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Structural relaxation around substitutional Cr³⁺ in pyrope garnet

¹ Institut de Minéralogie et Physique des Milieux Condensés (IMPMC), UMR CNRS 7590 Université Paris VI, Université Paris VII, IPGP, 4 Place Jussieu, 75252 Paris Cedex 05, France
² Institut Néel, CNRS—Université Joseph Fourier, 25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France

Correspondence: ^* E-mail: amelie.juhin{at}impmc.jussieu.fr

The structural environment of substitutional Cr³⁺ ion in a natural pyrope Mg₃Al₂Si₃O₁₂ has been investigated by Cr K-edge extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) coupled with first-principles computations. The Cr-O distance is close to that in knorringite Mg₃Cr₂Si₃O₁₂, indicating a nearly full relaxation of the first neighbors. The local C_3i symmetry of the octahedral Y site is retained during Cr-Al substitution. The second and third shells of neighbors (Mg and Si) relax only partially. Site relaxation is accommodated by strain-induced bond buckling, with angular tilts of the Si-centered tetrahedra around the Cr-centered octahedron, and by a deformation of the Mg-centered dodecahedra.

Key Words: Cr-pyrope • relaxation • XAS • ab initio • trace elements and REE • chromium • crystal structure • garnet • quantum mechanical calculations