The crystal structures of Mg₂Fe₂C₄O₁₃, with tetrahedrally coordinated carbon, and Fe₁₃O₁₉, synthesized at deep mantle conditions

Abstract

We simulated the redox decomposition of magnesium-siderite at pressures and temperatures corresponding to the top of the Earth’s D″ layer (135 GPa and 2650 K). It transforms into new phases, with unexpected stoichiometry. We report their crystal structure, based on single-crystal synchrotron radiation diffraction on a multi-grain sample, using a charge-flipping algorithm. Mg₂Fe₂(C₄O₁₃) is monoclinic, a = 9.822(3), b = 3.9023(13), c = 13.154(5) Å, β = 108.02(3)°, V = 479.4(3) ų (at 135 GPa). It contains tetrahedrally coordinated carbon units, corner-shared in truncated C₄O₁₃ chains. Half of the cations are divalent, and half trivalent. The carbonate coexists with a new iron oxide, Fe₁₃O₁₉, monoclinic, a = 19.233(2), b = 2.5820(13), c = 9.550(11) Å, β = 118.39(3)°, V = 417.2(5) ų (at 135 GPa). It has a stoichiometry between hematite, Fe2O3, and magnetite, Fe₃O₄. The formation of these unquenchable phases indicates, indirectly, the formation of reduced-carbon species, possibly diamond. These structures suggest the ideas that the mineralogy of the lower mantle and D″ region may be more complex than previously estimated. This is especially significant concerning accessory phases of fundamental geochemical significance and their role in ultra-deep iron-carbon redox coupling processes, as well as the iron-oxygen system, which certainly play an important role in the lower mantle mineral phase equilibria.