Ion exchange equilibria in a synthetic merlinoite

Merlinoite, a rare natural zeolite, was synthesized by hydrothermal treatment of a natural glass (rhyolitic pumice) with KOH solutions at 140°C and autogenous water vapour pressure. After chemical, X-ray and thermal characterisation, the ion exchange behaviour of merlinoite was investigated at 25°C, pre-exchanging it in Na+-form and studying the exchange equilibria in the presence of each of the following cations: K+, NH4+, Ba2+ or Ca2+. Thermodynamic parameters (equilibrium constant, Ka, and standard free energy of exchange, ΔG0), calculated with the help of a computer program, demonstrated merlinoite to be very selective for the former three cations unselective for the latter, in substantial accord with the behaviour of phillipsite, a zeolite having a X-ray diffraction pattern very close to that of merlinoite. A satisfying interpretation of ion exchange selectivity or unselectivity was, at last, worked out on the basis of a tentative cation location in the known extra-framework sites of merlinoite.