Soil hematites

This occurs in well-drained temperate soils, and is the reverse of Equation (8). In very well-drained warm soils, hematite can form. For... 

J. (1977) A1 substitution and differential disorder in soil hematites. Clays Clay Min. 25 373-374... 

As a first example, a sample from the topsoil and one from an underlying loess of a paleosol-loess sequence at Xiadong in the Jixian loess section (China), will be considered. The RT Mossbauer spectra of soil and loess show a series of central doublets, which can be attributed to illite. Apart from these doublets the sextet typical for soil hematite is observed (Fig. 3.45). 

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. 

Mbssbauer spectra measured by the Opportunity rover at the Meridiani Planum landing site (see Fig. 8.35) revealed four mineralogical components in Meridiani Planum at Eagle crater jarosite- and hematite-rich outcrop (see Eig. 8.34), hematite-rich soil, olivine-bearing basaltic soil, and a variety of rock fragments such as... 

Hematite in the soil is concentrated in spherules and their fragments, which are abundant on nearly all soil surfaces. Several trenches excavated using the rover wheels showed that the subsurface is dominated by basaltic sand, with a much lower abundance of spherules than at the surface. Olivine-bearing basaltic soil is present throughout the region. At several locations along the rover s traverse, sulfate-rich bedrock outcrops are covered by no more than a meter or so of soil. 

Fig. 8.36 Leyt Spectrum of the soil close to the crater rim where Opportunity entered and exited the crater. The basaltic soil is unusually high in hematite (but no indication of significant contribution Irom hematitic spherules). Middle rover tracks. Right 750 m diameter (. 75 m deep) eroded impact crater Victoria Crater, formed in sulfate-rich sedimentary rocks. Image acquired by the Mars Reconnaissance Orbiter High-Resolution Science Experiment camera (Hirise). The red line is the drive path of Opportunity exploring the crater. (Courtesy NASA, JPL, ASU, Cornell University)...

Fig. 8.37 Left, spectrum of an accumulation of hematite rich spherules (Blueberries) on top of basaltic soil (Sol 223-228 of the mission 1 Sol = 1 Martian day). The spectrum is dominated by the hematite signal. Estimations based on area ratios (bluebeiries/soil) and APXS data indicate that the blueberries as composed mainly of hematite. Right MI picture (3x3 cm ) of hematitic spherules (blueberries) on basaltic soil at Meridiani Planum...

The soil close to the crater rim, at the exit point of Opportunity, shows a high hematite content in the Mossbauer spectrum (Fig. 8.36), which can be attributed to the presence of hematite spherules blueberries). 

Pigna M, Colombo C, Violante A (2003) Competitive sorption of arsenate and phosphate on synthetic hematites (in Italian). Proceedings XXI Congress of Societa Italiana Chimica Agraria SICA (Ancona), pp 70-76 Quirk JP (1955) Significance of surface area calculated from water vapour sorption isotherms by use of the B. E. T. equation. Soil Sci 80 423-430 Rancourt DG, Fortin D, Pichler T, Lamarche G (2001) Mineralogical characterization of a natural As-rich hydrous ferric oxide coprecipitate formed by mining hydrothermal fluids and seawater. Am Mineral 86 834-851 Raven K, Jain A, Loeppert, RH (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environ Sci Technol 32 344-349... 

Morris, R.V. et al. 2006b. Mossbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars Opportunity s journey across sulfate-rich outcrop, basaltic sand and dust, and hematite lag deposits. Journal of Geophysical Research, 111, E12S15, doi 10.1029/2006JE002791. 

Iron and its various oxidation state species are common components of the environment. In addition to the oxides FeO and Fe203, it is found in minerals such as hematite, goethite, and ferrihydrite, and in a number of hydroxy and oxy compounds. Because of its common occurrence in the environment in general, and in soil in particular, the total iron content of soil is usually not a useful piece of information. 

Isomorphous substitution of iron oxides is important for several reasons. In the electronics industry, trace amounts (dopants) of elements such as Nb and Ge are incorporated in hematite to improve its semiconductor properties. Dopants are also added to assist the reduction of iron ores. In nature, iron oxides can act as sinks for potentially toxic M", M and M heavy metals. Investigation of the phenomenon of isomorphous substitution has also helped to establish a better understanding of the geochemical and environmental pathways followed by Al and various trace elements. Empirical relationships (e. g. Fe and V) are often found between the Fe oxide content of a weathered soil profile and the levels of various trace elements. Such relationships may indicate similarities in the geochemical behaviour of the elements and, particularly for Al/Fe, reflect the environment in which the oxides have formed (see chap. 16). 

Iron oxides are responsible for the vivid colours of many rocks and sods. The typical yello v-red to purple red colours of the so-called red beds are due to hematite (Torrent Schwertmann, 1987). The strong influence of hematite on soil colour is referred to in various languages and appears in the terms red earths, terra rossa and krasno-zems (see Chap. 15 16). 

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