Calcic zeolites

Because the compositions are basic, the expanding minerals are trioctahedral and they are apparently associated in all facies with chlorite. The occurrence of a regularly interstratified montmorillonite (saponite) -chlorite mineral, corrensite, is typified by an association with calcic zeolites and albite. Temperature measurement in the "hydrothermal" sequences at several hundred meters depth indicate that the ordered, mixed layered mineral succeeds a fully expandable phase between 150-200 C and this ordered phase remains present to about 280°C. In this interval calcium zeolites disappear, being apparently replaced by prehnite. The higher temperature assemblage above corrensite stability typically contains chlorite and epidote. 

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... 

Figure 33b. Calcic zeolites as a function of their compositions in Al-Si-Cax coordinates assuming Calcium end members. F = feldspar G = gismon-dine La = lawsonite Sc = scolectite Ph = phillipsite Ch = chabazite ...

The latter, according to Coombs, is an analcite-heulandite facies which contains other calcic zeolites in more basic rocks. The disappearance of analcite would occur near the heulandite-laumontite transition for calcic zeolites. Thus, calcic zeolites can continue to be stable at higher grades of diagenesis or epimetamorphism than alkali zeolites. 

Calcic zeolites, prehnite, pumpellyite, and pistacitic epidote are not mutually exclusive, but are related by a series of reactions such as... 

Soluble potassium may be made available to plants by the action of biological end products upon. the zeolite compounds of the soil. Maclntire has shown, however, that the economic additions of calcic and inorganic materials function to protect native soil potassium and give a repressive rather than a liberative chemical effect. 

---