Zeolite mineral assemblages

There are several other types of minerals commonly found in clay particle size mineral assemblages (i.e.,< 2 microns diameter, Krumbein and Pettijhon, 1938). Aside from quartz and amorphous materials, the two most important mineral groups are sepiolite-palygorskite and zeolites. These two groups are similar in that they both contain free 1 0 molecules in their structure. However the Si-0 linkage is quite different in each case. 

Figure 36. Representation of the zeolite-clay mineral assemblages found in a systeirf at 25°C and atmospheric pressure where Na is an intensive variable (perfectly mobile component) whereas A1 and Si are extensive variables or inert components of the system. G = gibbsite kaol = kaolinite Mo = montmorillonite Si = amorphous silica Anal = analcite.

In spite of statements sometimes made to the contrary, prehnite, pumpellyite, and lawsonite emphatically are not zeolites, and are not diagnostic of the zeolite facies. The zeolite facies might be defined as that set of mineral assemblages that is characterized by the association calcium zeolite-chlorite-quartz in rocks of favorable bulk composition. In this way, attention would be concentrated on bulk compositions typical of many volcanogenic sediments. A corollary would be the separate recognition of a clay-carbonate facies, as indicated above. Under such a restricted definition, the assemblage sodium-zeolite-quartz (normally... 

Many workers probably will prefer a loosely defined zeolite facies which includes all mineral assemblages characterized in rocks of appropriate composition by zeolites other than the analcime of silica-deficient environments. The latter can persist to magmatic or near magmatic temperatures. 

Graphical Analysis of Zeolite Mineral Assemblages from the Bay of Fundy Area, Nova Scotia... 

Zeolite occurrences in the Bay of Fundy area, Nova Scotia, are reported here for 2 localities. The mineral assemblages in the altered basalts at Cape Blomidon include (1) heulandite-laumontite, (2) anal-cite-heulandite, (3) quartz-analcite-heulandite, (4) analcite-mesolite-thomsonite, and (5) heulandite-mesolite-thomsonite. These data are summarized in Figure 2. 

Phase equilibrium studies on the stabilities of the three-phase mineral assemblages listed above and determinations of the chemical composition of aqueous solutions in equilibrium with these assemblages will provide data which can be used for a quantitative assessment of the conditions under which these and other comparable zeolite occurrences have formed. 

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. 

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. 

Although much information is available on this subject, it is not plentiful enough to draw any conclusions with certainty. The major problem with natural zeolites is that they occur frequently in multiphase assemblages making mineral separation difficult and thus identification and chemical information unsure. Only X-ray diffraction allows a proper mineral identification but this also is not certain due to the complexities of structural variation in zeolites which arise through chemical substitutions. In sum, chemical analyses of so-called single-phase zeolites are likely to be unreliable. 

The second division of the zeolite facies is based upon the appearance of albite as a diagenetic mineral, usually coexisting with analcite in the initial stages of its development, and also with the widespread development of montmorillonite-illite mixed layered mineral (30 to 90% expandable layers) coexisting with illite. The phase relations of this facies are indicated by Figure 35b. Assemblages can contain natrolite as above. They are ... 

In each of the different parageneses outlined here, the instability of a mineral can be denoted by its replacement with one or usually several minerals. The rocks in these facies are typified by multi-phase assemblages which can be placed in the K-Na-Al-Si system. This is typical of systems where the major chemical components are inert and where their masses determine the phases formed. The assumptions made in the analysis up to this point have been that all phases are stable under the variation of intensive variables of the system. This means that at constant P-T the minerals are stable over the range of pH s encountered in the various environments. This is probably true for most sedimentary basins, deep-sea deposits and buried sedimentary sequences. The assemblage albite-potassium feldspar-mixed layered-illite montmorillonite and albite-mixed layered illite montmorillonite-kaolinite represent the end of zeolite facies as found in carbonates and sedimentary rocks (Bates and Strahl,... 

There are zeolite-bearing rocks in which one mineral is apparently being replaced by another mineral under constant P-T conditions. This indicates a system in which certain chemical components appear to be perfectly mobile a system in which the total number of phases that can coexist at equilibrium is reduced as a function of the number of chemical components which ar e internal variables of the system. Two examples of this type of equilibrium concerning zeolites can be cited saline lakes and analcite-bearing soil profiles (Hay, 1966 Hay and Moiola, 1963 Jones, 1965 and Frankart and Herbillon, 1970). In both cases a montmorillonite-bearing assemblage becomes analcite or zeolite-bearing at the expense of the expandable phyllosilicate. Other phases remain constantly present. 

Na and Ca to play equivalent roles in zeolites, as well as K, and if we consider A1 and Si as the major variables combined with K and Na in the phyllosilicates, we can adequately represent the phases in a (Ca-Na)-K-Al-Si system where H O is in excess in the fluid phase. If the system has four chemical variables and the natural assemblages are frequently found to contain four authigenic minerals, we must assume that most chemical variables are inert or extensive variables of the chemical system which controlled the crystallization of the zeolite-clay mineral containing sediments. ... 

Sheppard and Cude report two sorts of assemblages those where zeolites are dominant, towards the lake edge and those where feldspar is dominant, towards the lake center. There are two groups of assemblages of diagenetic minerals which characterize these two zones ... 

Calculations that attempt to reproduce the observed vertical successions of zeolite minerals seen in various sedimentary piles (e.g., Miyashiro and Shido, 1970) typically assume that hydrostatic and lithostatic pressures are equal and that the pore fluid is pure water. The broad features of the successions can be predicted, but there is a bewildering overlap of zeolite subfacies (Coombs, 1971). In addition to the complexities already noted, it should be recalled that zeolites occur in an unusually rich variety of crystal structures, with several different major cations, extensive cation solid solution, significant Si/Al ratio differences in different structures, and frequent growth and persistence of metastable phases. The bulk chemical composition of the parent-rock is important in determining which zeolite mineral forms, and the Pco of the diagenetic environment may determine whether a non-zeolitic clay—carbonate assemblage persists (Zen, 1961). 

Largely to avoid involvement in definitions of metamorphism, and in the belief that assemblages of minerals reflect the physico-chemical conditions of their formation whether they be metamorphic, hydrothermal, or diagenetic, Coombs et al. (8,10) treated zeolitic assemblages in terms of Eskola s mineral facies concept (15) and broadened Turners definition of the zeolite facies to include at least all those assemblages produced under conditions in which quartz-analcime, quartz-heulandite, and quartz-laumontite commonly are formed. 

In 1966, Seki (36) proposed a new facies, the wairakite facies, characterized by the occurrence of wairakite-quartz with clay minerals, in the absence of prehnite or pumpellyite. On field, experimental, and thermodynamic grounds, this proposed facies evidently represents, at least for conditions where PH2o = Pfiuid, higher temperatures than the laumontite assemblages, and relatively low pressure. Subsequently, Seki (35) introduced separate mordenite, heulandite, and laumontite facies. Our earlier definition of the zeolite facies was intended to be broad... 

The spatial relationships of the minerals in assemblages 2, 4, and 5 indicate that all of the zeolites in each of these assemblages were growing... 

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