Discovery of MgTiSi2O7: a new high-pressure silicate with the weberite structure synthesized at transition-zone conditions
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The crystal structure and chemical composition of a crystal of MgTiSi2O7 synthesized in the model system MgTiO3–MgSiO3 at 15 GPa and 1600 °C have been investigated. The compound was found to crystallize with the weberite-3T structure type, space group P3121, with lattice parameters a = 6.3351(7), c = 16.325(2) Å, V = 567.4(1) Å3, and Z = 6. The structure was refined to R 1 = 0.059 using 2092 independent reflections, and can be described as a sequence of pairs of polyhedral layers (M and N) stacked along . As far as the cation sites are concerned, M and N layers have general formula AB3 and A3B, respectively, where B are the octahedrally coordinated cations (B1, B2, and B3), which mainly accommodate Si. The octahedral framework gives rise to three types of larger cavities occupied by eight-coordinated Mg (A1), Ti (A2), and mixed (Mg,Ti) (A3) atoms. Electron microprobe analysis gave the Mg0.99Si1.62Ti1.39O7 stoichiometry for the studied phase. The successful synthesis of this phase demonstrates that titanium can stabilize heretofore unknown Mg–Si-oxides, the major Earth and rocky planet-forming materials, and can provide new constraints on thermobarometry of wadsleyite/ringwoodite, and garnet-bearing assemblages.
KeywordsMagnesium silicate structure Titanium Weberite Crystal structure Microprobe analysis Synthesis
The research was supported by “progetto di Ateneo 2015, University of Firenze” to LB, by C.N.R., Istituto di Geoscienze e Georisorse sezione di Firenze, Italy, by the Foundation of the President of the Russian Federation (grant no. MK-1277.2017.5 to ES), and by the Russian Foundation for Basic Research (project no. 15-05-02051 to DP). ES thanks Geodynamics Research Center, Ehime University, Matsuyama, Japan, for support of her visit in 2016.
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