A natural shock-induced dense polymorph of rutile with alpha-PbO2 structure in the suevite from the Ries crater in Germany

A dense post-rutile titanium dioxide (TiO2) phase was discovered in shocked garnet gneisses from the Ries crater by reflected-light microscopy, laser rnicroRaman spectroscopy and micro-beam X-ray diffraction. The Raman spectr um consists of nine bands at wave numbers: 152, 175, 285, 315, 340, 358, 42 8, 532 and 575 cm(-1). These bands are identical to those of the alpha -PbO 2-structured polymorph of TiO2 synthesized in a dynamic laboratory experime nt. The diffraction pattern of the natural mineral revealed an orthorhombic lattice similar to the (alpha -PbO2) polymorph with the cell parameters a = 4,535(2) Angstrom. b = 5.499(2) Angstrom c = 4.900(2) Angstrom (space gro up Phen; columbite-type structure), density rho =4.34g/cm(2), where the num bers in parentheses are standard deviations in the last significant digits. This new polymorph is 2% denser than rutile. The rutile/alpha -PbO2 polymo rph phase transformation occurs exclusively at the grain boundaries between rutile and the shock-compacted host biotite and advances inwards in rutile . This textural relation establishes phase boundaries as the preferable fab ric settings for dynamic-induced high-pressure phase transitions. Ab initio calculations negates the formation of the alpha -PbO2 polymorph by inversi on of a parental fluorite-structured polymorph during decompression. Heatin g of the experimentally produced alpha -PbO2 polymorph above 500 degreesC s hows that it inverts back in a laboratory time scale to rutile. Hence. the survival of the alpha -PbO2-structured polymorph in naturally shocked rocks constrains the post-shock temperature of the TiO2-bearing assemblage at an upper bound of 500 degreesC. The presence of this dense phase is expected in the Earth's upper mantle below 123 km. The rutile/alpha -PbO2-structured polymorph phase transition in subducted crustal limbs in tile upper mantle should then be accompanied by considerable changes in the partitioning and fractionation of Nb and Ta between this TiO2 polymorph and the coexisting dense silicates. (C) 2001 Elsevier Science B.V. All rights reserved.