Weathering soil sequence

In terms of soil development and the development of soil horizons, the smectites and fine-grained micas are found in younger, less weathered soils. Kaolinite and amorphous clays are found in highly weathered soils. Considering a time sequence, at the beginning of formation, soil will contain more complex clays that weather to simpler forms over time. However, it is convenient to start with a description of the simpler layer silicate clays and then describe the more complex clays. 

A weathering sequence for phyllosilicates has been established which indicates that their relative stability in soils is as follows mica,... 

Thus the "stability sequence" is equivalent to increasing the intensity of weathering or to completing the process of equilibration under conditions of high water content, low concentration of alkali and silica ions and oxidation of iron in a soil profile. In fact the weathering process in a soil horizon sequence is much the same for pelitic rocks in all environments the dominant sequence is repeated with minor variations due to local conditions of pH or efficiency of the oxidation process. 

JACKSON (M.L.), TYLER (S.A.) and WILLIS (A.L.), BOURBEAU (G.A.), PENNINGTON (R.P.), 1948. Weathering sequences of clay size minerals in soils and sediments. Joum. Phys. Colloid Chem. J 2, 1237-60. 

JACKSON (M.L.), HZEUNG (Y.), COREY (R.B.), EVANS (E.J.) and VANDEN HEUVEL (R.C.), 1952. Weathering sequence of clay-size minerals in soils and sediments II. chemical weathering of layer silicates. Proc. Soil. 

As an example of these ideas, an activity-ratio diagram for control of Al(III) solubility by secondary minerals in an acidic soil solution will be constructed. The Jackson-Sherman weathering scenario14 indicates that when soil profiles are leached free of silica with fresh water, 2 1 layer-type clay minerals are replaced by 1 1 layer-type clay minerals, and ultimately these are replaced by metal oxyhydroxides. This sequence of clay mineral transformations can be represented by the successive dissolution reactions of smectite, kaolinite, and gibbsite ... 

Arsenic in soilds has been fractionated by Jackson s T28) procedure for soil phosphorus (15. 27). In this laboratory, a modification of Jackson s procedure is being used on sediment solids. A series of 1 molar solutions of NH4CI, NH4OH, acid ammonium oxalate (29) and HCl are used in sequence. The chloride fraction, or exchangeable fraction, contains weakly adsorbed, coulombically bound arsenic. The hydroxide fraction, contains iron and aluminum arsenate precipitates and surface precipitates (i.e. adsorbed arsenic species having both chemical and coulombic bonding to oxide surfaces). The oxalate, or reductant soluble fraction, contains arsenic occluded in amorphous weathering products. The acid, or calcium, fraction contains arseno-apatites. 

Maes, E., Vielvoye, L., Stone, W., and Delvaux, B. (1999). Fixation of radiocaesium traces in a weathered sequence mica -> vermiculite hydroxy interlayered vermiculite. Eur. J. Soil Sci. 50, 107-115. 

The minerals in the unweathered rock were identified with a petrographic microscope, and their weathering products by x-ray diffraction. Material to establish these sequences was obtained from exposures of bedrock, from pits dug on the side slope in the gorge area, and from hand auger holes along the ridge and in the swale. Pseudomorphs after oligoclase and biotite were picked from the soil samples. 

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