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Fe-Mn crusts are found throughout the ocean basins on hard-rock substrates. They form by precipitation from cold ambient bottom waters at depths typically of about 1000-3000 m. The crusts grow at extremely slow rates, about 1-7 mm/Ma, and contain high concentrations of strategically and economically important metals (e.g., Co, Ti, Ce, Zr, Ni, Pt, Mo, Te, Cu, W). It is essential to understand the physical properties of Fe-Mn crusts to (1) determine the mechanisms by which metals are sequestered from seawater, (2) contrast crust properties with substrate rocks so that exploration tools can be developed, and (3) characterize the breakability of the potential ore so that appropriate mining equipment can be engineered to minimize dilution by substrate rock. These properties vary with composition and environment of formation. Crusts are composed of Mn oxide and amorphous Fe oxyhydroxide. Thick crusts have inner layers that are impregnated with carbonate fluorapatite (CFA), which reduces porosity and permeability. Crusts generally contain sub-equal amounts of Mn and Fe, with Fe/Mn ratios of <1 in open Pacific Ocean crusts and >1 in continental margin and Atlantic and Indian Ocean crusts. In addition to the remarkably slow growth rates, crusts are unique in having extremely high specific surface areas (SSA), averaging 325 sq m/g. SSA decreases with time on exposure to air, up to 40% within 2 months. CFA-free crusts have a high mean porosity of 60%. All these properties aid in the acquisition of large quantities of metals from seawater. Metal sorption increases with increasing deviation from stoichiometry, which is determined by the O/Mn ratios of crusts. Physical properties that may aid in developing exploration tools, especially the in situ measurement of crust thickness, include seismic-wave velocity (P-waves and S-waves) and gamma radiation (475 mean net counts/minute). Of these, only gamma radiation falls outside the range displayed by a wide variety of substrate rock types (mean 130 net counts/minute). Consequently, gamma radiation may offer the best opportunity for in situ thickness measurements if seawater signal attenuation can be overcome. Physical properties that reflect mechanical breakability include wet bulk density, which varies from 1.83 to 2.44 g/cubic cm; seawater-saturated compressive, tensile, and shear strengths; Young's modulus; and angle of internal friction. These parameters, coupled with mechanical cutting and fragmentation experiments, indicate that crusts are relatively weak and behave similarly to medium-hard coal. This contrasts with substrate rocks, which vary from very weak (mudstone, hyaloclastite) to very strong (basalt, phosphorite). The physical properties of crusts overlap with those of the common substrate rock types, especially breccia and hyaloclastite, which makes the development of mining and exploration tools a challenge.
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