Amphibole weathering soils

Berner, R. A. and S. Schott (1982). Mechanism of pyroxene and amphibole weathering II. Observations of soil grains. Amer. ]. Sci. 282, 1214-1231. 

Figure 6.2 Primary minerals containing Fe (e.g., pyroxenes and amphiboles in basaltic volcanic rocks) are weathered, releasing Fe into solution, which is then precipitated and accumulated in oxides as illustrated when examining the composition and percentage of clay and free oxides in Hawaiian soils. (Modified from Sherman, 1952.)...

In Table 3, susceptibility to weathering increases down the list as fewer silicon-oxygen bonds need to be broken to release silicate. Consequently, quartz and feldspars especially, but also mica in temperate soils, are common inherited minerals in the coarse particle size fractions of soil (the silt and sand fractions, 0.002-2 mm). The amphiboles, pyroxenes, and olivine are much more easily weathered. Thus, soils derived from parent material with rock containing a predominance of framework silicates e.g. granite, sandstone) tend to be more sandy, while those derived from rocks containing the more easily weathered minerals tend to be more clayey. 

Olivines, pyroxenes, and amphiboles are typically rich in reduced iron (Fe " ), weathering rapidly in aerated soil environments as the Fe is oxidized to Fe " ... 

Observations of Feldspars from Soils, Geochim. Cosmochim. Acta, 43, 1173-1186. Berner, R. A., E. L. Sjoberg, M. A. Velbel, and M. D. Krom, (1980), Dissolution of Pyroxenes and Amphiboles During Weathering, Science 207, 1205-1206. 

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. 

A comparison of the three soil components shows that the rhizosphere tends to be depleted compared to the bulk soil for many minerals and at most sites. This trend is particularly well expressed for chlorite and amphiboles (Table 1). Vermiculite and plagioclases follow the same general trend when the rhizosphere is considered as a whole, although the trend is inverted at one site for each mineral (Table 1). Micas show no specific trend. On the other hand, K-feldspars rather tend to be less weathered in the inner rhizosphere. 

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 results taken as a whole converge and indicate a more intense mineral weathering in the rhizosphere compared to the bulk soil. The XRD results show a change in mineral assemblage that reflects increased mineral weathering in the rhizosphere, a reaction that mostly affects easily weathered minerals such as chlorite and amphiboles. A hypothesis submitted in the literature to explain the greater alteration in the rhizosphere links nutrient uptake by plants to a disequilibrium between the liquid and the solid phases that induces the release of interlayer elements such as K and Mg. However, the current study showed that K and Mg... 

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