Iron hydroxides silicate minerals

Aluminium oxide is the oldest ceramic material used in medicine. Bauxite and corundum are the main natural sources of aluminium oxide. Bauxite is a mixture of diaspore, gibbsite, iron hydroxides, clay minerals and quartz. It is formed by the tropical weathering of silicate rocks during which quartz and the elements sodium, calcium, magnesium and potassium are largely washed away. This is the reason why the remaining material becomes richer in the resistant elements titanium, iron and aluminium. The latter is extracted from this mixture in the form of aluminium hydroxide. In its turn this compound is converted into aluminium oxide by heating the mixture to 1200-1300 °C, this is called calcination. The hydroxide is thus made anhydrous. 

Recent sediments of water basins. In recent basins iron sediments consist mainly of the iron hydroxides Fe(OH)3 or Fe203-nH20, but in very rare cases silicates and carbonates of Fe ", pyrite, and hydrotroilite enter into the composition of the sediment all together they constitute reactive (mobile) iron, which actively takes part in the diagenetic processes. A mixture of clastic minerals, which decompose negligibly and take practically no part in the processes of diagenesis, constitute another group. 

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. 

Loughnan (1969) discussed the solubility in relation to pH of some of the common products of chemical weathering of silicate minerals, In general, the hydroxides of Na, K and Ca are soluble at all pH s, and Mg(OH)2 is soluble at pH < 10. Aluminium oxide is soluble at pH s < 4 and >10, whereeis SiOj is slightly soluble at pH < 9 and increasingly soluble at higher pH values. Titanium hydroxide is soluble at pH < 5, but TiOz is soluble only at pH < 2. The hydroxide of trivalent iron is soluble only below pH 2.5, but Fe(OH)2 is soluble below about pH 8.5. 

In nature, manganese can exist as common minerals, snch as carbonates, oxides, hydroxides, silicates, and to a limited extent as snlfides (Table 10.1). Next to iron, manganese is the most abundant element in the earth s crnst and is fonnd in variable concentrations in living organisms, water. 

Ueshima and Tazaki [104] describe mineral formation in the acidic polysaccharides associated with microbial cell surfaces. They find that polysaccharides, associated with extracellular polymeric substances (EPS), direct the preferential formation of nontronite, a sodium-iron (111) phyllosi-licate in simulation studies. It is suggested that the chain structure of the polysaccharides affect layer silicate orientation. They observed only Si-bearing amorphous iron hydroxides forming outside of the EPS. 

Associated minerals These mineral constituents do not impart plasticity to clay. Examples include silicate minerals like micas, quartz, feldspars etc. iron oxides and hydroxides like magnetite, hematite, maghemite, goethite, lepidoerocite etc. and aluminium oxides and hydroxides like corundum, gibbside, boehmite, diaspore etc. 

In Chap. 3, Robert E. Vandenberghe describes the applications of Mossbauer spectroscopy in earth science. With iron as the fourth most abundant element in the earth crust, Fe Mossbauer spectroscopy has become a suitable additional technique for the characterization of all kind of soil materials and minerals. In this chapter a review of the most important soil materials and minerals is presented. It starts with a description of the Mossbauer spectroscopic features of the iron oxides and hydroxides, which are essentially present in soils and sediments. Further, the Mossbauer spectra from sulhdes and carbonates are briefly considered. Finally, the Mossbauer features of the typical and most common silicate minerals are represented. Because the spectral analysis is not always a straightforward procedure, some typical examples are given showing the power of Mossbauer spectroscopy in the characterization of minerals. 

---