Manganese Nodules and Other

Iron-Manganese Nodules and Other Hydrogenous Minerals 

All figures are available on the companion website in color (if applicable). 

Two types of metal-rich hydrogenous deposits are formed on the seafloor iron-manganese oxides and polymetallic sulfides. The iron-manganese oxides have been deposited as nodules, sediments, and crusts. They are enriched in various trace elements, such as manganese, iron, copper, cobalt, nickel, and zinc, making them a significant repository for some of these metals. Most of the metals in the polymetallic sulfides are of hydrothermal origin. These sulfides have been deposited as metalliferous sediments aroimd hydrothermal systems and as rocks that infill cracks within former 

In this chapter, we consider the mineral composition of the hydrogenous minerals and how they fitrm. The evaporite minerals have already been covered in Chapter 17. The hydrothermal minerals (polymetallic sulfides) are discussed further in Chapter 19. 

---

CHAPTER 18 Iron-Manganese Nodules and Other Hydrogenous Minerals... 

Cronan D. S. (1976b) Manganese nodules and other ferromanganese oxide deposits, chap. 28. In Chemical Oceanography, 2nd edn. (eds. J. P. Riley and R. Chester). Academic Press, London, vol. 5, pp. 217-263. 

Lanthanide content [in ppm(wt)] of selected manganese nodules and other marine phases. 

Manganese ores are mined. On the ocean floor in the proximity of volcanoes there are so-called manganese nodules, which consist of oxides of manganese, iron and other heavy metals. They arise from deposition by microorganisms around a solid core (piece of mussel, shark tooth, etc.). 

An investigation of the origin and distribution of manganese nodules and the processes by which they selectively concentrate copper, nickel, and other metals was one of llie first major projects under the sea beds Assessment project of the IDOE program. At a workshop attended by over a hundred scientists from various countries, the most likely locations... 

Similar reaction steps can also be presented for other oxides. Obviously the above oxidation reactions involve a corresponding reduction of the metal oxides. Interestingly our results did not agree fully with the above reaction scheme (eq. 1 to 3) as the only mechanism operating in the oxidation of thiols. Because in each case the amount (mole) of thiol oxidised is not stoichiometric with respect to manganese nodule and it is 1.1 to 2.2 times greater than the mole of MnOj and FeOOH present in the manganese nodules. 

The economic potential of marine manganese deposits and the potential of these deposits as paleoenvironmental indicators are directly related to the chemistry of manganese, its associated transition metals and other key elements adsorbed on to the surface of these deposits. It is this complex interplay of these factors which makes the study of marine manganese deposits so intriguing. Unfortunately, research on deep-sea manganese nodules and crasts has been held hostage to the economic potential of these deposits. Time will tell if these deposits are to become a major source of metals for the world as was once thought. 

Under oxidising conditions, notably associated with deposition of iron hydroxides in the oceans, cerium is oxidised to Ce, with a consequent reduction in radius of 15% (Shannon 1976). This results in the precipitation of cerium hydroxides and phosphates in manganese nodules and a consequent dramatic depletion of Ce, compared to the other lanthanides, in seawater. This property has been used, for example, to estimate paleoredox conditions in ancient oceans. 

Manganese minerals are widely distributed oxides, silicates, and carbonates are the most common. The discovery of large quantities of manganese nodules on the floor of the oceans may become a source of manganese. These nodules contain about 24% manganese together with many other elements in lesser abundance. 

A rich supply of manganese lies in nodules of ore that litter the ocean floors (Fig. 16.9). These nodules range in diameter from millimeters to meters and are lumps of the oxides of iron, manganese, and other elements. However, because this source is technically difficult to exploit, manganese is currently obtained by the thermite process from pyrolusite, a mineral form of manganese dioxide ... 

Another important consideration pertains to the metal content of the deposit. A deposit with a content of iron of about 20% can have little value as an iron ore since there are several deposits with 30-50% iron. Earlier, a copper ore with a minimum of 5% copper was regarded or accepted as a copper ore. However, today, thanks to advancements in technology, rocks with as little as 0.5% copper are mined and processed economically despite the fact that the price of copper, in comparison with those of some other metals, might be showing a downward trend. It is possible that in the future, other resources, which are not considered to be worth exploiting today (such as the manganese nodules or the clays), would become acceptable ores for manganese, copper, nickel, cobalt, and aluminum. 

Finally, it may be added that it is the terrestrial resources which have served and will continue to serve as practically the sole suppliers of metals and materials. Among the other resources only the manganese nodules extracted from the ocean bed have drawn serious attention the world over for exploring economic methods of processing. 

MHO [Metallurgie Hoboken-Overpelt] A process for extracting manganese and other metals from nodules from the sea bed by extraction with hydrochloric acid ... 

Manganese is an important element in the aquatic environment. It is an essential micronutrient U 2) and is the subject of much interest because its oxides scavenge other heavy metals (3). Of particular interest are ferromanganese nodules, which are abundant in the aquatic environment. These nodules contain high concentrations of cobalt, nickel, copper and other heavy metals (4). 

The nodules range in diameter from 20 pm to 15 cm, with most between 1 and 10 cm. As shown in Table 18.4, they are composed predominantly of manganese and iron oxides (about 20% each). They are generally dark brown and have the appearance of dirty, lumpy potatoes or hamburgers, with some being spherical and others more discoidal. 

---