A multi-methodological study of the (K,Ca)-variety of the zeolite merlinoite

Abstract

A multi-methodological study of the (K,Ca)-variety of the zeolite merlinoite from Fosso Attici, Sacrofano, Italy was carried out on the basis of electron microprobe analysis in wavelength dispersive mode, singlecrystal X-ray diffraction (at 100 K), Raman and infrared spectroscopy. The chemical formula of the merlinoite from Fosso Attici is (Na0.37K5.69)Σ=6.06(Mg0.01Ca1.93Ba0.40)Σ=2.34(Fe3+ 0.02Al10.55Si21.38)Σ=31.9O64· 19.6H2O, compatible with the ideal chemical formula K6Ca2[Al10Si22O64]·20H2O. Anisotropic structure refinements confirmed the symmetry and the framework model previously reported (space group Immm, a = 14.066(5),b = 14.111(5), c = 9.943(3) Å at 100 K). Refinement converged with four cationic sites and six H2O sites; refined bond distances of the framework tetrahedra suggest a highly disordered Si/ Al-distribution. The Raman spectrum of merlinoite (collected between 100 and 4000 cm-1) is dominated by a doublet of bands between 496-422 cm-1, assigned to tetrahedral T-O-T symmetric bending modes. T-O-T antisymmetric stretching is also observed; stretching and bending modes of the H2O molecules are only clearly visiblewhen using a blue laser. The single-crystal near-infrared spectrum shows a very weak band at 6823 cm-1, assigned to the first overtone of the O-H stretching mode, and a band at 5209 cm-1, due to the combination of H2O stretching and bendingmodes.Avery broad and convoluted absorption, extending from 3700 to 3000 cm-1 occurs in the H2O stretching region, while the ν2 bending mode of H2O is found at 1649 cm-1. The powder midinfrared spectrum of merlinoite between 400-1300 cm-1 is dominated by tetrahedral T-O-T symmetric and antisymmetric stretches. Raman and Fourier-transform infrared spectroscopy spectra of merlinoite and phillipsite provide a quick identification tool for these zeolites, which are often confused due to their close similarity