The effect of calcination temperature on the physico-morphological properties of Indian Ocean manganese nodules has been investigated. Mossbauer, XRD, FT-IR spectroscopic and TG-DTA studies on the manganese nodules gave some idea of the phase transformations in the temperature range 110-1000 degrees C. Mossbauer studies reveal that the iron in the untreated nodule is mainly present as gamma-FeOOH along with minor amount of iron phosphate and/or silicate. No phase transformation has been observed in the calcined sample up to 500 degrees C and beyond that gamma-FeOOH phase transforms to alpha-Fe2O3 and delta-MnO2 to Mn2O3. A solid solution of Fe2O3 and Mn2O3 is formed above 700 degrees C and beyond that, some amount of Fe exists in magnesium manganese iron oxide form. Insertion of magnesium into the Mg-Mn ferrite structure becomes greater at greater than or equal to 900 degrees C.