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Mineralogical characterization of individual growth structures of Mn-nodules with different Ni+Cu content from the central Pacific Ocean

Abstract : The mineralogy of manganese nodules from the German license area in the eastern Clarion and Clipperton Zone (CCZ) of the central Pacific Ocean was studied using X‑ray diffraction. Their individual nanometer to micrometer thick genetically different (hydrogenetic/diagenetic) layer growth structures were investigated using high-resolution transmission electron microscopy. Relationships between the mineral phases and metal content (e.g., Ni+Cu) were assessed with electron microprobe analyzer. The main manganese phase detected in nodules of this study was vernadite, a nanocrystalline and turbostratic phyllomanganate with hexagonal layer symmetry. In layer growth structures of hydrogenetic origin, Fe-vernadite dominates. Layer growth structures of suboxic-diagenetic origin contain three vernadite forms, which are the main Ni and Cu carriers. These Mn-phases were identified on the basis of their structural layer-to-layer distances (7 and 10 Å) and on their capacity to retain these distances when heated. The first form is 7 Å vernadite, which is minor component of the nodules. The second is a thermally unstable ~10 Å vernadite collapsing between room temperature and 100 °C, and the third is a thermally stable ~10 Å vernadite collapsing between 100 and 300 °C. Todorokite was neither detected in bulk nodules nor in any of the individual suboxic-diagenetic growth structures. Because the mineralogical composition of the nodule is quite homogeneous (only different vernadite-types), it is suggested that the content of Ni and Cu in the individual growth structures is controlled by their availability in the environment during individual growth phases. A profile through a CCZ nodule revealed that the thermal stability of the vernadites change from younger (thermally unstable vernadites, collapsing <100 °C) to older growth structures (thermally stable 10 Å vernadites, collapsing >100 °C) of the nodule accompanied with changes in type and amount of interlayer cations (e.g., Mg, Na, Ca, K). The stability of the vernadites is probably due to re-organization and incorporation of metals within the interlayer of the crystal structure.
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Soumis le : vendredi 18 novembre 2016 - 12:13:31
Dernière modification le : mercredi 3 août 2022 - 04:03:47




Anna V. Wegorzewski, Thomas Kuhn, Reiner Dohrmann, Richert Wirth, Sylvain Grangeon. Mineralogical characterization of individual growth structures of Mn-nodules with different Ni+Cu content from the central Pacific Ocean. American Mineralogist, 2015, 100 (11-12), pp.2497 - 2508. ⟨10.2138/am-2015-5122⟩. ⟨hal-01399075⟩



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