Harmotome

Abstract: Some reactions have been studied at 80 °C between aqueous alkaline media and metakaolinite with and without added silica. The bases employed were TlOH, Ba(OH)2+ TlOH, Ba(OH)2+ LiOH, and Ba(OH)2+ NaOH. A similar study was made of kaolinite with aqueous Ba(OH)2+ LiOH. Kaolinite was less reactive than metakaolinite and yielded different reaction products. Non-zeolites formed included a barium silicate hydrate, barium aluminate, and a phase similar to cymrite. Three unidentified Tl-bearing species were also formed in minor yields. Zeolites of the following kinds were grown: those of edingtonite type containing Ba2++ Tl+ or Ba2++ Li+; variants of zeolite L, containing Ba2++ Tl+, Ba2++ Li+, or Ba2++ Na+; harmotome- or phillipsite-types containing Ba2++ Li+ or Ba2++ Na+; and zeolites like gismondite with Ba2++ Na+, like gmelinite and containing Ba2++ Na+, and like yugawaralite containing Ba2++ Li+. A lithium-bearing zeolite with no natural counterpart and in which the cations were Ba2++ Li+ was also grown.Properties of a number of these phases have been examined. From the present and earlier work, it appears that an important factor in promoting growth of chabazite-, edingtonite-, and phillipsite-type zeolites and of zeolite L is the presence of a sufficient amount of at least one of the cations K+ or Ba2+. On the other hand a sodic environment favours gismondite-, gmelinite-, and faujasite-type zeolites, and Linde A.

Abstract: Part I: Chemical and structural effects of cation-exchange Attempts were made to prepare, by appropriate exchange methods, homoionic samples of phillipsite, gismondite, harmotome, chabazite and gmelinite containing Ba, Ca, K, Na or Li-ions. Powdered natural samples were used as starting material. All samples were analysed chemically before and after the cation exchange. The results of the analyses demonstrated clearly that the „exchange capacity“ depends on the method used, the structure of the zeolite and the nature of the cation involved in the exchange. The analyses also disclosed the important fact that the ratio in Mole % of the sum of exchangeable cations: Al2O3 of the natural and of the exchanged samples is generally <1, and can be as low as 0.74. Examples are presented where cation exchange results in a substantial change in the framework structure. Part II: Dehydration behavior and structural changes at elevated temperatures Samples of the natural zeolites mentioned above, and of some of their cation exchange products were dehydrated in air of controlled humidity up to 600° C. The slopes of the weight loss curves of chabazite and gmelinite are continuous, whereas those of phillipsite, gismondite, and harmotome show a discontinuity between 90–190° C, indicating the existence of two discrete hydrated phases for each of these zeolites. High temperature x-ray studies of powdered samples confirmed this result. The high temperature hydrates of phillipsite, gismondite, and harmotome persist reversibly up to approximately 230° C. At higher temperatures, new probably anhydrous phases form. Gmelinite, at 240° C, transforms irreversibly to anhydrous gmelinite which is stable up to >700° C. The transition was studied by single crystal techniques. The chabazite structure remains intact up to >700° C. The absolute water content and the dehydration behavoir of the zeolites investigated are primarily dependent on the nature of the exchange cation. The structural changes at elevated temperatures are determined by the silica alumina framework. Part III: Hydrothermal stability* and interconversions The stability of phillipsite, gismondite, harmotome, chabazite, gmelinite, their exchange products, and of the synthetic Linde zeolites Faujasite and Type A was studied in the temperature range between 150° and 350° C at a constant pressure of 1000 atm of H2O. Between 180° and 260° C all examined Sodium and Calcium zeolites were metastable with respect to analcite (wairakite**). Phillipsite and sodium-rich zeolites generally converted to analcite (wairakite) directly. Caex-chabazite and Caex-gmelinite formed phillipsite, whereas Ca-gismondite and Ca-Type A formed natrolite as intermediate phases. Li-gmelinite converted to bikitaite***. (This represents the first successful preparation of natrolite and bikitaite. Attempts starting from gels or glasses have been unsuccessful so far.) Ba-gmelinite converted to harmotome at 250° C. This transformation was studied microscopically and by single crystal x-ray techniques. The transformations that take place on hydrothermal treatment as well as on low temperature cation exchange of zeolites (see Part I) indicate that, unlike the conditions prevailing in clays, the type of cation and the ratio of cations in the exchange positions have an important influence on the structure of the silicaalumina-oxygen framework. This explains two phenomena: The lack of solid solution between two potential end members of a solid solution series (for instance phillipsite-gismondite), and the large number of different zeolites in nature, where a great variety in the ratios of available alkali and alkaline earths ions must be expected. Any classification of zeolites becomes still more difficult in view of the fact that conversions among different groups (chabazite → phillipsite) and different structures (three-dimensional framework → fibre) take place relatively easily.