Spectral and chemical studies of iron and manganese oxyhydroxides in laterite developed on ultramafic rocks

Iron (Fe) oxyhydroxides (goethite and hematite) and manganese (Mn)-oxyhydroxides (lithiophorite, asbolane, lithiophorite-asbolane intermediate) are typically fine-grained and poorly crystalline in nature, and as such are difficult to identify by conventional X-ray powder diffraction. This study employs Raman spectroscopy and electron probe microanalysis (EPMA) to characterize Fe- and Mn-oxyhydroxides found in the Berong Ni-Co laterite deposit at Palawan Island, Philippines. Accurate identification of these minerals is important because these phases contain high Ni and Co contents. Goethite and hematite occur in a wide range of textures, which are related to their compositional variations with respect to Ni, Al, Mn, Cr, and Si. The change in the intensity of the Raman peaks can be linked to the variable concentrations of Ni, Al, Mn, Cr, and Si in goethite. These chemical variations affect the textural transformation of goethite from amorphous to cryptocrystalline. Lithiophorite, asbolane and their intermediates were properly distinguished using Raman spectroscopy. EPMA data shows that these Mn minerals contain appreciable concentrations of Ni, Co, Al, and Fe. The band shift from lithiophorite to asbolane end terms in the 486-593 cm(-1) domain indicates the substitution of Al in lithiophorite by Ni, Co, and Fe.