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Several compositions of mixed manganese¿¿magnesium ferrite, MgyMn1-yFe2O4 have been examined by neutron diffraction in order to determine the distribution of the manganese cations and the position of the oxygen anions in the spinel lattice. The compounds chosen, with y = 0.25, 0.50, 0.75 and 0.90, were quenched from both 1400° and 1000°C. For all the samples dealt with the results indicate that the distribution of the manganese cations is appreciably unaltered, i.e. approximately 90% of the manganese present remains fixed in the tetrahedral sites. The average bond length in the tetrahedral site for the various compositions, as determined from the oxygen position and the unit cell size, has been found to decrease, relative to the sum of the bonding ions, with increasing concentration of manganese. This change may be interpreted as a measure of the covalent bonding of the cations occupying these sites. On this basis, continued substitution of manganese for magnesium results in a greater covalent bonding on the tetrahedral sites, and an estimate has been made of the relative covalent bonding strengths of Mn2+ and Fe3+.