1H, 29Si and 27A1 MAS NMR as a tool to characterize volcanic tuffs and assess their suitability for industrial application

The type and quality of the information provided by the direct analysis of volcanic tuffs by H-1, Si-29, and Al-22 NMR were investigated. At this aim, five tuffs, characterized by different origin, bonding mechanism, and clast composition, were used as test materials. Results consistent with the different nature of the tuff matrix and mineral composition were obtained. While the relative content of Al in the crystal and amorphous phase was determined by Al-27 MAS and 3Q MAS NMR, the prevalent glassy or zeolitic nature of the matrix was assessed by Si-29 and H-1 MAS NMR. Zeolites present at levels as low as 15% w/w were detected by Si-29 MAS NMR, and in some tuffs, identification of their framework type was performed together with the determination of the Si/Al ratio and, for the first time, of their configurational entropy. Data obtained were coherent with those provided by X-ray fluorescence (XRF), X-ray powder diffraction (XPRD), thermogravimetric analysis (TGA), differential thermal gravimetry (DTG), cation exchange capacity (CRC) determinations, and scanning electron microscopy, used in both backscattering imaging mode (SEM) and for elemental analysis (SEM-EDS). Results show that, under favorable conditions, solid state NMR techniques can provide a comprehensive view of the chemical and physicochemical behavior of a tuff. A combined use of these techniques is suitable for characterization of tuffs on a routine basis, and can be particularly useful to decide if a material is suitable for industrial applications.