An Assessment of the Effects of Phosphatization on the Co Depletion of a Ferromanganese Crust from the Magellan Seamount Cluster

The mineral composition and chemistry of a Co-rich ferromanganese crust from Skornyakova Guyot in the Magellan Seamount Cluster is investigated by X-ray diffraction and ICP-OES to scrutinize the effects of phosphatization on the depletion of Co. The section of this crust is divided into four layers (layers I-1, I-2, II and III) from substrate to surface based on texture. The average Co concentrations of older layers I-1 and I-2 are 0.41% and 0.30%, respectively, whereas the average Co concentrations of younger layers II and III are 0.53% and 0.29%. The variation ranges of SiO2, Al2O3, CaO, P2O5, and Fe2O3 along the section are 1.5%–22.9%, 0.4%–6.9%, 2.7%–24.6%, 0.83%–15.26%, and 10.7%–28.4%, respectively, which are capable of diluting the Co concentrations significantly. The todorokite detectable by XRD does not occur in layers I-1 and I-2, which suggests that most of the depletion of Co in phosphatized crust cannot be attributed solely to the mineral transition from Fe-vernadite to todorokite during phosphatization, and other controls, including the dilution from detrital minerals and carbonate fluorapatite, need to be involved.