Clinoptilolite-rich materials from widespread pyroclastic and epiclastic deposits of northern Sardinia were investigated to assess their cation exchange properties and to find the most reliable experimental method to determine their cation exchange capacity (CEC). The CECs were evaluated using a cross exchange method (CEM) and a batch exchange method (BEM). The CEM resulted in values 30–35% lower than the theoretical or expected CEC calculated from the chemical analyses of the clinoptilolite-rich materials. The BEM resulted in CECs 6–12% lower than the expected CECs. Various parameters, such as the grain-size of powders (<64 µm and 125–250 µm) and replacement cation (Na+, K+, Li+, Cs+, NH4+, Ca2+, Mg2+, Sr2+) were evaluated in order to optimize the cation exchange process, i.e. enhance complete exchange. The particle size did not affect the exchange process appreciably. The type of replacement cation had a substantial effect on the experimental CEC determined by the BEM. The NH4+ and Cs+ replacement solutions resulted in the best experimental CECs ranging between 75% and 94% of the theoretical CEC with NH4+ as the replacement cation and 79% and 88% of the theoretical CEC with Cs+ as the replacement cation. The exchange efficiency was also measured as a function of ammonium concentration in the replacement solution (0.50, 1.00, 2.00 and 3.00 M). Experimental CECs ranged between 94% (0.5 M NH4Cl solution) and 99% (1 M NH4Cl solution) of the theoretical CEC for one epiclastic rock sample and between 79% (3 M NH4Cl solution) and 87% (2 M NH4Cl solution) of the theoretical CEC for one pyroclastic rock sample.

To get a better understanding of the mechanisms of selective retention of cations by clay minerals, we have studied the physicochemical state and the location of immobilized cations. The approach developed is based on the concomitant study of the exchange isotherms and the compensating cations/clay structure interactions, using far infrared (FIR) spectroscopy. For that, cations are used as spectroscopic probes to characterize the selective sites.

Exchanged K, Rb-, K, Cs-, K, NH4- and NH4, Cs-Llano vermiculite samples were prepared. The exchange isotherms illustrate the higher selectivity of Llano vermiculite in the following order: K+ < Rb+ < Cs+. Desorption experiments show that a 2 N MgCl2 solution can extract a small fraction of immobilized Cs+. The concomitant analyses of the exchange phenomena at microscopic (X-ray diffraction) and molecular (far and middle IR spectroscopy) levels show that: (1) only ∼65% of the interlamellar hydrated Mg2+ of Llano vermiculite is exchanged; (2) cations are randomly distributed in the interlamellar spaces; and (3) retention is strongly related to the distance between compensating cations and oxygen atoms of the ditrigonal cavity.

The low hydration energy of selectively retained cations induces strong cation/clay interactions, which give FIR absorption bands of compensating cations. The FIR absorption bands of smaller cations shift towards lower wavenumbers when the proportion of the larger cations increases, whereas the wavenumber of the larger ones is constant. This reproducible scenario shows that larger cations act as wedges and expand layers, thus increasing the distance between the smaller cations and the layers. Calculation of the distances dM-O inner and dM-O outer shows that selectively-retained cations are six-coordinated in these dehydrated systems. The decrease of the difference between dM-O outer and dM-O inner from K+ to Cs+ may explain the observed selectivity of Llano vermiculite.

Sedimentary zeolite occurrences are widespread in Central and Western Anatolia, Turkey. Erionite occurrences in Central Anatolia have significant health implications for inhabitants of the region. The widespread occurrences of zeolites are generally associated with volcano-sedimentary rocks and consist of low-temperature forms. The aim of the work was to define specifically the formation mechanism and chemical characteristics of these volcano-sedimentary deposits, and particularly, the stability conditions for erionite. The first step was to construct chemical potential diagrams and calculate thermodynamic data for erionite and Ca-saponite. Then, equilibrium activity diagrams were calculated for the zeolites and related minerals in the system of Ca-Na-K-Mg-Fe-Al-Si and H2O. Stability diagrams for log [aCa2+/(aH+)2] — log [aNa+/aH+] and log [aCa2+/(aH+)2] — log [aK+/aH+] for various saturation phase activities of Al3+ and SiO2 (aq) were plotted for sedimentary conditions. The coexisting phases and chemical characteristics of the each deposit were evaluated by examination of the activity diagrams. Deposits which do not include some of the common sedimentary zeolites, possibly have high Al3+ activity (equal to or greater than gibbsite saturation) or low SiO(aq) activity (less than quartz saturation) during formation. In addition, erionite was found to be very sensitive to the alkalinity of the system and is stable in only a limited range of thermochemical conditions.