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

摘要

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 R-1 = 1.26%, wR(2) = 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) angstrom (Z = 4). The refined structure of the Mg2Cr2O5 phase has four (Mg, Cr)O-6 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 A(2)(2+)B(2)(3+)O(5) phase, it would be an important factor for the B3+ cation to have high octahedral-site preference, resulting in only A(2+) cation being accommodated in the tight trigonal prism site. Our study also suggests that mLd-type phase with (Mg, Fe2+)(2)Cr2O5 composition would crystallize as one of decomposed phases of chromitites, when the chromitites were possibly subducted into the mantle transition zone.