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, single-crystal 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(Al10.55Si21.38)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 visible when 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 bending modes. 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 mid-infrared 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.