Abstract: The formation of uniform spherical hematite particles, obtained by hydrolysis of ferric chloride at elevated temperatures, follows the mechanism of phase transformation from β-FeOOH to α-Fe_2O_3 and recrystallization from tiny α-Fe_2O_3 particles to spherical colliodal α-Fe_2O_3 uniform particles. A new viewpoint on the formation of uniform colloidal particles is suggested, that is, if uniform particles are to be formed by the mechanism of phase transformation-aggregation recrystallizationt, the secondary aggregations of tiny particles must be avoided after the first burst of aggregates. The relationship between initial time of reaching critical supersaturation with the aging temperature, pH and concentration of ferric chloride can be described by a three-parameter logarithmic linear equation.

Key words: Phase transformation, Aggregation, Recrystallization, Critical supersaturation, Hydrolysis