Mineralogical control on transition metal distributions in marine manganese nodules

Abstract

Electron microprobe and X-ray diffraction data for north Pacific manganese nodules reveal that the transition metal distributions are controlled by the mineralogy. Microlayers rich in 10Å-manganates generally have high Mn Fe ratios and positive correlations between Ni, Cu and Mn, and between Co and Fe. Microlayers rich in vernadite, on the other hand, show low Mn Fe ratios, and Co, Ni and Cu all show positive correlations with Mn. The 10Å-manganates form mainly in porewaters with high Mn Fe ratios. The Ni 2+ and Cu 2+ ions are post-depositionally incorporated into the interlayers of the manganates, whereas Co 3+ is substituted for Fe 3+ in ferric oxyhydroxides. In seawater with a low Mn Fe ratio, on the other hand, the adsorption of positively charged ferric oxyhydroxides on negatively charged [MnO 6] octahedral layers suppresses the growth of 10Å-manganates, enhancing the formation of vernadite. Positively charged hydroxides of Co 3+, Ni 2+ and Cu 2+ are also adsorbed on the [MnO 6] layers. These mechanisms of mineral formation and metal uptake are corroborated by data for other oceanic non-hydrothermal manganese nodules and crusts.