Phyllosilicates weathered

F. Paris, P. Bonnaud, J. Ranger, M. Robert, and F. Lapeyrie, Weathering of ammonium- or calcium-saturated 2 1 phyllosilicates by ectomycorrhizal fungi in vitro. Soil Biol. Biochein. 27 1237 (1995). 

Clays are layer silicates (phyllosilicates) of particle size less than about 4 pm, produced by the weathering of aluminosilicate rocks. Clay minerals fall roughly into two structural classes the kaolinite type, based on paired sheets of tetrahedral (SiC>44-) and octahedral [A10n(0H) g " or... 

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

Although many authors insist upon the sequential degradation of phyllosilicates, i.e., taking the same initial material and transforming it into the various types of expandable minerals in the weathering process, (Heaver and Jackson, notably) this is undoubtedly not the only mechanism by which these clay minerals are formed in soils and possibly not the dominant one. Studies on the weathering of granites and more basic rocks... 

Figure 19. Sequence of relative phyllosilicate stabilities shown as a function of weathering intensity (modified from Jackson, 1959).

Isomorphic substitution is especially significant in the case of clay minerals (hydrous aluminum silicates), which are formed by the weathering of the primary silicates of igneous rocks. Most of the clay minerals belong to the phyllosilicates, but there are some that are of fibrous and amorphous structure. 

Paris, F., Bonnaud, P., Ranger, J. Lapeyrie, F. (1995). In vitro weathering of phlogopite by ectomycorrhizal fungi. I. Effect of K" " and Mg deficiency on phyllosilicate evolution. Plant and Soil, 177, 191-205. 

All studies concerning the weathering of phyllosilicates provide clear evidence that the dissolution of clay minerals and micas is very slow under natural conditions (Correns, 1963 Polzer and Hem, 1965 Calvera and Talibudeen, 1978 t Serstevens et al., 1978 Lim and Clemency, 1981 Carroll-Webb and Walther, 1988). Hence they are characteristic secondary weathering products occurring in soils and sediments. 

Biotite (Pe2+. bearing micas) Phyllosilicates Granitic and high-grade metamorphic rocks R Stable in only the youngest or least weathered soils precursor of other 2 1 soil clay minerals and Fe-oxides source of K... 

Halloysite Phyllosilicates Volcanic ash granitic (feldspathic) saprolite R Ephemeral in intensely weathered soils... 

HIVd Phyllosilicates Acid, highly weathered soil surface horizons u Variable CEC (degree of A1 interfilling) high anion adsorption... 

Montmorillonite A phyllosilicate mineral that typically forms microscopic crystals making a clay. It is the main constituent of the volcanic ash weathering... 

The XAFS study of Zn in a contaminated dredged sediment by Isaure et al. (2002) used a combination of methods (EXAFS, pXAFS, pPIXE, powder XRD, electron microprobe analysis, ICP-AES analysis, and selective chemical extractions) to identify three primary Zn-containing minerals (sphalerite, willemite, zincite) plus zinc associated with Fe-oxyhydroxide and phyllosilicates that was released by chemical weathering. 

Smectites Most clay minerals in deep-sea sediments are detrital phases from continental weathering. These minerals compose the bulk of the nonbiogenic sediments in the <2 pm fraction and include clay-sized quartz and the phyllosilicates kaolinite, illite, chlorite, and smectite. The smectite... 

Comparable results for plagioclases and K-feldspars were obtained by Courchesne and Gobran (1997) and April and Keller (1990). Both studies have been conducted in the field. Their results indicate that easily weathered minerals such as amphiboles and expandable phyllosilicates were depleted in the rhizosphere as compared to the bulk soil (Courchesne and Gobran, 1997). April and Keller (1990) also showed that the rhizoplan and the rhizosphere were depleted in biotite, a K-bearing mineral. Similarly, Kodama et al. (1994) and Hinsinger et al. (1993) conducted pot experiments that indicated a relative enrichment in vermiculite in the rhizosphere. 

The three examples of the effects of pedochemical weathering on the surface structures in soil clays just described illustrate the complexity of the reactive solid materials in natural soils. To these examples can be added many others, including the formation of iron oxyhydroxide or calcium carbonate coatings on the external surfaces (as opposed to interlayer surfaces) of phyllosilicates, the development of thick envelopes of colloidal organic matter on aggregates of metal oxides and aluminosilicates, and the... 

Figure 1.11. Some effects of pedochemical weathering on phyllosilicate surfaces. Weathering produces interlayer hydroxy-polymers, interstratification, and external-surface organic and inorganic polymer coatings on smectite. On kaolinite, organic and iron oxide coatings are produced by weathering. After Jenne ...

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