Mineralogy of Cobalt-Rich Ferromanganese Crusts from the Perth Abyssal Plain (E Indian Ocean)

Abstract

Mineralogy of phosphatized and zeolitized hydrogenous cobalt-rich ferromanganese crusts from Dirck Hartog Ridge (DHR), the Perth Abyssal Plain (PAP), formed on an altered basaltic substrate, is described. Detail studies of crusts were conducted using optical transmitted light microscopy, X-ray Powder Diffraction (XRD) and Energy Dispersive X-ray Fluorescence (EDXRF), Differential Thermal Analysis (DTA) and Electron Probe Microanalysis (EPMA). The major Fe-Mn mineral phases that form DHR crusts are low-crystalline vernadite, asbolane and a feroxyhyte-ferrihydrite mixture. Accessory minerals are Ca-hydroxyapatite, zeolites (Na-phillipsite, chabazite, heulandite-clinoptilolite), glauconite and several clay minerals (Fe-smectite, nontronite, celadonite) are identified in the basalt-crust border zone. The highest Ni, Cu and Co contents are observed in asbolane and Mn-(Fe) vernadite. There is significant enrichment of Ti in feroxyhyte−ferrihydrite and vernadite. The highest rare earth element (REE) content is measured in the phosphate minerals, less in phyllosilicates and Na-phillipsite. The geochemical composition of minerals in the DHR crusts supports the formation of crusts by initial alteration, phosphatization and zeolitization of the substrate basalts followed by oscillatory Fe-Mn oxyhydroxides precipitation of hydrogenous vernadite (oxic conditions) and diagenous asbolane (suboxic conditions).Minerals 2019, 9, 84 2 of 33 hydrothermal fluids), the two major types of ferromanganese crusts are distinguished: (I) hydrogenetic cobalt-rich ferromanganese crusts; (II) hydrothermal crusts and encrustations (sometimes called stratabound manganese oxides) [15][16][17][18]. Hydrothermal crusts that precipitate directly from low temperature hydrothermal fluids (few tens of degrees up to 200 • C), usually grow significantly faster, even up to 1600-1800 mm/Ma [19]. Ferromanganese crusts in some locations form through a combination of fluid sources and thereby exhibit a mixed origin, primarily either hydrogenetic, diagenetic or hydrothermal-hydrogenetic [15,18,20,21].Co-rich ferromanganese crusts formation is dominated by hydrogenetic processes. The precipitation from bottom waters is extremely slow, with growth rates of 1-5 mm/Ma. The thickest crusts occur in the depth interval between 800-2500 m and show the highest concentrations of critical metals [22]. Some authors limit this depth to the anoxic zone at approximate depths of 1000-1500 m. Below these depths, the thickness of crusts decrease [23][24][25].On the youngest rock outcrops, crusts form mainly patina-thin layers. Most crust surfaces are either botryoidal, smooth or rough, that form under conditions of strong bottom currents. In the thick crusts, 2 to 8 visible or macroscopic layers may be distinguished. The internal layers are usually black and massive, while outer parts are slightly lighter, laminated and more porous [13]. However, crusts with four major layers of oxide layers have been found. The outer-and innermost are massive and show light colors, whereas the...

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Investigating Nanoscale Electron Transfer Processes at the Cell-Mineral Interface in Cobalt-Doped Ferrihydrite Using Geobacter sulfurreducens: A Multi-Technique Approach

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