The paper presents new data on the crystal structure behaviour of the Rb boroleucite phase under Si⁴⁺→ B³⁺ substitution and on the density of glasses and crystals. Polycrystalline samples were prepared by glass crystallisation. The crystal structures of the Rb boroleucite, RbBSi₂O₆ and two of its silica rich solid solutions, Rb_0·96(B_0·77Si_{2·1 8})_2·95O₆ and Rb_0·92(B_0·46Si_{2·4 2})_2·88O₆, have been determined by the Rietveld method. They are isostructural with boroleucite, KBSi₂O₆ (cubic system; I3d). When B³⁺ is substituted for Si⁴⁺, vacancies are formed at the B and Rb sites in the structure of the resulting silica Rb leucite solid solutions.