Zeolite Synthesis and Stability

It is not our purpose here to analyze the voluminous literature dealing with synthetic zeolites, suffice it to be said that the subject is vast and as yet incompletely explored. However, citations from several recent summary papers allow the general trends of observed stability and crystallization to be outlined in a reasonably accurate fashion. 

The most important factor in zeolite synthesis in the laboratory, or factory, is the rate of crystallization. Composition and concentration of the liquid solution acting on the solids is important to the process as is the absolute necessity of maximum disorder of the Si-O-Al bonds in the initial solids reacted (Zhdanov, 1970). It is thus evident that not only bulk chemical (equilibrium) factors are important in the initial crystallization of zeolites but also the. relative free energies of the reactants. It is apparent that zeolite equilibria are essentially aqueous i.e., that silicate equilibrium or approach to it is attained through reaction with solutions, and thus the solubilities of the solids present are of primary importance. If materials are slow to enter into solution they are essentially bypassed in the rapid crystallization sequence (Schwochow and Heinze, 1970 Aiello, et al , 1970). In most studies the zeolites precipitated from solution appear to respond to the laws concerning chemical activity of solutions (Zhdanov, 1970). 

One important aspect of experimental studies is the ease with which alkali or other ions are exchanged in a given zeolite structure. It would appear that complete solid solution in a given crystal structure is frequently possible (Zhdanov, 1970 Breck, 1970 Taylor and Roy, 1964  

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. 

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