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 spectrum consists of nine bands at wave numbers: 152, 175, 285, 315, 340, 358, 428, 532 and 575 cm(-1). These bands are identical to those of the alpha -PbO2-structured polymorph of TiO2 synthesized in a dynamic laboratory experiment. 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 group Phen; columbite-type structure), density rho =4.34g/cm(2), where the numbers in parentheses are standard deviations in the last significant digits. This new polymorph is 2% denser than rutile. The rutile/alpha -PbO2 polymorph 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 fabric settings for dynamic-induced high-pressure phase transitions. Ab initio calculations negates the formation of the alpha -PbO2 polymorph by inversion of a parental fluorite-structured polymorph during decompression. Heating of the experimentally produced alpha -PbO2 polymorph above 500 degreesC shows 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 the 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.