Abstract: Mixtures of magnetite and goethite were formed by the slow oxidation of mixed FeCl2-AlCl3 solutions in an alkaline environment at room temperature. The compositions of the products ranged from almost exclusively magnetite in Al-free systems to goethite only at Al/(Al + Fe) ≈ 0.3. The magnetic phase consisted of a partly oxidized (Fe2+/Fe3+ < 0.5), Al-substituted magnetite. The unit-cell edge length a of the magnetite decreased with increasing Al (Al = 0–0.37 per formula unit, corresponding to 0–14 mole % Al) and decreasing Fe2+ in the structure as described by the empirical relationship a(A) = 8.3455 + 0.0693 Fe2+ - 0.0789 Al. A correlation between the experimentally determined a and that calculated from the unit-cell edge lengths of end-member magnetite, maghemite, and hercynite was highly significant (r = .96) although shifted by about 0.01 Å. Mössbauer spectra showed Al to have entered preferentially the tetrahedral rather than the octahedral sites at low Al substitutions (<0.15 per formula unit), perhaps because of steric reasons. With increasing Al substitution the crystal size of magnetite decreased and structural strain increased, indicating that the structure had a limited capability to incorporate Al under these synthesis conditions. The capacity of the goethite structure to tolerate more Al may explain why goethite replaced magnetite at higher Al concentrations.