Iron minerals, weathering

Osmond JK, Ivanovich M (1992) Uraninm series mobilization and surface hydrology. In Uranium-series disequihbrium. Ivanovich M, Harmon RS (eds) Clarendon Press, Oxford, p 259-289 Payne XE, Davis JA, Waite XD (1994) Uranium retention by weathered schists—the role of iron minerals. Radiochim Acta 66/67 297-303... 

Nordstrom, D. K., 1982, Aqueous pyritc oxidation and the consequent formation of secondary iron minerals. In Acid Sulfate Weathering. Soil Science Society of America Special Publication 10, 37-56. 

Almost all rocks contain at least some iron. The more important minerals, in which Fe is a major constituent, are given with their Fe contents in Table 16.1. In all of these minerals except magnetite, iron is exclusively or predominantly in the bivalent state. During weathering, the iron is released from these minerals and secondary , pedogenic iron minerals are formed. The most important ones are Fe-contain-ing clay silicates and Fe oxides but under reducing conditions, Fe carbonates, sulphides and phosphates may also be formed. 

Galvez, N. Barr6n,V. Torrent,. J. (1999 a) Effect of phosphate on the crystallization of hematite, goethite, and lepidocrodte from ferrihydrite. Clays Clay Miner. 47 304-311 Gancedo, J.R. Grada, M. Marco, J.F. Pala-dos, J. (1988) Comparative Mossbauer and SEM study of the corrosion reaction of iron and weathering steel in SO2 polluted atmospheres. Hyperfine Interactions 41 637-640... 

Payne, T. E., Davis, J. A. Waite, T. D. 1994. Uranium retention by weathered schists. The role of iron minerals. Radiochimica Acta, 66/67, 297-303. 

Iron occurs in two oxidation states, the divalent (Fe ) ion or trivalent (Fe ) ion, and sedimentary rocks contain iron in these various forms with ferric oxides being the most common. When iron is weathered out of the rocks, it is not retained in solution but, depending upon conditions, it is redeposited as oxides or hydroxides. In addition, Fe can replace aluminium in some silicate minerals. An important chemical feature of iron (in solution) is its tendency to form complexes with organic materials. Such complexes are considerably more stable and consequently survive in solution or in the soil for longer periods of time. Specific examples of Fe-organic complexes will be discussed in later sections. 

Silica, alumina, iron, and the various base cations that are dissolved by primary mineral weathering can precipitate as new low-temperature minerals—a process known as neoformation. This should be distinguished from weathering processes, termed alteration, in which part of the parent mineral structure is inherited by the weathering product. Examples of alteration are given in the previous section. 

In this chapter, the aim is to identify and quantify the iron mineral phases present in South African coal fractions by the use of Mossbauer spectroscopy, in conjunction with various other analytical techniques. Because the atomic weight of the carbon content in coal is low, Mossbauer spectroscopy is a convenient, and to a certain degree unique, analytical tool in the identification of iron-bearing minerals in coal with iron contents as low as 1%. With an understanding of the iron mineral phases present in the as-mined coal, the fete of these minerals during transportation, weathering, oxidation, and combustion or gasification can be better understood. 

F.B. Waanders, E. Vinken, A. Mans, A.M. Mulaba-Bafubiandi, Iron minerals in coal, weathered coal and coal ash—SEM and Mossbauer results, Hyperfme Interact. 2003, 148-149, 21-29. 

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