### General aspects

According to the recommended nomenclature for zeolite minerals (Coombs et al. 1998), there are more than eighty distinct zeolite species. Table 1⇓ (see Appendix below) is a listing of the zeolite minerals discussed in this chapter. Although zeolites are not as geologically abundant or widespread as many other silicate mineral groups, there is perhaps more interest in the crystal structures of zeolites than in any other mineral group, as evidenced by the number of reported crystal structure refinements (Table 2⇓).

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 Table 1. 
Alphabetical list of zeolites and zeolite-like minerals covered in this chapter with their, chemical formulas, space group, cell parameters, channel description, and FD (T/1,000A3). (Channel descriptions from Meier et al. (1996); cell parameters and chemical formulas from Gottardi and Galli 1985; and new data taken from this chapter.) Meier at al. (1996) use the following symbols for channel descriptions: bold numbers represent the number of tetrahedra defining the channel, the channel free diameter is given in A, the number of stars represent the number of channels in a given direction, the connectivity of channels is given by “|” to represent connected channels or “⇔“ to represent non-connected channels.



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 Table 2. 
Number of crystal structures reported in ICSD (1995) for selected mineral groups.



Oxygen and silicon are the two most abundant elements in the Earth’s crust, followed by Al, Fe, Ca, Na, Mg, K. Along with H, Ba, and Sr, these are also the major elements found in most zeolite minerals. In fact, many similarities can be drawn between the feldspars, the most abundant mineral group in the Earth’s crust, and zeolites. The two most important principles in understanding zeolites (or any mineral group) are that charge balance must be maintained (i.e. the sum of the formal valence charges of the ions in the chemical formula must …

Crystal Structures of Natural Zeolites

CCS, Carbon Capture and Storage, is considered a promising technology to abate CO2 emissions from point sources. The present review deals with the principle of post-combustion capture techniques, including thermal or pressure swing principles, adsorption or absorption, and electrical swing or membrane separation processes. Opportunities and challenges are assessed. In the first section of absorption processes, several commercial technologies are compared and complemented by the aqueous or chilled ammonia (NH3) process, and a dual or strong alkali absorption. The second section deals with adsorption where fixed beds, circulating fluidized beds and counter-current bed configurations will be discussed, with particular focus on the different adsorbents ranging from zeolites or activated carbon, to more complex amine-functionalized adsorbents, nanotubes or metal organic frameworks (MOFs), and alkali-promoted oxides. Thirdly, membrane processes will be analysed. The review will finally summarize challenges and opportunities. Several research groups confirmed that absorption is the most mature post-combustion capture process: among the assessment of post-combustion CCS, 57% apply absorption, 14% rely on adsorption, 8% use membranes, and 21% apply mineralization or bio-fixation. This conclusion was in-line with expectations since absorption gas separation has been largely applied in various petrochemical industries. All other systems need further development prior to large scale application.

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Post-combustion carbon capture

Electrical properties of nanoscaled host/guest compounds

The effect of NH3 on the ionic conductivity of dehydrated zeolites Na beta and H beta

Proton mobility in H-ZSM5 studied by impedance spectroscopy

New information is presented relating to the hydrothermal stability, lattice parameters, and adsorptive, electrical, and catalytic properties of synthetic and natural faujasite. Present concepts concerning the nature and relationship of synthetic and natural faujasite are restated to be consistent with the experimental evidence and the developed physical model. The major structural and physicochemical properties reflect the close similarities and smooth gradations expected of substitutional members of a continuous series. The existing division of the range (2–6) of SiO2/Al2O3 mole ratios (S/A) at 3 into two compositional subranges is shown to be unjustifiable and rather misleading. Individual compositions from these two subranges do not represent distinctly different zeolite species; instead, it is demonstrated that they are members of a continuous series with smoothly changing properties over the studied range of SiO2/Al3O3. Some of the properties of the natural mineral faujasite are found to be very similar to those of the synthetic analogs with the same SiO2/Al2O3 mole ratio.

Dennis N. Stamires

Papers

Properties of the Zeolite, Faujasite, Substitutional Series: A Review with New Data