Fe and Si were codeposited on Si(lll) with an understoichiometric atomic ratio of 1:3 at substrate temperatures between 580 and 720 degrees C in order to investigate the nucleation and growth of silicide precipitates under growth conditions similar to molecular beam allotaxy. Medium energy ion scattering and transmission electron microscopy were used to determine cluster sizes, phases and shapes. The surface density of clusters decreases and the average cluster size increases with increasing substrate temperature. Using Venables’ theory on surface diffusion, we found an activation energy for surface diffusion of 0.76 +/- 0.10 eV for Fe on Si(111) during coevaporation. Our results show that a FeSi2 cluster containing more than three Fe atoms may be regarded as stable on the surface. Clusters of the high-temperature alpha- and of the low-temperature beta-phase were found side by side on the same samples.