alpha-PbO2-type TiO2: From mineral physics to natural occurrence

alpha -PbO2-type TiO2 (space groups Pbcn, a thermodynamically predicted hig h-pressure polymorph,ators using shock and static compression experiof ruti le). was synthesized by a number of investig ments. Recently, in situ. high -pressure and high-temperature studies employing the multi-anvil device and white-beam (using a synchrotron radiation source) energy-dispersive method indicated that the transformation pressure is lower for nanophase material (similar to4 GPa and 900 degreesC) than for the bulk (similar to6 GPa and 850 degreesC). In addition, the phase boundary of rutile/alpha -PbO2-type T iO2 changes from a negative to a positive slope with increasing temperature . This timely knowledge provides indicative pressure-temperature (P-T) cons traints on the natural occurrence of alpha -PbO2-type TiO2, recently identi fied by analytical electron microscopy as an epitaxial nanometer-thick slab between twinned rutile bicrystals in almandine-rich garnet of diamondifero us quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany. The stab ility field of "bulk" alpha -PbO2-structured TiO2 shows that the minimum st abilization pressure of transition is similar to6 GPa and could have been u p to 2 GPa lower as a result of the nanophase effect. This suggests burial of continental crustal rocks to depths of at least 130-200 kilometers. Thus , alpha -PbO2-type TiO2 inclusions in garnet may be a useful P-T indicator in the diamond stability field. Furthermore, the possibility of finding alp ha -PbO2-type TiO2 or even a higher-P polymorph (e.g., baddeleyite-structur ed TiO2) at impact sites of meteorite craters is increased, in view of the recent identification of post-stishovite (isostructure of rutile) SiO2 poly morphs in the meteorite Shergotty, and the alleged identification of alpha -PbO2-type TiO2 by Raman spectroscopy in shocked gneisses from the Ries Met eorite Crater, Germany.