Spectroscopy of natural silica-rich glasses

Abstract

Natural-silica-rich glasses (impactites, tektites and obsidians) have been investigated with infrared (IR), Raman and optical spectroscopy. Comparison with artificial glasses with silica-rich and alkali-rich compositions is made. The vibrational data of these compounds are discussed in relationship with their structures, particularly with respect to (i) Si-O-Si bonding differences, (ii) SiO₄-ring arrangements, (iii) lattice disorder. IR spectra are strongly dependent on silica content: frequencies of the ν₃ and ν_D bands increase with the silica content. A general finding in the Raman spectra of tektites, is the relationship between silica, alumina, sodium contents and the presence of vibrational bands peaked at very specific energies. Raman spectra of Lybian desert glass (LDG), Darwin glass (DG) and vitreous silica are almost identical with the typical doublet at 440-490 cm⁻¹ whereas in tektites the band at 440 cm⁻¹ has relatively less pronounced doublet structure. A common character of Raman spectra of tektites and obsidians is the appearance of broad bands centered around 1000 and 1600 cm⁻¹ due to substitutions of silicon by metals. Tektites have a strong absorption band at 1100 nm which originates from Fe²⁺ ions. In the other glasses, this absorption is slightly shifted towards 1110-1130 nm. Additional sharp features of impactites and obsidians at 1380, 2210-2250 nm are completely absent in the absorption spectra of tektites. These bands are the signature of molecular water trapped inside the structure.