Magnetic resonance and magnetization studies of natural volcanic glasses: Lipari obsidians (abstract)

Magnetic resonance spectra of Fe³⁺ and Mn²⁺ ions in natural volcanic glasses, black obsidians from Lipari (Island of Southern Italy), have been investigated at 9 (X band) and 35 GHz (Q band). The spectrum in the X band is found to exhibit at g=4.3 the classical signal of isolated Fe³⁺ ions.¹ At g=2 it is found the superposition of a superparamagnetic signal and a ferromagnetic resonance arising from the small and not so small magnetite particles previously found.² It was also observed at X band a granular dependence of the g=2 line shape, similar to that observed in polycrystalline powders. At Q band there were two signals, one due to iron clusters at g=2 and a weak hyperfine sextet attributed to Mn²⁺ free ions. The magnetization for a given temperature (from 1.75 to 20 K) and very high magnetic fields (up to 150 kOe) shows the lack of hysteresis loops and the absence of remanent magnetization, both indicative of superparamagnetism. However, the magnetization curves do not superpose to a Langevin function L[M(H/T)], which reflects the presence of inhomogeneous ferrimagnetic magnetite particles.