Parent rocks

There are no unequivocal weathering reactions for the siUcate minerals. Depending on the nature of parent rocks and hydrauhc regimes, various secondary minerals like gibbsite, kaolinite, smectites, and iUites are formed as reaction products. Some important dissolution processes of siUcates are given, for example, by the following reactions (19). 

Bentonite is a rock rich in montmorillonite that has usually resulted from the alteration of volcanic dust (ash) of the intermediate (latitic) siliceous types. In general, reUcts of partially unaltered feldspar, quartz, or volcanic glass shards offer evidence of the parent rock. Most adsorbent clays, bleaching clays, and many clay catalysts are smectites, although some are palygorskite [1337-76 ]. 

Ur gestein, n. primitive rock parent rock (Ceram.) native kaolinic rock (from which kaolin is obtained by elutriation). -gewicht, n. standard weight, -granit, m. primitive granite, -heber, m. author inventor orig inator, founder. 

The condition of any soil represents a stage in the changing process of soil evolution. Soils develop, mature and change with the passage of time. Whereas the time required for a true soil to develop from the parent rock of the earth may be thousands of years, rapid changes can result in a few years when soils are cultivated, irrigated, or otherwise subjected to man s manipulation. The type of soil that develops from the parent material will depend upon the various physical, chemical and biological factors of the environment. 

Soil reaction (pH) The relationship between the environment and development of acid or alkaline conditions in soil has been discussed with respect to formation of soils from the parent rock materials. Soil acidity comes in part by the formation of carbonic acid from carbon dioxide of biological origin and water. Other acidic development may come from acid residues of weathering, shifts in mineral types, loss of alkaline or basic earth elements by leaching, formation of organic or inorganic acids by microbial activity, plant root secretions, and man-made pollution of the soil, especially by industrial wastes. 

C weathered parent rock R solid parent rock... 

The deduced average input 5 Ca is not identical to the average volcanic rock value, but is lower by about 0.4%o (Fig. 14). This suggests that weathering may discriminate between calcium isotopes. This inference is supported by the few available data on soils (Fig. 14 Skulan and DePaolo 1999 Skulan 1999 Schmitt et al. 2003b). The soils generally have 5 Ca values that are higher than the volcanic rocks. Data to allow direct comparison of soils to parent rock material are not available. 

The amount and type of chemical weathering that occms depends on the chemical composition of the parent rock and the weathering solution, as well as the environmental conditions. As shown in Figure 14.2, the high-temperatme igneous silicates are more prone to chemical weathering than the low-temperature minerals. Climate is important because it determines temperature and water availability, both of which control... 

Iodine is essential in the mammalian diet to produce the thyroid hormone thyroxine deficiency in humans causes goitre. Collectively, deficiencies of iodine, iron, zinc and vitamin A in humans are thought to be at least as widespread and debilitating as calorie deficiencies (Welch and Graham, 1999). The main source of iodine in soils is oceanic salts rather than parent rock, and so deficiency is most widespread in areas remote from the sea (Fuge, 1996). In principle deficiency is easily corrected with dairy supplements. However in practice this is not always feasible. Addition of iodate to irrigation water has successfully corrected widespread iodine deficiency in parts of China where the usual methods of supplementation had failed (Cao et al., 1994 Jiang et al 1997). However there is not much information on the behaviour of iodine in soil and water systems. 

The degree of transformation can be quantified analytically through the ratio of Fe in the oxides (commonly extracted with the strong reductant sodium dithionite (Fed)) to the total amount of Fe (Fct) and also by the ratio of Fe"/Fe ", because the iron located in the primary minerals of the parent rock is predominantly Fe". With age, the ratio Fea/Fct gradually approaches unity and the total Fe"/Fe " approaches zero (Leigh, 1996). Therefore, both can serve as an indicator of the maturity of a soil as a function of time (chronosequence) (e.g. Italy Arduino et al., 1984 Nepal Baum-ler et al. 1991, Spain Simon et al. 2000 USA Barret, 2001 Egli et al. 2001). The... 

Tab. 16.5 Correlation coefficients with Fe and accumulation factors with respect to parent rock...

Element Laterites from basalts (n SI) Accumulation factor with respect to parent rock Lateritic soils of Western Australia (n 39) Range of Correlation coefficient with concentration Fe concentration ... 

Most obsidian sources are chemically homogeneous, with variations in composition on the order of a few percent or less. However, the individual sources have different trace-element compositions as a reflection of the compositions of parent rocks and changes taking place in the magma chamber prior to eruption. The major elements are restricted to a relatively narrow range of composition but the abundances of trace elements can differ by orders of magnitude between obsidian sources. If the variations within sources are smaller than the differences between sources, then the provenance of obsidian artifacts can be successfully established. 

Primary clay, known as kaolin, is found in the same place as the parent rock. Kaolin is formed by the weathering of feldspar. Kaolins are coarse in particle size and therefore nonplastic compared to most sedimentary clays. Kaolins are relatively free of mineral impurities such as iron. 

Secondary clay has been transported from the site of the parent rock. As a result, this clay has fine particles and might be contaminated with iron, quartz, mica, and carbonate compounds. Ball clays are secondary clays. They are higher in iron content, more fusible, finer in particle size, and more plastic than kaolin clays. 

If sediment was collected from a particular waterway, the distribution of the element of interest between different components of the sediment was found to vary with the degree of exposure to air and the temperature of any drying stages (Rapin et al., 1986 Kersten and Foerstner, 1986). The minor elements present in sediments (and soils) are not uniformly distributed. Part can be present as mineral fragments derived from the original parent rock, while other parts can be associated with distinct component phases such as carbonate compounds, hydrous oxides of Fe, Al, Mn and organic matter. Some fractions are loosely sorbed on particle surfaces or are held on ion exchange sites. 

Soils exhibit a large variety of characteristics that are used to classify them for various purposes, including crop production, road construction, and waste disposal. The parent rocks from which soils are formed obviously play a strong role in determining the composition of soils. Other soil characteristics include strength, workability, soil particle size, permeability, and degree of maturity. 

Subdivision (dispersion) is also common in geologic systems. In this case dispersion involves weathering of parent rock (physical disintegration and/or chemical dissolution with re-precipitation) followed by suspension of the particles in liquid. The physical disintegration can be caused by a range of events [49] ... 

Soils may be developed on either residual or transported material, but irrespective of which is involved, a soil may always be traced back to the parent rocks from which it has formed. This means that all of the minerals that occur in rocks may also occur in soils, in addition to those formed by pedogenic (i.e., soil-forming) processes. There are thousands of different minerals, but the main minerals and groups of minerals encountered in most soils form a much smaller set, numbering around twenty or thirty common types (Table 11.1). 

---