Raman, FT-IR and XRD investigation of natural opals

Opals are naturally occurring hydrous silica materials (SiO2*nH2O), characterized by different degrees of crystallinity and crystal structure. Because of their optical properties, opals have been largely used in jewelry and as decorative elements in artworks. For this reason, a complete characterization and a provenance study of this kind of materials is mandatory in order both to avoid frauds and to reconstruct ancient and modern trade routes of gems. In this work, we present a combined spectroscopic (Raman, FTIR) and X-ray powder diffraction (XRD) investigation of nine opals from the main deposits around the world (Australia, Madagascar, Slovakia, Mexico, Honduras and Ethiopia). Four of these samples are the rare and precious fire opals, characterized by an intense red-orange color. Ethiopia, Honduras and Mexico opals showed spectra and diffraction patterns typical of Opal-CT, generally associated to volcanic genesis, while Australia, Madagascar and Slovakia opals are Opal-A type, associated to sedimentary origin. Unexpectedly the fire opal from Brazil behaves as a CT one. The presence of CO2 was detected only in the latter group, and exceptionally in the Honduras sample; FTIR-FPA imaging showed carbon dioxide to be homogeneously distributed inside the gems. Opals-CT are CO2-free and give much more complex FT-IR spectra in the NIR region where H2O combination modes occur. The obtained results are discussed in terms of relevance of the above experimental techniques for geosourcing opals, and contribute to increase the database of the chemical-physical properties of opals.