A shallow origin of so-called ultrahigh-pressure chromitites, based on single-crystal X-ray structure analysis of the high-pressure Mg₂Cr₂O₅ phase, with modified ludwigite-type structure

The crystal structure of the high-pressure Mg₂Cr₂O₅ phase was studied by single-crystal X-ray diffraction (XRD) analysis for the recovered samples. The 61 parameters including anisotropic displacement parameters of each atom and site occupancies of Mg and Cr in cation sites were refined with R₁ = 1.26%, wR₂ = 4.33%, and S_fit = 1.265 for 470 unique reflections. The results show that the structure of the recovered Mg₂Cr₂O₅ phase is the same as modified ludwigite (mLd)-type Mg₂Al₂O₅ [space group: Pbam (no. 55)], and the lattice parameters are a = 9.6091(2), b = 12.4324(2), c = 2.8498(1) Å (Z = 4). The refined structure of the Mg₂Cr₂O₅ phase has four (Mg,Cr)O₆ octahedral sites and a MgO₆ trigonal prism site, and is similar to but distinct from that of CaFe₃O₅-type Mg₂Fe₂O₅ phase, which has two octahedral sites and a bicapped trigonal prism site with two longer cation-oxygen bonds. The isotropic atomic displacement parameter of the trigonal prism site cation in mLd-type Mg₂Cr₂O₅ is relatively small compared with that of CaFe₃O₅-type Mg₂Fe₂O₅, suggesting that the trigonal prism site is less flexible for cation substitution than that of CaFe₃O₅-type structure. To stabilize mLd-type A₂²⁺B₂³⁺O₅ phase, it would be an important factor for the B³⁺ cation to have high octahedral-site preference, resulting in only A²⁺ cation being accommodated in the tight trigonal prism site. Our study also suggests that mLd-type phase with (Mg,Fe²⁺)₂Cr₂O₅ composition would crystallize as one of decomposed phases of chromitites, when the chromitites were possibly subducted into the mantle transition zone.