The process of formation of silicon nanoclusters in a silicate matrix by the focused electron beam of high power and their cathodoluminescent properties were studied in this paper. The size of the silicon clusters and their concentration depend on the electron beam power and on the properties of the silicate matrix (density, porosity, composition). In this paper we studied the evolution of cathodoluminescent properties in bulk silicon oxide, porous silicon oxide glass, silicagel (n·SiO2·m·H2O) and opal. The result of the modification of silica is the appearance of the CL bands at 2.3eV in green region, and the bands at 1.4 and 1.1eV. The CL intensity of these bands increases during the interaction with electron beam. The volt-ampere characteristics of the modified areas of the silica are non-linear and depend on the time of the interaction and the power of the electron beam. We relate the appearance of these new CL bands with formation of oxygen-deficit defects and silicon clusters.