Chlorite in soils

Chlorites in soil occur as primary minerals derived from mafic rocks and as secondary minerals from the weathering of biotite, hornblende, and other amphiboles and minerals (Bamhisel, 1977). Chlorites are 2 1 1 minerals consisting of 2 1 mica structure in addition to an interlayer hydroxide sheet. Chlorites have low CEC and surface areas. 

The 2 1 layer type with hydroxide interlayer is represented by dioc-tahedral chlorite in soil clays (Figs. 1,3). The unit cell chemical formula of this mineral can be expressed ... 

Bailey, S. W. 1975. Chlorites. In Soil Components. Vol. 2 Inorganic Components, ed. J. E. Gieseking, 191-263. New York Springer-Verlag. 

Kohut, C. K., and C. J. Warren. 2002. Chlorites. In Soil Mineralogy with Environmental Applications, ed. J. B. Dixon and D. G. Schulze, 531-553. Soil Science Society of America Book Series 7. Madison, WI Soil Science Society of America. 

Chlorites in soils and sediments may have incomplete brucitic or gibbsitic interlayer sheets... 

Differential thermal curves for some common chlorites are reproduced in Figure 28, from which it will be observed that particle size can markedly affect the appearance of the curve. The variability among different species is also noteworthy, and it is impossible to define a mean curve for chlorite. Curves for some pseudochlorites (Figure 29), the synthetic ones with incomplete brucite layers, show some similarities to the curves for the chlorite minerals themselves. Some clay-sized chlorites in soils do not, however, give pronounced peaks (Mackenzie [1956]), apparently because of oxidation of Fe to Fe in the lattice (Bain [1972]). 

Bain, D. C., 1972. Oxidation of chlorites in soil clays and effect on DTA curves. Nature Phys. Sci. 238 142. 

Chlorine dioxide is a very reactive compound and may exist in the environment for only short periods of time (see Section 6.3.2). Chlorine dioxide is readily soluble as a dissolved gas. However, chlorine dioxide can be easily driven out of aqueous solutions with a strong stream of air. The partition coefficient between water and C102(g) is about 21.5 at 35 °C and 70.0 at 0 °C (Aieta and Berg 1986 Kaczur and Cawlfield 1993 Stevens 1982). Transport and partition of chlorine dioxide in soils and sediments will not be significant. Chlorine dioxide is expected to be reduced to chlorite ions in aqueous systems (see Section 6.3.2.2). 

Because chlorite is an anion, sorption of chlorite ions onto suspend particles, sediment, or clay surfaces is expected to be limited under enviromnental conditions. Thus, chlorite ions may be mobile in soils and leach into groundwater. However, chlorite (ions or salts) will undergo oxidation-reduction reactions with components in soils, suspend particles, and sediments (e.g., Fe, Mn ions see Section 6.3.2.2). Thus, oxidation-reduction reactions may reduce the concentration of chlorite ions capable of leaching into groundwater. 

No information was located in the literature on the transformation and degradation of chlorine dioxide or chlorite (ions or salts) in sediment and soils. However, chlorine dioxide and chlorite ions should degrade rapidly in soil in an analogous manner to the reactions described in water (see Section 6.3. 2.2). 

JOHNSON (L.J.), 1964. Occurrence of regularly interstratifled chlorite-vermiculite as a weathering product of chlorite in a soil. Amer. 

Barnhisel, R. I., and Bertsch, P. M. (1989). Chlorites and hydroxyl-interlayered vermiculite and smectite. In Minerals in Soil Environments, Dixon, J. B., and Weed, S. B., eds., Soil Science Society of America, Madison, WI, pp. 729-788. 

The X-ray method affords a reasonable estimate of the structural formula based on chemical data. Using X-ray data, Ball calculated the structural formulas for twenty-six weathered soil and vein clay chlorites from North Wales. Tetrahedral Al ranged from 1.0 to 1.7 which is similar to the values for chlorites in shales. Octahedral Fe ranged from 0.8 to 2.4, with all but two values being less than 1.6 these values are much lower than those calculated (X-ray) for shale samples but almost identical to the shale values based on chemical determination of the Fe content. 

Not only can trioctahedral chlorites form in soils but there appears to be relatively little difference in composition between them and chlorites in sediments. This indicates that the trioctahedral chlorites in sediments can not automatically be assumed to be detrital. 

Most of the chlorite-like material formed in soils is dioctahedral rather than trioctahedral. In the process of weathering, illite and muscovite are stripped of their potassium and water enters between the layers. In these minerals and in montmoril-lonites and vermiculites, hydroxides are precipitated in the interlayer positions to form a chlorite-like mineral (Rich and Obenshain, 1955 Klages and White, 1957 Brydon et al., 1961 Jackson, 1963 Quigley and Martin, 1963 Rich, 1968). Al(OH)3 and Fe(OH)3 are likely to be precipitated in an acid to mildly basic environments and Mg(OH)2 in a basic environment. The gibbsite sheets in the soil chlorites are seldom complete and the material resembles a mixed-layer chlorite-vermiculite. The gibbsite may occur between some layers and not between others or may occur as islands separated by water molecules. 

Although partially organized dioctahedral chlorites form readily in soils, there are relatively few reported in sedimentary rocks. (More dioctahedral chlorites probably exist than have been recognized.) Swindale and Fan in 1967 reported the alteration of gibbsite deposited in Waimea Bay off the coast of Kauai, Hawaii, to chlorite but no data were obtained on its composition. Dioctahedral chloritic clays have been reported forming in recent marine sediments however, the identification is indirect and the interlayer material is relatively sparse (Grim and Johns, 1954). 

Vermiculite and vermiculite layers interstratified with mica and chlorite layers are quite common in soils where weathering is not overly aggressive. (A few references are Walker, 1949 Brown, 1953 Van der Marel, 1954 Hathaway, 1955 Droste, 1956 Rich, 1958 Weaver, 1958 Gjems, 1963 Millot and Camez, 1963 Barshad and Kishk, 1969.) Most of these clays are formed by the removal of K from the biotite, muscovite and illite and the brucite sheet from chlorite. This is accompanied by the oxidation of much of the iron in the 2 1 layer. Walker (1949) has described a trioctahedral soil vermiculite from Scotland formed from biotite however, most of the described samples are dioctahedral. Biotite and chlorite with a relatively high iron content weather more easily than the related iron-poor dioctahedral 2 1 clays and under similar weathering conditions are more apt to alter to a 1 1 clay or possibly assume a dioctahedral structure. 

As mixed-layer I/S clays become smectite-rich, their ion exchange and swelling properties approach those of the pure smectites. Other three-layer clays, including the chlorites and vermiculites, also commonly occur in soils in mixed-layer form (e.g., mixed-layer chlorite-smectite [-vermicu-lite]) (cf. Wilson and Nadeau 1985 Drever 1988). 

Kaolinite is present in large quantities in soils of higher elements ofrelief. In the less acidic soils of lower slopes, where solutions containing silica, magnesium and iron can enter from the slopes, the quantity of montmorillonite, especially chloritized montmorillonite and chlorite, increases. The biogeochemical cycle is generally depressed (see Table I). 

While these minerals are not common in soils, they can be used as prototypes for the many variations on the 2 1 structure that occur in soils. Common ones that will be described here are (a) smectite, (b) vermiculite, (c) illite, and (d) chlorite. Each of these mineral names represents a clay group and a reasonably well-defined range of chemical compositions. 

Chlorite dismutase (Cld) contains a heme b active site, analogous to peroxidases, and occurs in perchlorate and chlorate respiring bacteria.28 Perchlorate (CIO ) is a rocket fuel and used in pyrotechnics and ammunition. It has been found in soil and ground water only recently.29 Due to its size similarity to iodide, perchlorate inhibits the thyroid gland irreversibly. Microbes have evolved to utilize the oxidizing power of perchlorate. These organisms reduce perchlorate to chlorate and chlorate to chlorite... 

The concentration of magnesium in soils generally lies in the range between 0.5 g/kg for sandy soils and 5 g/kg for clay soils. The levels of magnesium are higher in clay soils due to the presence of weatherable ferromagnesian minerals, such as biotite, serpentine, and olivine and also the carbonate mineral dolomite. It is also present in secondary clay minerals, such as chlorite and vermiculite. 

Magnesium is an important constituent of many primary and secondary aluminosilicate minerals (with the exception of the feldspars). Magnesium in mafic (Mg2+- and Fe2+-rich) minerals often leads to the formation of chlorite and mont-morillonite clay minerals in soils. 

Chlorites occur extensively in soils and are 2 1 1 layer silicates (Fig. 5.6). The positively charged and substituted brucite sheet between the negatively charged mica-like sheets restricts swelling, decreases the effective surface area, and reduces the effec-... 

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