Co-Rich Mn Crusts

Co-rich Mn crasts may be defined as hydrogenous manganese crasts having Co contents 1% (Manheim 1986 Mangini et al. 1987 Hein et al. 

Mn/Fe 2.0. 5Mn02 is the principal manganese mineral present and they have a growth rate of 1-2 mm Ma. The crasts have attracted economic interest as a potential source of Co and, to a lesser extent, Pt. The areas in which these crasts generally form lie well above the CCD. Crasts therefore tend to form in regions of strong bottom current activity which can prevent the deposition of calcareous ooze by erosiom 

Halbach and Puteanus (1984) showed that the dissolution of calcareous tests in the water column plays a key role in the incorporation of Fe into these crasts. The calcareous tests contain about 500 ppm Fe. The flux of Fe to the surface of the crasts derived from the release of colloidal Fe oxyhydroxide particles on dissolution of the calcareous tests was estimated to be about 15 ag cm a which is almost equivalent to the flux of Fe in the concretions of 22.4 - 44.8 ag cm a. The rate of incorporation of Fe into the crasts is therefore related to the position of the ly socline. Based on such considerations, these authors concluded that the metal supply from the water column to the crasts 

Pt is also significantly enriched in Co-rich cmsts (Heinetal. 1988,1997 Halbach etal. 1989). However, the mechanism of enrichment in the cmsts is not well understood. Hodge et al. (1985) argued that Pt is oxidized from the PtCb state in seawater to the tetravalent state in manganese nodules. This process was thought to be responsible for the anomalously high Pt/Pd ratios in nodules (50-1,000) compared to seawater (4.5). However, Halbach et al. (1989) considered that this process would not be possible 

However, Stiibenetal. (1999) subsequently plotted stability field diagrams for Pt and Pd and showed that Pt is present in seawater dominantly as Pt(OH)2° and is close to saturation under seawater conditions. From this, it was argued that Pt is enriched in Co-rich cmsts mainly as a result of the preferential adsorption of Pt(OH)2° onto the surface of Mn and Fe oxy hydroxide minerals. Pd, on the other hand, lies at the boundary of the stability fields of PdCb and Pd(OH)2° and is probably undersaturated in seawater. Since PdCb can not be adsorbed on Mn oxy hydroxides which have a negative surface charge, this would explain the so-called negative Pd anomaly in marine manganese deposits. 

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Fig. 11.15 Vertical section of a Co-rich Mn crust from the flanks of a guyot on the Ogasawara Plateau N.W. Pacific (25°18.9 N, 143°54.8 E 1515 m) collected by dredge during a cruise of the GSJ with R.V. Hakurei-maru in 1986. The substrate (not clearly seen) is phosphatized limestone. The crust is more than 20 cm across, about 10 cm thick and has a knobby surface texture. Element contents are Mn 21.7%, Fe 18.9%, Co 0.81%, Ni 0.31%, Cu 0.04%, Pb 0.26% and Pt 0.29 ppm. The upper layer of the crust displays the highest Co content and the bottom layer the highest Pt (0.78 ppm). 6Mn02 is the principal mineral present with minor quartz and plagioclase. Photograph courtesy of A. Usui, GSJ.

Fig. 11.16 Eh, pH diagram for Cu calculated for the chemical conditions prevailing in the deep sea (after Glasby and Schulz 1999). Note that, at pH 8, the metastable form of Cu is Cu(OH)2 at an Eg, > +0.48 V. However, if the Eh, drops to +0.4 V corresponding to the Eh of seawater, the anionic species, CuC, becomes the more stable species. It is believed that the dominance of CuCl, in seawater in the oxygen minimum zone accounts for the low Cu contents in Co-rich Mn crusts.

Application of Isotopic Studies of Co-rich Mn Crusts to the Study of Present-day Deep-ocean Circulation... 

The types of mining systems likely to be used for the recovery of deep-sea nodules and Co-rich Mn crusts are not well known. For nodules, Lenoble (2004) has described a system to mine the French area. This... 

Fig. 11.27 Sketch of a proposed vehicle for mining Co-rich Mn crusts (after Hein et al. 2004, Fig. 9).

Natural iron oxides are commonly formed by weathering of iron-rich primary minerals (silicates, spinels) in an environment where other metal cations are also present in these processes. For this reason, natural iron oxides may contain different amounts of various cations incorporated into their structure. Due to their abundance in the Earth s crust as well as chemical and physical properties similar to those of iron, the most common metal found in natural iron oxides is Al [181-192], but the presence of many others, such as Mg [193,194], Si [195-197], Ti [194,198], V [199], Cr [ 186,192,200], Mn [ 192,194,200,201 ], Co [192,200], Ni [186,192,200,202,203], Cu [200], Zn [192,200], and Ge [204] has been observed. 

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