Alkali zeolites

Using the most simple expression of the chemical formulae for the minerals cited above, a chemical system can be constructed using the components Na-K-Al-Si-l O. Initially we will assume that hydrogen and oxygen are always combined in a 2 1 ratio. This is a valid approximation for zeolite structures but not for phyllosilicates, a point to be discussed later. 

Since the major chemical reactions take place through the agency of an aqueous fluid, the system can be considered to be saturated with respect to water. H O is always the major component of an omnipresent fluid phase during the attainment of equilibrium and it is therefore considered a component in excess. We are left with a four component system, Na-K-Al-Si where, for unspecified P-T conditions over a short range, there will be a maximum of four phases coexisting. 

The two major sources of information available are quite different and, as a first approximation, require different interpretations. We will first consider the composition and chemiographic relations of the silicate phases involved as they can be deduced from analyses of natural minerals. Then we will look at the mineral assemblages reported in soils, sediments 

As has been pointed out in the first part of this section, it is common to find two or more high-silica zeolites associated. This could be considered an example of metastable association but, equally, this could be considered an indication of a tendency toward a stable assemblage 

He = heulandite Cl = clinoptilolite Mor = mordenite M = mica. Solid solution or continuous compositions are assumed present between analcite and mordenite-clinoptilolite. 

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The gzz values were assigned to a given oxidation state of the adsorption site on the basis of spectroscopic and chemical evidence. For transition metal ions, the oxidation state was deduced from reactions of the type M1"- u+ + 02 - Mn+02, which were ascertained by a decrease in the EPR signal of Moxidation state has been taken. For the +1 oxidation state observed only in alkali zeolites there is a large range of gzz values 2.054-2.166 (Table X), which has been used in Fig. 3. It appears,... 

The occurrence of kaolinite is generally erratic but in the terrigenous sediments (Muffler and White, 1969) it can apparently react with dolomite to form the assemblage calcite + chlorite between 120-180°C. Expandable chlorite was noted in shear zones, and iron-rich chlorite is common in most of the rocks becoming more evident at greater depths. In the terrigenous rocks observed, the apparent alumina content of chlorite decreases with depth. Alkali zeolites have been observed at temperatures up to 100°C in the deeply buried rocks. 

Alkali zeolites (especially the more common species natrolite, anal-cite, phillipsite, erionite, scoleite, heulandite, clinoptilolite and... 

A wide variety of zeolites are known to form in saline lakes where the species present is dependent upon the chemistry of the solutions. Rapid zeolite formation is aided by the existence of the volcanic glass and high water salinities. Potassium feldspar occurs with the common alkali zeolites (Hay and Moiola, 1963 Hay, 1964 Hay, 1966 Sheppard and Gude, 1969, 1971), however, albite is not evident as a diagenetic mineral in saline lakes. 

Many investigations have reported the presence of zeolites at the deep ocean bottom (Biscaye, 1965 Heath, 1969 Bonatti, 1963 Sheppard and Gude, 1971 Jacobs, 1970 Morgenstein, 1967 among others). Most of the alkali zeolites are represented except the silica-poor species natrolite and analcite. Rex and Martin (1966) indicate that detrital potassium feldspar is not stable under ocean floor conditions. Zeolites are found in most ocean basins where wind-carried volcanic ash predominates over detrital river-born clay mineral sediments. In these sediments phillipsite is particularly evident and it is known to continue to grow in the sediment column to depths of more than a meter (Bernat, t al.,... 

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. 

Experimental phase equilibria studies by Campbell and Fyfe (1965) Thompson (1971)and Liou (1971a) indicate an approximate 180°C. lower stability for albite in the presence of quartz and analcite from 12 to 2000 atmospheres pressure. A calculated stability for analcite at 3Kb is about 120°C. (Campbell and Fyfe, 1965), conditions equivalent to rock pressures at 7.5Km depth. However, if water pressure is lower than total, litho-static pressure, the termal stability of a very hydrous, low-density mineral such as analcite can be significantly lowered (Greenwood, 1961). The experimental transformation of alkali zeolites to analcite at 100°C. and 2-3 atmospheres pressures was demonstrated by Boles (1971). The alumina content of the alkali zeolites used in this latter study was found to influence that of the analcite produced, and this independently of the amount of crystalline quartz added to the initial materials. 

Probably the only general statement which can be made about the experimental studies on zeolites is that the majority of published data is inapplicable directly to natural minerals. This is due either to the excessively high temperatures under which the experiments are performed, outside of the physical limits of zeolite stability, or to short time spans of observation which do not allow the silicates to come to equilibrium with the fluids of the experiments. Those studies designed to determine zeolite stability indicate that the most silica-poor alkali zeolite, analcite, is not stable above 180°C. More silica-rich species will be found below this temperature. However, the reasons for the crystallization of one or another of the silica-rich alkali zeolites are not yet elucidated. 

The identification of a specific zeolite species with a particular genesis or environment of formation is very difficult if natural mineral occurrence is used as the sole criteria. Most alkali zeolites are found at one place or another in most low temperature geological situations. Various authors have cited various physical and chemical factors which would control the sequence or particular species of alkali zeolite found in nature. Silica and alkali activities in solution are of great importance in surface and buried deposits (Sheppard and Gude, 1971 Honda and Muffler, 1970 Hay, 1964 Coombs, t al.. 1959 Read and Eisbacher, 1974). 

Temperature is also of great importance, apparently decreasing the Si-content of the zeolites as it increases. Temperature and time, or approach to equilibrium, appear to determine the type of zeolite assemblages found and thus an understanding of these factors permits the tentative establishment of two alkali zeolite "facies" subdivisions the diagenetic and analcite type (Hay, 1966 Moiola, 1970 Iijima and Utada, 1970 Iijima, 1970 Studer, 1967 Coombs, 1970 Seki, 1969 Miyashiro and Shido, 1970). 

The major aluminous clay minerals, alkali zeolites and feldspars which are most commonly associated in nature can be considered as the phases present in a simplified chemical system. Zeolites can be chemiographically aligned between natrolite (Na) and phillipsite (K) at the silica-poor, and mordenite-clinoptilolite at the silica-rich end of the compositional series. Potassium mica (illite), montmorillonite, kaolinite, gibbsite and opal or amorphous silica are the other phases which can be expected in... 

Figure 33a. Alkali zeolites as a function of their compositions in Al-Si-(Na,K)2 coordinates assuming Na-K end-members. Nat = natrolite Anal = analcite Ph = phillipsite Er = erionite F = feldspar S = stilbite ...

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. 

It is also probable that albite becomes stable when this restricted solid soltuion exists, since analcite-albite and alkali zeolites of restricted composition have been reported together (Hay, 1966). The existence of albite implies a decreased silica content in analcite and such a variation is commonly reported in rocks having experienced moderate temperatures. 

Figure 34. Alkali zeolites projected into a portion of the Na-K-Si coordinates. Anal = analcite Ph = phillipsite solid solution Ze = alkali zeolites undifferentiated Alb = albite KF = potassium feldspar Q = quartz Si = amorphous silica, a) low, b) medium, and c) high temperature facies. Shaded areas are two-phase fields.

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. 

The first two parageneses described above, where alkali zeolites are stable, correspond to the "diagenetic" zeolite facies of Coombs (1970). 

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. 

Kaol = kaolinite Mo = beidelitic montmorillonite M 38 muscovite (or illite) ML = mixed layered minerals Anal = analcite Nat = natrolite Ze = alkali zeolites. Alkali zeolite tie-lines for specific species are not given, j) less than 100°C. 

These associations are noted by Hay (1966) as being found in sequences of sedimentary rocks or altered pyroclastics buried to depths greater than 3,000 meters and generally less than 10,000 meters. However, the limits are actually vague and the identifications imprecise. The relatively frequent occurrence and persistence of albite or potassium feldspar and alkali zeolite in such rocks leads one to believe that they can coexist stably in nature. This could be, however, a misleading conclusion based upon too few observations. The elimination of the silicic, alkali zeolites and the persistence of montmorillonite is known to exist in series of deeply buried rocks (Ii-jima, 1970 Moiola, 1970 Iijima and Hay, 1968). 

Let us assume for the moment that a tetrahedral representation is adequate. For the case in hand, it can, in fact, be solely represented by a triangular face of the system (Ca, Na)-K-Al-Si. These coordinates contain the phyllosilicates reported by Sheppard and Gude, mica, illite and a montmorillonite as well as potassium feldspar. Alkali zeolites are found towards the fourth pole where Ca and Na are present (Figure 35). 

The bulk composition of the sediments must be normally near the potassium-rich side of the zeolite facies since analcite is not reported in these sediments and potassium feldspar apparently coexists with or replaces the alkali zeolites. 

A consideration of natural occurrence and chemical composition of alkali zeolites allows a certain refinement of the zeolite facies concept previously proposed. The key factor is the grouping of the alkali zeolites into a continuous solid solution series. Other possible coexisting phases of similar composition are sodium and potassium feldspar, natrolite and analcite. The extent of solid solution decreases with temperature, possibly also with pressure. This effect allows the sequential series zeolite-K feldspar, zeolite-analcite-K feldspar, analcite-K feldspar-albite and eventually two feldspars to the exclusion of analcite, the alkali zeolite with the highest stability limits. 

If we consider the process of recrystallization of the acidic volcanic glass at surface conditions in the soil or sedimentary environment, the assemblage feldspar (plus alkali zeolites in many instances) -montmorillonite free silica can be deduced from the diagram- on the basis of bulk composition. This assemblage is common in such natural materials. 

The second facies is marked by the instability of the fully expanding dioctahedral phases and the existence of a kaolinite-illite tie-line (Figure 48b). In this facies the siliceous alkali zeolites (other than analcite) become unstable, the compositional range of the trioctahedral expanding phases is reduced and aluminous 14 8 chlorite-"allevardite"... 

The transition from alkali zeolite and analcite to albite-bearing rocks appears to occur near 180°C at depths of several hundred meters and 120°C at 5 Km. This information is known only at two P-T points and therefore might be considered as open to question to a certain extent, especially in the zone between the two data points. 

HAYHURST (D.J.) and SAND (L.B.), 1975. Experimental kinetics related to alkali zeolite paragenesis. Geol. Soc. Arne. Annual Meeting Abst. 

EF material free, alkali exchanged zeolites are used as quite mild basic catalysts. Light alkali and alkaline earth metal zeolites, such as Na-X, Na-Y [165], alkali-MOR, Na-A and Ca-A [166], have a mild Lewis acid behavior and do not appear to have strong basic character. The same occurs for Na-silica-alumina [167]. However, heavy alkali metal zeolites such as Cs-Y actually act as base catalysts, or rather as acid-base catalysts, for example for toluene side-chain alkylation. Stronger basic character arises from impregnation of alkali zeolites with alkali salts, later... 

Ni(C0)4 in dealuminated NaY (Si/Al > 400) shows one band at 2046 cm , similar to that of tetrahedrally coordinated Ni(CO)4 in THF solution. No change of the Ni oxidation state and no loss of CO ligands after adsorption of Ni(CO)4 in alkali zeolite Y are detected with XANES and EXAFS spectroscopies. However, the appearance of four IR bands, which shift when the Ni(CO>4 loading or the alkali cations are varied, indicates an interaction of the type -OC—Ni, where = Na or Li. A reactive Ni(CO)3 in-... 

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