Akaogiite: An ultra-dense polymorph of TiO2 with the baddeleyite-type structure, in shocked garnet gneiss from the Ries Crater, Germany

Abstract

Akaogiite, an ultra-dense polymorph of TiO2 was encountered in heavily shocked garnet-cordieritesillimanite gneiss in the Suevite breccia of the Ries meteorite impact crater in Germany. The chemical formula of akaogiite is TiO2, containing (wt%) TiO2 97.6-97.7; Nb2O5 0.15-0.20, and FeO 0.11-0.14. The empirical formula is: Ti0.998Fe0.002Nb0.001O2. 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) Å, b = 4.896(3) Å, c = 4.933(3) Å, and β = 99.17(6)°; space group P21/c; molar volume = 16.82(2) cm3/mol; density = 4.72 g/cm3. Akaogiite coexists in the shocked gneiss with graphite-diamond phase transition assemblage, a second dense TiO2 polymorph with the scrutinyite-type structure (TiO2-II), liquidus idiomorphic jadeite inclusions in melt pockets in garnet, and a new FeTiO3-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 <500 °C, thus correcting previous estimates. The name was approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2007-058). The name is for Masaki Akaogi, Professor at the Department of Chemistry, Gakushuin University in Tokyo, Japan.