Granitic gneiss weathering

Results of a field study at Bear Brook Watershed, Maine, were compared with laboratory kinetic experiments using size-fractionated soils from the same location. It is a forested, glaciated region with thin podzolic soils and granitic gneiss bedrock. Fractional order dependence (m = 0.5) of weathering rales on H 1... 

Figure 19. Typical example of how uranium is released to the environment (redrawn and completed after Pfeifer et al. (1994)). By weathering and erosion U-rich ore quality material (pitchblende veins in granitic gneiss) is deposited on the slope beneath a cliff. The soil and plants devel-opping on it are enriched in uranium (up to 2500 ppm in the soils vs. 3 ppm in the reference soil outside the touched area, 50 m to the south-east, marked with r 100 mg/kg U in contaminated plants, - see Appendix A.8). The waters from this type of environment contain between 10 and 30 jxg/l U (cf. Fig. 18, Section 2.2 and Appendix A. 1).

The granite-gneiss has been strongly kaolinized due to weathering. The weathering zone shows secondary enrichment of REE. The average REE-composition is given in Table 2.11. 

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. 

Quartz is a widely distributed mineral species consisting of silicon dioxide (silica, SiOj) (Table 7.6). It is one of the most common minerals and is found in many varieties with very diverse modes of occurrence. Quartz is a primary constituent of rocks such as granite, quartz, porphyry, and rhyolite. It is also a common constituent in many gneisses (laminate rocks) and crystalline schists (foliated rocks). It is also, in a sense, mobile since by the weathering of silicates, silica passes into solution and is redeposited in cavities, crevices, and along joints of rocks of all types. Thus, it is not... 

Clays were formed many millions of years ago by the weathering of rocks such as granite and gneiss. They are all silicoaluminates and their use depends on chemical composition and crystal structure. 

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