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A shock-induced polymorph of anatase and rutile from the Chesapeake Bay impact structure, Virginia, U.S.A.

U.S. Geological Survey, National Center, Reston, Virginia 20192, U.S.A.

Correspondence: ^* E-mail: jjackson{at}usgs.gov

A shock-induced polymorph (TiO₂ II) of anatase and rutile has been identified in breccias from the late Eocene Chesapeake Bay impact structure. The breccia samples are from a recent, partially cored test hole in the central uplift at Cape Charles, Virginia. The drill cores from 744 to 823 m depth consist of suevitic crystalline-clast breccia and brecciated cataclastic gneiss in which the TiO₂ phases anatase and rutile are common accessory minerals. Electron-microprobe imaging and laser Raman spectroscopy of TiO₂ crystals, and powder X-ray diffraction (XRD) of mineral concentrates, confirm that a high-pressure, -PbO₂ structured polymorph of TiO₂ (TiO₂ II) coexists with anatase and rutile in matrix-hosted crystals and in inclusions within chlorite. Raman spectra of this polymorph include strong bands at wavenumbers (cm^–1) 175, 281, 315, 342, 356, 425, 531, 571, and 604; they appear with anatase bands at 397, 515, and 634 cm^–1, and rutile bands at 441 and 608 cm^–1. XRD patterns reveal 12 lines from the polymorph that do not significantly interfere with those of anatase or rutile, and are consistent with the TiO₂ II that was first reported to occur naturally as a shock-induced phase in rutile from the Ries crater in Germany. The recognition here of a second natural shock-induced occurrence of TiO₂ II suggests that its presence in rocks that have not been subjected to ultrahigh-pressure regional metamorphism can be a diagnostic indicator for confirmation of suspected impact structures.

Key Words: Phase transition • anatase • rutile • polymorph • shock metamorphism • Chesapeake • impact • crater

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