Alteration in granites

DEEP WEATHERING AND ALTERATION IN GRANITES - A PRODUCT OF COUPLED... 

Ledesert, B., Berger, G., Meunier, A., Genter, A. Bouchet, A. 1999. Diagenetic-type reactions related to hydrothermal alteration in the Soultz-sous-Forets granite, France. European Journal of... 

Fig. 11. (a) Thermodynamic reaction pathway for the initial oxidative alteration of the spent fuel matrix at pH 8, calculated by using the PHREEQC code (adapted from Bruno etaL 1995). (b) Thermodynamic reaction pathway for the alteration of schoepite in granitic/bentonite groundwater at pH 8, calculated by using the PHREEQC code (adapted from Bruno et al. 1995 with permission). 

Most halloysite appears to be the result of supergene processes and, as noted by Ross and Kerr (1934), leaching by sulphuric acid, produced by the alteration of pyrite, appears to be one of the more common weathering processes. Alunite is commonly associated with halloysite (Ross and Kerr, 1934 Swineford et al., 1954). Sulphuric acid is commonly used to dissolve ilmenite in the manufacture of Ti02 (Willets and Marchett), 1958). It is possible that in nature it serves the same purpose. In the alteration of granitic rocks and pegmatites, feldspar is more likely to alter to halloysite... 

The KTB deep drilling project resulted in two of the deepest research boreholes drilled in the Earth s cmst (Lodemann et al., 1998 Moller et al., 1997). The boreholes were 4,000 m and 9,101 m deep and are located on the western margin of the Bohemian Massif (Weber, 1992). The area contains a variety of rock types that have been tectonically emplaced and thermally altered. Several granitic instmsions are in close proximity to the site and large fracture-thmst belts trend through the area (Figure 16). The site contains both altered sedimentary and crystalline rocks. 

Ferry JM (1979) Reaction mechanisms, physical conditions, and mass transfer during hydrothermal alteration of mica and feldspar in granitic rocks from South-Central Maine, USA. Contrib Mineral Petrol 68 125-139... 

China Stone. Partly decomposed granite, consisting of feldspathic minerals and quartz, it is used as a flux in pottery bodies. Examples in the UK are Cornish Stone and Manx Stone. The former is available in various grades, e.g. Hard Purple, Mild Purple, Hard White and Soft White the feldspars are least altered in the Hard Purple, alteration to secondary mica and kaolinite being progressively greater in the Mild Purple, Hard White, and Soft White the purple stones are so coloured by the small amount of fluorspar present. Manx Stone (from Foxdale, Isle of Man) is virtually free from fluorine. 

It is thought that submarine hydrothermal alteration of basalt took place in the Besshi-subtype mineralization area in Sanbagawa. However, no study on the hydrothermal alteration of basalt (basic schist) has been carried out and it is clearly necessary. The contact metamorphosed rocks and sulfide ores caused by Tertiary granitic intrusion are found in deeper part of the Besshi mine and Makimine mine (Tatsumi, 1952 Kase, 1972). 

Host rocks in the Hitachi area suffered regional metamorphism, contact metamorphism by Cretaceous granitic rocks and hydrothermal alteration associated with sulfide mineralization. 

Figure 11.23 shows the isochron obtained by Marshall and De Paolo (1982) for the granite batholith of Pikes Peak (Colorado). The effectiveness of the double-spike technique is evident, especially when we see that aliquot-spiked samples do not fall on the best-fit interpolant (York s algorithm York, 1969). The obtained age (1041 32 Ma) is consistent with that previously obtained with Rb-Sr whole rock analyses (1008 13 Ma see Marshall and De Paolo, 1982, for references). The initial ratio ( Ca/ Ca)o of 151.0 is identical, within the range of uncertainty, to upper mantle values, indicating negligible contamination by old crust components the relative K/Ca abundance in the earth s mantle is about 0.01, a value too low to alter the primordial (" Ca/" Ca)o composition. 

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