The stabilities of gibbsite,boehmite, aluminous goethites and aluminous hematites in bauxites,ferricretes and laterites as a function of water activity,temperature and particle size

Stability relationships between Al-goethite, Al-hematite, boehmite and gibbsite are presented in terms of water activity H₂O], temperature (T), grain size and bulk-composition in the system Fe₂O₃-Al₂O₃-H₂O at a total pressure of 1 bar.Al-goethite and Al-hematite are treated as ideal solid solutions, the former of the end-members FeOOH (goethite) and AlOOH (diaspore) and the latter of the end-members Fe₂O₃ (hematite) and Al₂O₃ (corundum). Using log K_sp provided by the literature for the various phases involved, the common associations observed in laterites, bauxites and ferricretes do not have stability fields over geologically reasonable intervals of H₂O] and T. Consequently a new internally consistent set of log K_sp values is proposed and used, and allows such associations to have actual stability fields in the appropriate diagrams. The new log K_sp values used in the calculations are such that the solubilities of the end members are greater than those commonly listed. This is in agreement with natural observations which show that such minerals, are generally poorly crystallized and of very small size. The assumption of an ideal solid solution in aluminous goethite and aluminous hematite combined with the new log K_sp values leads to prediction of composition limits for these two minerals which agree well with observed values. The fact that an ideal solid solution must extend continuously from one end-member to the other is masked by the appearance of other stability fields (e.g. gibbsite or boehmite) which cross and overlap a part of the solid solution stability fields of AlOOH-FeOOH and Fe₂O₃-Al₂O₃.