Raman, FT‐IR and XRD investigation of natural opals

Abstract

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

Pressure–induced crystallization of biogenic hydrous amorphous silica

“…5a) is almost identical to that of opal-A (Sodo et al, 2016). The broad band can be attributed to the Si-O-Si symmetric stretching mode (Sharma et al, 1981).…”

“…4). X-ray diffraction patterns of naturally occurring opals classified into opal-A also show only one broad peak (Sodo et al, 2016). Taking into consideration the results of FTIR and X-ray diffraction analyses, the diatom frustule used in the study is classified as hydrous amorphous silica.…”

A Review of the Classification of Opal with Reference to Recent New Localities

“…Conventionally, opals are classified into three types [10]: opal-A (further divided into opal-AG (opal) and opal-AN (hyalite)), opal-CT and opal-C [11,12]. For nearly 50 years, the classification of opals proposed by Jones and Segnit [1], based on X-ray powder diffraction (XRD), has been widely adopted, e.g., [13][14][15]. The pertinent features of the XRD classification are:…”

Recent advances in linear and non‐linear Raman spectroscopy. Part XI

“…Sodo et al used Raman, FT‐IR, and XRD to investigate 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 colour . Stangarone and co‐workers studied the Raman modes in Pbca enstatite (Mg 2 Si 2 O 6 ) and gave an assignment based on quantum mechanical calculations to interpret experimental results.…”

“…Four of these samples are the rare and precious fire opals, characterized by an intense red-orange colour. [145] Stangarone and co-workers studied the Raman modes in Pbca enstatite (Mg 2 Si 2 O 6 ) and gave an assignment based on quantum mechanical calculations to interpret experimental results. The calculated data show excellent agreement with the experimental data from literature with an absolute average difference of ∼5 cm −1 .…”