The features of structure and isomorphic substitutions in flux-synthesized V-containing beryls and V-containing beryllian indialite

Abstract

An X-ray diffraction study of isostructural V-containing beryl (V-Iz) and V-containing beryllian indialite (V-BI) (the general formula M ^VI₂ T2 ^IV₃ T1 ^IV₆ O₁₈)¹ synthesized in the Al₂Be₃Si₆O₁₈-Mg,Ca/F,Cl flux system with V₂O₅ impurity has been performed. It is shown for V-Iz that the isomorphic replacement of some part of Al³⁺ in the octahedron with larger V³⁺ and Mg²⁺ cations and a small fraction of Be²⁺ ions with Al³⁺ in the interring tetrahedron T2, as well as the introduction of alkaline ions into R voids, leads to an increase in the average bonds lengths in the M position and increases the unit-cell parameter a at an insignificant increase in the parameter c. In V-Bi, the heterovalent replacement of some part of Mg²⁺ ions by V³⁺ ions, with complex isomorphism in all structural positions, is accompanied by the isovalent Mg²⁺ → Fe²⁺ scheme. These substitutions cause some decrease in the parameter c of V-Bi with the preservation of its parameter a. It is found that T1 tetrahedra in V-BI are much closer to ideal than in V-Iz, T2 tetrahedra are distorted much more weakly, and M octahedra are distorted somewhat more strongly. However, the degree of distortion of the ring tetrahedron T1 in V-BI reaches 0.9 rel. %. This fact indicates that V-BI and Cr-BI have a similar disposition to congruent melting, which significantly inferior that of “pure” species.