Much more than a glass: the complex magnetic and microstructural properties of obsidian

Obsidian is a natural volcanic glass in which nanocrystalline and microcrystalline phases can coexist with the glassy one. In this paper, magnetic properties of Monte Arci obsidians are investigated by an experimental approach commonly applied to synthetic nanostructured materials and rarely to natural ones, and correlated with the mineralogical composition and microstructure. Among the different crystalline phases, the iron-containing components are found to be responsible for a great variety of magnetic behaviors, including paramagnetism, antiferromagnetism, ferromagnetism and superparamagnetism. The combined use of Powder X-Ray Diffraction (PXRD), 57Fe Mössbauer Spectroscopy, DC magnetometry and Transmission Electron Microscopy (TEM/HRTEM) provides new insights in the Monte Arci obsidian: (i) the presence of magnetite nanoparticles spread into the glassy matrix; (ii) the presence of an antiferromagnetic phase responsible for a discontinuity at about 45 K; (iii) exchange bias phenomena, for the first time revealed in obsidians, due to the coupling between the nanostructured ferrimagnetic phase and the antiferromagnetic one; (iv) Goldanskii-Karyagin effect (GKE) associated with biotite.