Chapter 14.2 - Parallels and Distinctions Between Clay Minerals and Zeolites

Abstract

Abstract Zeolites and clay minerals, especially smectites, share many properties, including large cation exchange capacities; open, low-density structures; reversible hydration and dehydration; and their common fine-grained occurrence. The last 30 years saw a greatly increased realization of the importance of zeolites in natural systems. For example, the zeolites clinoptilolite/heulandite, laumontite, phillipsite, erionite, chabazite, mordenite, and analcime are now known to occur in large amounts throughout the world. In spite of the similarities between clay minerals and zeolites, large differences in their structures give rise to significant property distinctions. Clay minerals have two-dimensional layer structures, often disordered in the stacking direction, with weak bonding between individual layers. Exchangeable cations and water molecules occur between individual layers. In contrast, zeolites have three-dimensionally connected framework structures with large extra framework sites occupied by exchangeable cations and water molecules. The hydration state of both classes of minerals is directly dependent on the partial pressure of water (relative humidity) around the materials. Some clay minerals, such as smectites, can undergo essentially infinite swelling in water, a process known as osmotic swelling. In contrast, zeolites have a fixed upper hydration limit and undergo only limited structural expansion on hydration. Many of the properties related to hydration and dehydration of smectites are distinctly quantized or stepwise, as a result of hydration occurring in somewhat discrete ‘layers’ of water molecules. Although zeolite hydration is often much more regular, the nature of the bonding between water molecules, exchangeable cations, and the zeolite structure means that many zeolites contain water molecules with a range of very different energetics, giving rise to gradational, multi-step hydration behaviour. In addition, partially dehydrated zeolites can have very large hydration enthalpies, often on the order of two to three times the enthalpy of condensation of water, compared with values for smectite less than two times that of water. Thus, many natural zeolites, more so than clay minerals, have the potential to be hydrated in environments with very low water activities. The large structural changes accompanying hydration and dehydration of smectites give rise to hysteresis, but this is typically observed only for those zeolites exhibiting hydration/dehydration-induced structural transitions, such as laumontite. All of these differences between smectites and zeolites impact the manner in which these two classes of minerals interact with water in the environment.