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The changes observed in the structure and phase composition of titanium-containing glasses in the magnesium aluminosilicate system at the equimolar ratio of MgO and Al2O3 during their heat treatment are investigated using the small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction analysis. Nickel oxide additives (from 0.5 to 5.0 mol %) are introduced into the glasses. It is shown that heat treatment of the studied glasses is accompanied by three-phase separation. As a result, two amorphous phases precipitate in the high-silica matrix. One phase (magnesium aluminotitanate) is enriched with magnesium, aluminum, and titanium oxides, whereas the other phase (magnesium aluminate) is enriched with aluminum and magnesium oxides. Nickel oxide is incorporated into both phases. Moreover, nickel oxide reduces the crystallization ability of the magnesium aluminotitanate phase and magnesium aluminotitanates do not crystallize in glasses containing 5 mol % NiO. During subsequent heat treatments, regions of the nickel magnesium aluminate phase serve as precipitation centers of the magnesium aluminate phase, and, consequently, nickel oxide affects the microinhomogeneous structure of glasses at the final stages of heat treatment. This leads in particular to a decrease in the size of spinel microcrystals. The temperature of the onset of crystallization in regions of existence of the nickel magnesium aluminate phase (spinel precipitation) is lower than that in glasses free from nickel oxide. It is assumed that spinel microcrystals are solid solutions of nickel aluminum and magnesium aluminum spinels.