The growth of Fe₃O₄ and FeOOH crystals was investigated. Growth modifiers were used to alter the formation of iron oxides from ferrous hydroxide precipitates. Multifunctional carboxylic acids were found to have a strong influence on morphology of FeOOH. Dicarboxylic acids, containing two and seven carbons, changed the characteristics of α-FeOOH. These changes included alteration of the isoelectic point of the particulates and changes in particle size and shape. EDTA was found to alter the phase of FeOOH formation, favoring the synthesis of γ-FeOOH over α-FeOOH at temperatures below 50°C. The effects of multifunctional carboxylic acids were dependent upon the time of addition, and the presence of other growth modifiers. The changes in FeOOH formation were postulated to occur due to interaction between the acid molecules and Green Rust II, a common intermediate in iron oxide growth.

The growth of Fe₃O₄ was found to be sensitive to solution pH, and the form of the iron starting materials. Examination of reaction intermediates by x-ray diffraction showed that other Crystalline phases formed prior to the production of Fe₃O₄. Different intermediate phases occurred depending on the amount of hydroxide in the reaction, and differences in Fe³⁺ starting materials. The production of different intermediate phases affected the morphology of Fe₃O₄. Early precipitation of Fe₃O₄ led to small particles, while formation of crystalline Fe(OH)₂ led to large crystals. Formation of a mixture of Green Rust II and Fe(OH)₂ early in the oxidation process led to formation of multiple nuclei, and produced smaller average particles with a wide particle size distribution. Fe₃O₄ particles prepared from α-FeOOH seed crystals were spherical, while Fe₃O₄ particles prepared from FeSO₄ alone were octahedral crystals.