gamma-Fe2O3 nanocrystallites dispersed in an amorphous silica matrix have been successfully synthesized for the first time by mechanical activation of a chemistry-derived precursor at room temperature. At the initial 10 h of mechanical activation, nanocrystallites of Fe3O4 were triggered to form a highly activated matrix. With increasing mechanical activation time, phase transformation from Fe3O4 to gamma-Fe2O3 occurs. gamma-Fe2O3 phase was well established when the precursor was mechanically activated for 30 h. Further increasing the mechanical activation time to 40 h induced the formation of alpha-Fe2O3. The mechanical activation-grown gamma-Fe2O3 nanocrystallites are similar to10-12 nm in size and they are well dispersed in the silica matrix as observed using TEM. They demonstrated superparamagnetic behavior at room temperature when measured using a vibrating sample magnetometer (VSM). In addition, the gamma-Fe2O3 derived from 30 h of mechanical activation exhibited a saturation magnetization of as high as 62.6 emu g(-1). (C) 2002 Elsevier Science B.V. All rights reserved.