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The crystal structure and chemical composition of a crystal of Mg(Mg,Cr,Si) 2 O 4  post-spinel phase synthesized in the model system MgCr 2 O 4 -Mg 2 SiO 4  at 16 GPa and 1600 °C have been investigated. The compound was found to crystallize with a distorted orthorhombic calcium-titanate (CaTi 2 O 4 ) structure type, space group Cmc2 1 , with lattice parameters a = 2.8482(1), b = 9.4592(5), c = 9.6353(5) Å, V = 259.59(1) Å 3 , and Z = 4. The structure was refined to R 1  = 0.018 using 345 independent reflections. The loss of the inversion center is due to the ordering of cations at the octahedral sites: Cr is mainly hosted at the M1 site, whereas Si at the M2 site. Such an ordered distribution induces a distortion thus provoking a change in coordination of Mg, which becomes sevenfold-coordinated instead of the usual eightfold coordination observed in post-spinel phases. Electron microprobe analysis gave the Mg[(Cr 0.792 Mg 0.208 ) (Si 0.603 Mg 0.397 )]O 4  stoichiometry for the studied phase. The successful synthesis of this phase can provide new constraints on thermobarometry of wadsleyite/ringwoodite-bearing assemblages.

Letter. Structural and chemical characterization of Mg[(Cr,Mg)(Si,Mg)]O 4 , a new post-spinel phase with sixfold-coordinated silicon