Mn-substituted iron oxides were synthesized by coprecipitating Fe(NO3)3 and Mn(SO4) solutions with NH4OH, adjusting the suspensions to pH 4 or 6, and then keeping the suspensions at 55°C for 62 days. The Mn mole fraction of the final products ranged from 0 to 0.3. X-ray powder diffraction patterns showed that goethite and hematite formed in each Fe-containing system. Groutite formed in systems having initial Mn mole fractions ≥0.35. Only manganite and hausmannite formed in the pure Mn systems. The oxalate-soluble Fe in the samples increased as the Mn mole fraction increased and was slightly larger for the pH 6 series.

For samples that contained the largest Mn mole fraction, the b and c dimensions of the goethite unit cell were shifted toward those of groutite, and the b and c dimensions of the groutite unit cell were shifted toward those of goethite. Assuming the Vegard rule holds for the unit-cell c dimension, the goethite accommodated a maximum Mn mole fraction of 0.34, and the groutite accommodated a maximum Fe mole fraction of 0.31. The unit-cell dimensions of hematite did not vary systematically with the mole fraction of Mn in solution, probably because little Mn substituted into the hematite structure.