title: An ultradense polymorph of rutile with seven-coordinated titanium from the Ries crater.
abstract: We report the discovery of an ultradense post-rutile polymorph of titanium dioxide in shocked gneisses of the Ries crater in Germany. The microscopic diagnostic feature is intense blue internal reflections in crossed polarizers in reflected light. X-ray diffraction studies revealed a monoclinic lattice, isostructural with the baddeleyite ZrO2 polymorph, and the titanium cation is coordinated with seven oxygen anions. The cell parameters are as follows: a = 4.606(2) angstroms, b = 4.986(3) angstroms, c = 4.933(3) angstroms, beta (angle between c and a axes) = 99.17(6) degrees; space group P2(1)/c; density = 4.72 grams per cubic centimeter, where the numbers in parentheses are standard deviations in the last significant digits. This phase is 11% denser than rutile. The mineral is sensitive to x-ray irradiation and tends to invert to rutile. The presence of baddeleyite-type TiO2 in the shocked rocks indicates that the peak shock pressure was between 16 and 20 gigapascals, and the post-shock temperature was much lower than 500 degrees C.
authors.affiliation: Max-Planck-Institut für Chemie, J. Joachim-Becher-Weg 27, 55128 Mainz, Germany. goresy@mpch-mainz.mpg.de
authors.forename: A
authors.initials: A
authors.lastname: El Goresy
authors.forename: M
authors.initials: M
authors.lastname: Chen
authors.forename: L
authors.initials: L
authors.lastname: Dubrovinsky
authors.forename: P
authors.initials: P
authors.lastname: Gillet
authors.forename: G
authors.initials: G
authors.lastname: Graup