly shock-metamorphosed(S5)meteorite due to the presence of transformation of plagio-clase to maskelynite(Xie et al.2001a;2011).This mete-orite contains a few very thin shock-produced melt veins,and up to 14 high-pressure phases were identified inside oradjacent to these veins,among them,the ringwoodite,majorite,lingunite,magnesiowu stiteandmajorite-py-ropes.s.are abundant constituent high-pressure phases,buttuite,xieite,chenmingite,wangdaodeite,hemleyite,asi-mowite,poirierite,hiroseite and elgoresyite are new high-pressure minerals(Xie et al.2001a,2002,2011,2019;Chen et al.2003,2008;Ma et al.2018;Xie and Chen 2016,Bindi et al.2017,2019,2020,2021;Tomioka et al.2021).Polished thin sections were prepared from fragments ofthe Suizhou meteorite.All observations and physical andchemical analyses are performed in-situ on thin sections.The mineral assemblages in polished sections of the sam-ples were investigated by optical m