LAUSR

Hydrogenetic cobalt-rich ferromanganese crusts (cobalt-rich crusts, CRCs) are expected as potential mineral deposits for rare earth elements and platinum group elements as well as cobalt, nickel, and manganese (Manheim 1986; Rona 2003). Previous investigations have revealed that CRCs are widespread on seamounts in the western Pacific and that some of the guyots with several 100 km2 in the summit area and 1000–2000 m in summit depth are covered with CRCs up to 20 cm thick (Usui and Okamoto 2010; Okamoto and Usui 2014). For the commercial development of CRCs, it is necessary to establish mining technologies for the CRCs that are thinly and widely distributed on the deep seafloor. Therefore, it is essential to understand the mechanical properties of CRCs. Larson et al. (1987) obtained the density, Shore hardness, compressive strength, tensile strength, Young’s modulus, and elastic wave velocity of the CRCs and substrate rocks in the Hawaiian Archipelago; they noted “the limited number of samples minimized the number of tests ... the data contained in this report should be considered preliminary and subject to change as more data become available”. Thereafter, Yamazaki et al. (1995) reported the compressive strength of CRCs and substrate rocks, and then Wu and Chen (2013) reported the shear strength of CRCs. However, experimental studies on the mechanical properties of CRCs are insufficient because material sampling from the deep seafloor requires considerable time and effort. These previous studies showed that the uniaxial compressive strength (UCS) of CRCs is up to 20 MPa, and, therefore, mechanical cutting is probably suitable for the mining of CRCs. Larson et al. (1987) reported the cuttability and friability of the CRCs; however, they used a single type of a cutting bit and qualitatively investigated the cutting resistance on the rough surface of the CRCs. In addition, there is little knowledge of the abrasivity of CRCs which is an essential information for the mining of CRCs on the deep seafloor. In this study, porosity, P-wave velocity, and UCS were obtained from the CRCs and substrate rocks sampled from the seafloor in the northwestern Pacific, and the relation between the mechanical properties was investigated. Abrasivity of the samples was examined using the CERCHAR abrasivity index (CAI) which is commonly used for rocks. Friability of the samples was investigated from the particle size distribution of debris after crushed. In addition, the relation between cutting resistance and cutting depth, and the applicability of a cutting theory were examined through linear cutting tests of the CRCs.