Mineralogical transformations in polymetallic nodules and the change of Ni, Cu and Co crystal -chemistry upon burial in sediments

Polymetallic nodules from the Clarion and Clipperton Zone of the equatorial Pacific Ocean were studied using X-ray diffraction, X-ray absorption, Fourier -transformed infrared spectroscopy and transmission electron microscopy. This study includes nodules found at the sediment surface as well as subsurface (14-16 cm sediment depth) and deeply buried (530- 985 cm sediment depth) nodules. The surface and subsurface nodules are currently under oxic conditions whereas the deeply buried nodules are under suboxic conditions. Surface nodules consist mainly of turbostratic phyllomanganates (7 A ? and 10 A? vernadite and Fe-vernadite); todorokite is a minor phase, if present at all. In contrast, subsurface and especially deeply buried nodules predominantly consist of todorokite, which increases in abundance with depth in the sediment. Thus, upon burial of nodules within the shallow sediment, phyllomanganates transform to todorokite, probably through the combined action of time and change in the ambient chemical conditions. Nodules from deeper sediment depth ( 500 cm) consist primarily of todorokite and additionally show signs of dissolution. The transformation of phyllomanganates to todorokite and their further dissolution upon nodule burial under suboxic conditions induces modifications in the crystal -chemistry of Ni, Co, and Cu. In surface nodules, Ni and Co are incorporated in the octahedral sheets of phyllomanganates, whereas Cu mainly is located at the edges of those phyllomanganate sheets. In buried nodules Cu and to a lesser extent Ni are incorporated in todorokite by forming outer -sphere complexes within the tun- nels. However, Ni is predominantly incorporated within the octahedra of the newly formed todorokite structure. Co is also enriched in the octahedra of todorokite as a result of dissolution of hydrogenetic vernadite and re -incorporation in the more stable Mn-phase formed during the diagenetic transformation. Co enrichment under suboxic conditions after burial within the sediments is noteworthy since Co in surface nodules is characteristic for oxic conditions. (C) 2020 Published by Elsevier Ltd.