Ferro and paramagnetic resonance studies of natural volcanic glasses: Teotihuacan obsidians

Magnetic resonance spectra of iron and manganese in black obsidians from Teotihuacan (Central Mexico) were investigated at 9 GHz (X band) and 35 GHz (Q band). The specimens were found to be magnetically very heterogeneous. The X band spectrum exhibited, at g=4.28, the classical paramagnetic signal of isolated high spin Fe(III) ions. At g=2.0 a ferromagnetic resonance occured, associated with magnetite-like particles previously characterized by Mössbauer spectroscopy. At Q band there are two signals, a superparamagnetic signal at g=2.00 and a weak hyperfine sextet, superimposed on the g≅2.00 signal, attributed to isolated Mn(II) free ions. At X band a strong grain-size dependence of the overall line shape was also observed. The temperature, frequency, and grain-size dependence of the spectral lines of these natural glasses indicate, in addition to isolated high spin Fe(III) and Mn(II) ions, the presence of superparamagnetic clusters of magnetite and a range order, in these obsidians, larger than the common short-range order of the glassy state.