Weathering of granites

Michel J (1984) Redistribution of uranium and thorium series isotopes during isovolumetric weathering of granite. Geochim Cosmochim Acta 48 1249-1255... 

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... 

Corrosion may be broadly defined as the degradation of a material due to interaction with its environment. From this broad definition, it is evident that the material need not be man-made for instance the "weathering of granite tors on moorlands may be considered as corrosion of their crystal constituents by the atmosphere (rain, sun, wind, etc). In this chapter, however, it is only metallic corrosion that will be considered. 

Middleburg J. J., van der Weijden C. H., and Woitties J. R. W. (1988) Chemical processes affecting the mobility of major, minor and trace elements during weathering of granitic rocks. Chem. Geol. 68, 253-273. 

Gilkes, R.J., Scholz, G. Dimmock, G.M. (1973) Lateritic deep weathering of granite. Journal of Soil Science 24, 523-536. 

The clay minerals are silicates and aluminosilicates with sheet-like structures. They result from the weathering of granite and other rocks. The layers have enormous inner surfaces that can absorb large amounts of H2O. Clay mixtures often occur as minute platelets with a very large total surface area. When wet, the clays are easily shaped. When heated to high temperatures, they lose H2O when fired in a furnace, they become very rigid. 

At weathering of granite from Gunheath Pit in Cornwall kaolinized granite has been obtained, containing the sheet silicate kaolinite with the composition AI,i(0H)gSi, 0 (, and quartz. 

Uranium is classified as a lithophile element, and its abundance in granitic rocks is about double its average crustal abundance. In fact, the weathering of granites is probably the major primary source for the uranium that presently occurs in sedimentary host rocks. Although reported to have all valence states from 2 + to 6 +, only the 4 + and 6 + states are important in minerals. There is some evidence for a 5 -t state both as a solution species and in some of the uranium oxides, but its role in mineral structures is not confirmed. 

Uranium in epigenetic sandstone deposits is believed to have come from such varied sources as weathering of granitic rocks, siliceous tuffs or other uraniferous rocks in the source area for the sandstone devitrification of tuffaceous sediment in or interbedded with the sandstone hydrothermal solutions from nearby magmas and recycling and redistribution of earlier-formed uranium deposits. 

Clay is a natural, earthy mixture of very small crystals of certain silicate sheet minerals. These minerals form by the weathering of granite, the rock that composes the backbone of mountain ranges. Clay minerals easily adsorb water, and wet clay is moldable. The wet platelike crystals adhere to one another to give a plastic, or easily deformable, mass. It is this plasticity of wet clay that allows the potter to form usefiil and artistic shapes. 

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