Weathering gneiss

Flake Mica. Flake mica is mined from weathered and hard rock pegmatites, granodiorite, and schist and gneiss by conventional open-pit methods. In soft, residual material, dozers, shovels, scrapers, and front-end loaders are used to mine the ore. Often kaolin, quartz, and feldspar are recovered along with the mica (see also Clays Silicon compounds). 

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. 

Metamorphic rocks. Quartzite, schist, gneiss, and other metamorphic rocks and metasediments vary in their hydrological properties, but in most cases are fractured and transmit water. In rainy zones weathering may produce clays that occasionally clog the fractures. 

A weathered quartz-feldspar-biotite gneiss (Adapted from http //geology.csupomona.edu/). 

Goldich S. S. and Gast P. W. (1966) Effects of weathering on the Rb-Sr and K-Ar ages of biotite from the Morton gneiss, Minnesota. Earth Planet. Sci. Lett. 1, NIl-NIS. 

Many of the above observations can be illustrated through discussion of an example of weathering taken from Goldich (1938) (see also Krauskopf 1967). The parent rock in this humid, temperate climate example is a quartz-feldspar-biotite gneiss. The mineralogic and oxide composition of this rock and its weathered product soil is given in Table 7.2. 

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... 

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