**1. Introduction**

The genesis, uses and environmental implications of iron oxides and ores remain a topic of interest to academics, industrial players and environmentalist. Oxides of iron are chemical compounds widely spread naturally, comprising iron and oxygen. Here, two components of oxides are widely spread naturally. They are relevant to humans and useful in most geological

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and biological activities [1]. In addition, they are useful as pigments and catalyst in industries and hemoglobin in blood circulation. Iron oxides that are economically viable in natural and beneficiated forms are considered as second to oil and gas in relation to demand and utility in the global market. They occur as divalent compound, trivalent compound and a combination of both. The interplay and conversion of these components from one form to another are essentially controlled by bacterial species. These bacteria use iron and reduce trivalent iron oxides to divalent form during metabolism.

beneficiation. The leached out iron could be carried downwards to be precipitated as bodies of ochre and iron oxide coatings within the existing sedimentary rocks. This product is a soft earth mixture of hematite, limonite and goethite from which various red and yellow ochre

Genesis, Uses and Environment Implications of Iron Oxides and Ores

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The main product is a dark red hematite variety. Some ore deposits consist of magnetite with occasional hematite; varying amounts of sulphide occur mostly as pyrite and pyrrhotite with minor amount of chalcopyrite. The ore is composed of ferriferous oolites and grains of quartz bounded by a cement of clay minerals or of chlorites and carbonates. The oolites consist of iron hydroxides or limonite, with variable proportions of silica, alumina and phosphorus. The strikes of the orebodies are sometimes in accordance with the iron formation and sometimes discordant. The high-grade ore could be magnetite that is partially oxidized at the outcrops to

Deposits of iron may be categorized under the following: magmatic, sedimentary and metamorphic. In some regions of the world, the major sources of high-grade iron ore are derived from magmatic iron and metasomatic hydrothermal iron deposits. These especially the skarn-type iron deposits are mainly associated with intermediate-felsic igneous rocks, with only a minor proportion related to mafic intrusions. The iron redox cycle is a substantial process that exists in most terrestrial environments, which can be conducted by both abiotic and microbial processes. In anoxic, pH-neutral environments, microbial Fe(II) oxidation is driven by either nitrate-reducing bacteria, photoferrotrophic bacteria or neutrophilic microaerophilic bacteria [11, 12].

Microbial Fe(III) reduction forms the other component of the Fe cycle, generated by intracellularly by magnetotactic bacteria [12] or outside of the cell wall by dissimilatory iron-reducing bacteria. These combines with the oxidation of organic substrate or hydrogen with the reduction of poorly crystalline, short-range ordered Fe(III) minerals (e.g. ferrihydrite) [13]. This can lead to the development of many different iron mineral phases and compounds including

CO3

products of reduction depends on geochemical parameters, inclusive Fe(III) reduction rate,

Based on redox condition, magnetite can donate or accept electron in different metabolic processes involving Fe. In this respect, knowledge of the mineralogical outcomes of biomineralization characteristics can help provide signatures of microbial reactions with fluid, rocks, mineral deposits and subsequent diagenesis. Banded iron formation (BIF) is a chemically produced rock of sedimentary origin. This was precipitated in Precambrian time, comprising intercalated microcrystalline quart, iron oxides and silicates rich in iron. In line with deposi-

The constituents are discontinuous silica- and iron-rich bands with similar mineralogical properties of Fe, chert and carbonate minerals. Arc/back-arc basins or intra-cratonic rift zones host the Algoma type, and the latter is hosted by clastic carbonate rocks linked with shallow marine environments. As the main mineral constituents of BIFs, magnetite has been broadly reported for BIFs' depositional systems, mineralization characteristics and possible microbial involvements. Chemical and sedimentary methods in normal seawater could produce marine

). The mineralogical composition of these

hematite, with minor quantities of silicate minerals, anthophyllite and chlorite [11].

and iron oxide coatings can be extracted [10].

goethite, magnetite, green rust and siderite (Fe2

pH, temperature and the availability of electron shuttle [14, 15].

tional settings, the BIFs fall into Algoma type and superior type [16–18].

Different geological conditions control the spread of iron ore deposits worldwide. They occur in basins of sedimentation, with eroded, deep-seated intrusive and where deep tropical weathering conditions prevail. Magnetite deposits occur in the deeply dissected regions of plutonic intrusions in North America [2, 3].

Several authors have provided information in the environmental outcome of exploration and exploitation of iron ore. These include pollution of the atmosphere and ecosystem courses of water. Pollution of the atmosphere involves release of poisonous gases such as nitrous oxide, carbon dioxide, carbon monoxide and sulfur dioxide. Pollution of the ecosystem involves release of metal load into courses of water. While there is limited remediation programme on the former, the latter has gained considerable attention, and elaborated ochre is a product of the aqueous oxidation of iron and oxides of iron [4–7].
