Summary of Natural Occurrences

Natural occurrences in hydrothermal areas show that the replacement of analcite + quartz by albite probably takes place near 150-180°C (Coombs, et al., 1959) at several hundred meters depth. The observed upper limit of analcite appears to be 100-125°C in deeply buried rocks in Japan 5Km depth). In other rocks for which no temperature data are available analcite can be found to coexist with sodium feldspar (High and Picard, 1965 Iijima and Utada, 1966 Iijima and Hay, 1968 Otalora, 196A Callegari and Jobstribitzer, 1964 Gulbrandsen and Cressman, 1960). Several authors have indicated that analcite replaces other zeolites in buried sequences of rocks (Moiola, 1970 Sheppard, 1970 Iijima and Hay, 1968 Iijima, 1970 Gude and Sheppard, 1967) but this is certainly not the rule since analcite is frequently associated with other zeolites as a primary mineral in soils sediments and sedimentary rocks (Hay, 1966). 

From the clay mineral-zeolite associations found at low temperatures, it is apparent that kaolinite as well as potassium mica occur rarely with alkali zeolites. Such assemblages are known for highly alkaline waters in continental lakes (Hay, 1966 Sheppard and Gude, 1969) where montmoril-lonite is nevertheless the predominant clay mineral. At higher temperatures, where most alkali zeolites become unstable but analcite persists, mont-morillonite will be present up to 100°C and a mixed layered mineral above this temperature. 

It can be seen that alkali zeolites, those predominantly sodi-potassic, are most often found in low temperature, low pressure environments. 

Frequently two or more species are found together in the same geologic sample. As pressure-temperature conditions become more severe, the mineralogy becomes more simple, feldspar appears and finally within the limits of clay mineral stabilities only calcic zeolites are found. However, the calcic minerals are generally confined to rocks of basic 

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It would appear from the above summary of natural occurrences that quartz is the most stable form of silica at near-surface conditions but that other metastable phases, representing initially poorly organized material, predominate in the natural occurrences or newly formed silica. Experiments demonstrate the persistence of metastable amorphous or cryptocrystalline hydrated Si02 at low temperature (Kittrick, 1969 Krauskopf, 1956, 1959) and slow conversion at higher temperatures (above 100 bars) (Frondel, 1962 Heydemann, 1964 Carr and Fyfe, 1958 Mlzutanl, 1970). 

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