Amphibole weathering

Berner R. A., Sjoberg E. L., and Schott J. (1980). Mechanisms of pyroxene and amphibole weathering, I Experimental studies. In Third International Symposium on Water-Rock Interaction Proceedings, Edmonton Alberta Research Council. 

Schott, J., Berner, R. A., and Lennart-Sjoberg, E. L. (1981). Mechanism of pyroxene and amphibole weathering. I. Experimental studies of Fe-free minerals. Geochim. Cos-mochim. Acta 45, 2123-2135. 

Schott, J, R, A. Berner, and E. L. Sjoberg (1979), Mechanism of Pyroxene and Amphibole Weathering—I. Experimental Studies, Geochim. Cosmochim. Acta 43, 1161 1171. 

Schott J, Berner RA (1983) X-ray photoelectron studies of the mechanism of iron silicate dissolution during weathering. Geochim Cosmochim Acta 47 2233-2240 Schott J, Berner RA, Sj0berg EL (1981) Mechanism of pyroxene and amphibole weathering - I. Experimental studies of iron-free minerals. Geochim Cosmochim Acta 45 2123-2135... 

Brantley, S. L. and Chen, Y. (1995). Chemical weathering rates of pyroxenes and amphiboles. In Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. Brantley, eds), Mineralogical Society of America Washington, EXT, Reviews in Mineralogy 31,119-172. 

The Hostrock and Backfill Material. Most crystalline igneous rocks, including granite and gneiss, are composed of a comparatively small number of rock forming silicate minerals like quartz, feldspars (albite, microcline, anorthite etc.) micas (biotite, muscovite) and sometimes pyroxenes, amphiboles, olivine and others. Besides, there is a rather limited number of common accessory minerals like magnetite, hematite, pyrite, fluorite, apatite, cal cite and others. Moreover, the weathering and alteration products (clay minerals etc.) from these major constituents of the rock would be present, especially on water exposed surfaces in cracks and fissures. 

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. 

Iron(II) silicates are widely distributed in nature. Most of the iron in the primary magmatic rocks is in the form of Fe(II) silicates (olivines, pyroxenes, amphiboles, andbiotites). When exposed on the Earth s surface, they undergo oxidative hydrolytic weathering to give iron(III) hydroxyoxides such as goethite, the overall stoichiometry being described by equation (1). This form of iron is very insoluble, but subsequent microbial reduction to iron(II) makes it available to the biosphere. 

Velbel M. A. (1992) Geochemical mass balances and weathering rates in forested watersheds of the southern Blue Ridge III. Cation budgets and the weathering rate of amphibole. Am. J. Sci. 292, 58-78. 

Banfield J. F. and Barker W. W. (1994) Direct observation of reactant—product interfaces formed in natural weathering of exsolved, defective amphibole to smectite evidence for episodic, isovolumetric reactions involving structural inheritance. Geochim. Cosmochim. Acta 58, 1419-1429. 

Figure 1. High-resolution transmission electron microscope image of goethite from weathered amphibole. Note the nanometer-scale porosity that separates oriented nanociystals. Similar aggregates were reported by Smith et al. (1983, 1987) in botiyoidal goethite (Banfield and Barker, unpublished data).

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

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