A supercritical fluid technique was used to prepare hexagonal nanoplatelets of magnetite. Ferrocene was used as the Fe source, and sc-CO2 acted as both a solvent and oxygen source in the process. Powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and magnetic measurements were used to characterize the products. It was found that the morphology and structure of the product strongly depended on the reaction conditions.