Prismatic polycrystalline magnetic particles of iron oxalate were grown by allowing the aggregation of the precipitating crystallites that were produced by a reaction between aqueous iron salts and oxalic acid solutions. The experiments were done at room temperature and under different pH conditions. The particle-growing process was followed by óptical microscopy and was then digitally recorded. The effect of the presence of a static magnetic field on the coprecipitation process was also analyzed. It was found that both pH and applied field, considerably influence the aggregation of the crystallites. This is clearly shown in the mean size and surface quality of the particles. It was observed that high pH values produce larger particles and that the applied field yields particles with a smoother surface than those obtained without the application of the field. The polycrystalline particles grow with the formation of clusters whose spatial distribution can be characterized by scaling relations. From this scaling behavior, some information about the spatial correlations occurring during precipitation can be obtained.