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Cobalt-rich Ferromanganese Crusts: Global
distribution, composition, origin and research
activities
Cobalt-rich ferromanganese crusts occur throughout the global ocean
on seamounts, ridges, and plateaus where currents have kept the rocks swept
clean of sediments for millions of years. Crusts precipitate out of cold ambient
seawater onto hard-rock substrates forming pavements up to 250 mm thick.
Crusts are important as a potential resource for primarily cobalt, but also for
titanium, cerium, nickel, platinum, manganese, thallium, tellurium, and others.
Crusts form at water depths of about 400-4000 m, with the thickest and most
cobalt-rich crusts occurring at depths of about 800-2500 m, which may vary on a
regional scale. Gravity processes, sediment cover, submerged and emergent
reefs, and currents control the distribution and thickness of crusts. Crusts occur
on a wide-variety of substrate rocks, which makes it difficult to distinguish the
crusts from the substrate using remotely sensed data, such as geophysical
measurements. However, crusts can be distinguished from the substrates by
their much higher gamma radiation levels. The mean dry bulk density of crusts
is 1.3 g/cm 3 , the mean porosity is 60%, and the mean surface area is extremely
high, 300 m 2 /g. Crusts generally grow at rates of 1-6 millimetres per million
years. Crust surfaces are botryoidal, which may be modified to a variety of
forms by current erosion. In cross-section, crusts are generally layered, with
individual layers displaying massive, botryoidal, laminated, columnar, or
mottled textures; characteristic layering is persistent regionally. Crusts are
composed of ferruginous vernadite (dMnO2) and X-ray amorphous iron
oxyhydroxide, with moderate amounts of carbonate fluorapatite (CFA) in thick
crusts and minor amounts of quartz and feldspar in most crusts. Elements most
commonly associated with the vernadite phase include manganese, cobalt,
nickel, cadmium, and molybdenum, and with the iron oxyhydroxide, iron and
arsenic. Detrital phases are represented by silicon, aluminium, potassium,
titanium, chromium, magnesium, iron, and sodium; the CFA phase by calcium,
phosphorus, strontium, yttrium, and carbon dioxide; and a residual biogenic
phase by barium, strontium, cerium, copper, vanadium, calcium, and
magnesium. Bulk crusts contain cobalt contents up to 1.7%, nickel to 1.1%, and
platinum to 1.3 parts per million (ppm), with mean iron/manganese ratios of 0.4
to 1.2. Cobalt, nickel, titanium, and platinum decrease, whereas iron/manganese,
silicon, and aluminium increase in continental margin crusts and in crusts with
proximity to west Pacific volcanic arcs. Vernadite- and CFA-related elements
decrease, whereas iron, copper, and detrital-related elements increase with
increasing water depth of crust occurrence. Cobalt, cerium, thallium, and maybe
also titanium, lead, tellurium, and platinum are strongly concentrated in crusts
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