Paragenesis, feldspars

Pyroxenes (as feldspars) occupy a position that is chemically central in the realm of rock compositions (Robinson, 1982). They are therefore found ubiquitously and virtually in any kind of paragenesis developing in the P-T-Xconditions typical of earth s mantle and crust. Table 5.31 hsts the main occurrences. 

The observed paragenesis contains a significant amount of feldspars. Although feldspars are not considered to crystallize in the numerical simulations (Jacquol 2000 Durst Vuataz 2001), our experimental results suggest that formation of clay minerals is controlled by previously crystallized feldspars. Therefore, the presence of feldspars is of importance for the crystallization of secondary minerals and the evolution of porosity in the stimulated reservoir of the HFR site. 

Reported assemblages in sedimentary rocks containing analcite and potassic feldspar plus an alkali zeolite (Hay, 1966 Sheppard and Gude, 1968, 1969 Moiola, 1970) lead to the construction of an intermediate paragenesis which denotes a more restricted range of alkali zeolite solid solution, notably the instability of the alkali-rich phase phillipsite. 

Finally, when analcite is the only alkali zeolite stable, one finds the association analcite- albite-potassium feldspar (Figure 34c). This is the highest temperature zeolite paragenesis. It might be added that natro-lite may be stable also but one cannot decide, using the available data, how it fits into zeolite parageneses. From experimental studies, the analcite-albite-K feldspar paragenesis exists up to 180°C at low pressures (Pu = P total). However, Iijima (1970) indicates that its limit is... 

Due to the conflicting experimental results, we will rely upon natural occurrence and we will place an 80°C limit on the initial zeolite paragenesis where solid solution is maximum. An intermediate stage exists up to temperatures near 100°C at high water pressure where analcite, potassium feldspar and alkali zeolite can coexist as can analcite and albite. Analcite persists up to 180°C where the upper limit of the alkali zeolite facies is reached. 

Fig. 12. Paragenesis and relative mass of authigenic minerals in hydrolysis of coexisting K-feldspar and albite (after Helgeson et al., 1969b).

Morey, G. W., and W. T. Chen, 1955. The action of hot water on some feldspars. Am. Mineral. 40 997. Muir, I. D., 1962. The paragenesis and optical properties of some ternary feldspars. Norsk Geol. Tids. 42 477. 

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