Abstract Glass samples from alkali-trachytic pumice of the Lower Member (LM, level LM1) and Upper Member (UM) of the Neapolitan Yellow Tuff at Campi Flegrei Caldera were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spec- troscopy. The EPR data show that the relative occupancy of the different iron sites varies in samples from the Lower Member LM1 and Upper Member of the eruptive sequence. The LM1 glasses are characterized by a more homogeneous distribution of the cationic site population of iron compared with UM and by a different Fe 3+ /Al 3+ substitution in the C 2v symmetrical sites of the silicate framework. 29 Si MAS NMR spectra on glasses indicate that LM1 glasses are more depolymerized than UM glasses, indicating the existence of structurally distinct magma batches. 27 Al MAS NMR spec- tra indicate the occurrence in UM glasses of aluminum in both tetrahedral and octahedral coordination; the observed Al VI is believed to be a primary feature of the glass, since 27 Al CP-MAS NMR experiments do not reveal detectable esa-hydrated Al. The Al in sixfold coordination could be ascribed either to the more abundant alkali–OH termina- tions in UM, in which alkalies would be extracted from their charge-balance role for Al, thus altering its fourfold structural position, or to permanent compaction during fast cooling and decompression of high-pressure melts. 23 Na CP-MAS NMR experiments indicate the existence of Na–OH groups in both LM1 and UM glasses, relatively more abundant in the latter. Although preliminary, the re- Editorial responsibility: S. Carroll R. Petrini (✉) · G. Contin Dipartimento di Scienze della Terra, Universita’ di Trieste, via Weiss 8, I-34100 Trieste, Italy C. Forte · C. Pinzino Istituto di Chimica Quantistica ed Energetica Molecolare, CNR, via Risorgimento 35, I-56126 Pisa, Italy G. Orsi Osservatorio Vesuviano, via Manzoni 249, I-80123 Napoli, Italy Bull Volcanol (1999) 60:425–431 © Springer-Verlag 1999 ORIGINAL PAPER R. Petrini · C. Forte · G. Contin · C. Pinzino · G. Orsi Structure of volcanic glasses from the NMR-EPR perspective: a preliminary application to the Neapolitan Yellow Tuff Received: 1 July 1998 / Accepted: 1 October 1998 sults indicate the possible application of atomic-scale stud- ies to variables that determine the regimes of explosive vol- canism. Key words Nuclear magnetic resonance · Electron paramagnetic resonance · Volcanic glasses Introduction Silicate glasses are of great interest for earth scientists, be- cause they are assumed to freeze in the structure of silicate melts. Most of the physicochemical and rheological proper- ties of silicate melts, such as the conductivity, density, vis- cous flow, elemental partitioning and tracer self-diffusion, which are of primary importance in understanding magmat- ic processes, are dictated by silicate speciation in the melt structure (e.g. Mysen 1990). In particular, the structure of glasses in the products of explosive volcanism retains im- portant information on the dynamics of magma ascent and the different eruptive styles, including magma fragmenta- tion and fluid-melt interactions. Magnetic resonance tech- niques, such as nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), allow a direct de- termination of the local chemical environment of magneti- cally active nuclei, such as the isotopes 29 Si, 27 Al and 23 Na, and paramagnetic species, such as Fe 3+ or ions of most of the -d and -f transition elements. Several magnetic reso- nance studies have been performed on synthetic Al-silicate glasses under laboratory-controlled conditions (e.g. Stebbins 1995) and furnish the basis for the interpretation of spectra for glasses of natural composition. The NMR analyses of complex systems, such as natural igneous glass- es, commonly yield poorly resolved linewidths, since there is a relatively broad range of silicon and aluminum environ- ments (e.g. bond angles, bond lengths). Nevertheless, im- portant information can be obtained from these techniques on the structural properties of silicate melts. We report on the attempt to apply NMR and EPR to vol- canic glasses separated from pumice and pumice fragments