Geology and near infrared signature of opal: new data from synthesis

Opals are hydrated amorphous to poorly crystalline silica phases that form in a wide range of geological environments. Recent studies have shown that the shape of water-related absorption bands in the near infrared domain can be used as a potential proxy for deciphering opal genesis processes. However, the physicochemical parameters controlling the shape of the absorption, and thus its use as a criterion for genesis studies, have not yet been experimentally constrained. We examined whether temperature, pH, and the resulting microstructures independently influence the CRC (Concavity Ratio Criterion), which quantifies whether the low-frequency side of the absorption band is concave or convex. In this study, amorphous silica analogous to opal-A was synthesized under controlled laboratory conditions using the Stöber routine, varying in temperature (25-70 °C) and pH (≈0.6-12). The resulting materials were imaged by scanning electron microscopy to assess microstructural changes, and by near-infrared spectroscopy (4000-8000 cm^-1) to investigate the speciation and bonding state of water. The results show that temperature primarily influences particle size and size distribution, with only a minor effect on the Near InfraRed (NIR) spectral signature. In contrast, pH exerts a strong control on both silica structure and water speciation, leading to significant variations in CRC values. These experimental results demonstrate that the CRC is mainly governed by fluid chemistry rather than microstructure. The CRC therefore constitutes a robust proxy for constraining the chemical conditions of opal formation, with important implications for interpreting terrestrial deposits and remote sensing observations of hydrated silica on planetary surfaces, including Mars.