Soil iron-bearing minerals

Minerals. Iron-bearing minerals are numerous and are present in most soils and rocks. However only a few minerals are important sources of iron and thus called ores. Table 2 shows the principle iron-bearing minerals. Hematite is the most plentiful iron mineral mined, followed by magnetite, goethite, siderite, ilmenite, and pyrite. Siderite is unimportant in the United States, but is an important source of iron in Europe. Tlmenite is normally mined for titania with iron as a by-product. Pyrite is roasted to recover sulfur in the form of sulfur dioxide, leaving iron oxide as a by-product. 

In general, minerals in sedimentary and meta-morphic rocks contain ferrous iron (Velde, 1985) which is destined to become iron oxide under conditions of weathering. Oxidation under surface conditions has a tendency to produce iron in the ferric state. Most often the process takes iron out of the silicates and puts it into an oxide phase. In the uppermost layers of mature soils, iron oxide and various silicates, usually non-iron-bearing, are produced. In silicates containing iron, the majority is in the ferric state. The extent of the transformation of iron oxidation state is a rough measure of the maturity of the soil. In the extremely weathered soils one finds only ferric iron and aluminum oxides and hydroxides. These soils are typically red. 

Iron oxides are the most abundant metallic oxides in soils (Schwertmann and Taylor, 1989 Cornell and Schwertmann, 1996 Bigham etal., 2002). Iron oxides usually form via solution from Fe + ions released from Fe(II)-bearing silicates and sulfide minerals on weathering (Oades, 1963 Schwertmann and Taylor, 1989 Cornell and Schwertmann, 1996). Once formed in soil and other natural environments, the mineral phase, composition, and distribution of iron oxides can be continually modified by the alteration of their environments (Schwertmann and Taylor, 1989). Therefore, the formation and transformation of pedogenic iron oxide mineral phases depend on the pedo-environmental conditions under which they have formed. 

Iron oxides and hydroxides are the most important iron-bearing constituents of soils, sediments and clays. To characterize the samples, i.e. the identification of the different minerals present and the determination of their morphology and chemical composition, a variety of standard techniques are commonly used such as X-ray and electron diffraction, chemical analyses, optical and electron microscopy, infrared spectroscopy and thermal analysis (DTA, DTC,...). Most of these techniques are further applied in conjunction with selective dissolution or other separation methods in order to obtain more specific information about particular components in the complex soil system. In addition to all those characterization methods, MS has proven to be a valuable complementary technique for the study of these kinds of materials and in particular for the characterization of iron oxides and hydroxides which are usually poorly crystallized. 

---