Mossbauer spectra of olivines of composition (MgxFe1−x)2SiO4(x?0.3 to 0.9) have been examined in the temperature range 4.2 to 300 K, and found to exhibit superparamagnetic relaxation effects originating from clusters of Fe2+ions. Relaxation times were determined by comparison with theoretical relaxation spectra calculated from the stochastic model of Blume and Tjon, which was modified to include the asymmetry parameter and general orientations of the hyperfine field direction with respect to the crystal axes. From the variation of the relaxation time with temperature, the mean cluster‐size diameters of the Fe‐rich regions were found to be in the range 20 to 40 A˚, and to increase with increasing Fe content. The distribution of cluster diameters was asymmetric, with higher probabilities for small diameters. Potential applications with regard to the determination of the thermal histories of rocks containing olivine are discussed.