Sources of trace metals in Ferralsols in New Caledonia

One third of New Caledonia is occupied by soils deriving from ultrabasic rocks anomalously rich in trace metals such as Cr, Ni, Mn and Co. The location of the metals present in three samples from a toposequence of Ferralsols was studied by combining selective dissolutions (CaCl2, Cu(NO3)(2), hydroxylamine, dithionite-citrate-bicarbonate), examinations with a transmission electron microscope (TEM) equipped with an EDAX device, and X-ray diffraction and infrared spectroscopy. The behaviour of the metals during dissolution was found to depend on both the type of metal and the location of the samples in the toposequence. In the lowland soil samples derived from transported materials, about 50% of total Mn and Co and a small fraction (< 1%) of total Ni were dissolved in the hydroxylamine reagent. By TEM, the manganese oxide minerals present in one of the lowland samples were found to have chemical compositions similar to those of lithiophorite-asbolane mixed layers identified previously in the subsurface horizons of the laterites capping the landscape where the toposequence occurs. Whatever the location of samples, goethite, i.e. the major constituent of these soils, was found to be the scavenger of the largest proportions of Ni and of about 40% of total Cr. However, this mineral was present as two distinct morphological facies, and each of them had a statistically different chemical composition. The small needle-shaped goethite, which was abundant in the lowland soils only, was richer in Ni and Si and poorer in Al than the larger acicular one. Our data show that the potential mobility of trace metals in these soils is likely to be controlled by the most soluble phases identified, i.e. the Co-rich Mn oxides and the Ni-rich needle-shaped goethites.