Ferruginous concretions in a Late Cretaceous karst bauxite: composition and conditions of formation

The karst bauxites from the Apulian carbonate platform (southern Italy) have an ooidic texture. The concretions, which are formed at the expense of the kaolinitic matrix, are composed of Al-hematite in the core and of Al-hematite and boehmite in the banded cortex. The Al-hematite in the core has an average formula (Al0.12Fe0.88)2O3 and the Al is within the range of values observed in hematite formed through in situ epigenetic replacement of kaolinite. The Al substitution in hematite involves a water-unsaturated environment, possibly promoted by fluctuations of the groundwater table followed by intense drainage and evaporation. In the banded cortex, boehmite may form instead of Al-hematite at lower water activity values. Further, the boehmite stability increases for high values of the aAl3+/a3H+ ratio, which is mostly controlled by the pH of the solution. The stability of the Al-hydroxide decreases with increasing pH.