The effect of Fe(II) on the formation of iron oxide particles, synthesized from pyrite cinders lixivium via hydrothermal process, is investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The results indicate that Fe(II) can significantly affect the formation of iron oxide particles and the properties of the synthesized iron oxide particles. When n(Fe(II))/n(Fe(III)) varies from 0 to 0.1, all the samples are crystallized to hematite (alpha-Fe2O3), without any other phase. Particle size of the prepared hematite gradually increases with the increase of nFe((II))/nFe((III)) ratio, while the shape of the prepared hematite changes from sphere into platelet. When the n(Fe(II))/n(Fe(III)) increases to 0.12, both (alpha-Fe2O3) and Fe3O4 are formed and the shape of the product becomes spherical. As for the formation mechanism, it is found that Fe(II) can accelerate the phase transformation. The transformation process without Fe(II) is dissolution re-precipitation. However, in the transformation process with trace Fe(II), the phase transformation is dominated by solid-state transformation, though dissolution re-precipitation is still function. In addition, the formation of platelet-type hematite particles is attributed to that Al3+ ions are adsorbed onto {0001} face. Crown Copyright (C) 2011 Published by Elsevier By. All rights reserved.