Nature and properties of iron rich glaebules and mottles from some south-west Australian soils

This paper compares the mineralogical and chemical composition, morphology and strength of co-existing matrix, mottles, weak glaebules and strong dense glaebules in some soils from south-western Australia. The mineralogy and morphology of mottles and weak glaebules are very similar to soil matrix, whereas most dense glaebules differ substantially from the soil matrix. These dense glaebules are very hard (more than 200 kg weight is required to break them) and consist mostly of hematite and maghemite with minor amounts of corundum, boehmite and quartz. In the comparatively more porous and weak glaebules, that are the more common forms occurring in lateritic soils, kaolinite, gibbsite and quartz are the major minerals with small amounts of goethite, hematite and anatase. Generally the strength of mottles congruent to soil matrix <glaebules <dense glaebules.Mottles contain similar to 3 times more dithionite-citrate-bicarbonate (DCB) Fe (median = 1.8%) and similar to 2.5 times more oxalate Fe (median = 614 mu g/g) than soil matrix, with smaller values congruent to 5% of the active Fe ratio [(oxalate Fe/DCB Fe) . 100]. Both types of glaebules contain much more Fe (5 to 10 times) than matrix with median values of HCl and DCB extractable Fe of 12.7 and 6.5%, respectively. The active Fe ratio for glaebules is also very high with a median value of 23%.The DCB Al content of mottles is about 2.5 times that of the soil matrix, whereas the oxalate Al content was about the same for both mottles and soil matrix. The amount of Al extracted from glaebules by DCB, oxalate and pyrophosphate reagents is significantly higher than for mottles and soil matrix samples. Oxalate extracted 1.5 times more Al compared to DCB reagent.Manganese was not concentrated in most mottles relative to co-existing matrix, and in glaebules it was only present in small amounts (median value of 104 mu g/g for HCl Mn).Al-substitution in goethite in mottles was very similar to values for goethite in the soil matrix. The mineralogy of weak glaebules was similar to that of the soil matrix. Corundum and maghemite were only present in strong magnetic glaebules and were not present in the soil matrix.