Akaogiite: An ultra-dense polymorph of TiO(2) with the baddeleyite-type structure, in shocked garnet gneiss from the Ries Crater, Germany

International audienceAkaogiite, an ultra-dense polymorph of TiO(2) was encountered in heavily shocked garnet-cordierite-sillimanite gneiss in the Suevite breccia of the Ries meteorite impact crater in Germany. The chemical formula of akaogiite is TiO(2), containing (wt%) TiO(2) 97.6-97.7; Nb(2)O(5) 0.15-0.20, and FeO 0.11-0.14. The empirical formula is: Ti(0.998)Fe(0.002)Nb(0.001)O(2). Akaogiite is optically distinguishable from rutile only in reflected light microscopy, through its slightly higher brightness and the intense royal blue color of its internal reflections in crossed nicols. The mineral grains consist of countless randomly oriented submicrometer particles. In situ micro-beam X-ray diffraction established akaogiite to be monoclinic with the baddeleyite-type structure. Cell parameters are a = 4.606(2) angstrom, b = 4.896(3) angstrom, c = 4.933(3) angstrom, and beta = 99.17(6)degrees; space group P2(1)/c; molar volume = 16.82(2) cm(3)/mol; density = 4.72 g/cm(3). Akaogiite coexists in the shocked gneiss with graphite-diamond phase transition assemblage, a second dense TiO(2) polymorph with the scrutinyite-type structure (TiO(2)-II), liquidus idiomorphic jadeite inclusions in melt pockets in garnet, and a new FeTiO(3)-polymorph with the Li-niobate structure. The high-pressure assemblage constrains the equilibrium peak-shock pressure to be <= 22 GPa and a post-shock temperature