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

An X-ray diffraction study of isostructural V-containing beryl (V-Iz) and V-containing beryllian indialite (V-BI) (the general formula M2VIT23IVT16IVO18)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.