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Well-crystallized hexagonal hematite (a-Fe2O3) platelets were synthesized by hydrotheal process,using a highly concentrated ferric hydroxide as precursor.The precursor was prepared by adding ammonia to the ferric sulfate solution which was obtained by leaching pyrite cinders with sulfuric acid.Structure and morphology of the synthesized products were investigated by X-ray diffraction,scanning electron microscope,transmission electron microscope and selected area electron diffraction.The results reveal that the reaction temperature has significant effects on the structure,size and shape of the synthesized hematite particles.Typical hexagonal hematite platelets,about 0.4-0.6 μm in diameter and 0.1 μm in thickness,were prepared at 230 ℃ for 0.5 h.Al3+,contained in the sulfuric acid leaching solution as an impurity,plays an extremely important role in the formation of hexagonal hematite.In addition,a possible mechanism about the formation of hexagonal hematite platelets was proposed.