Near infrared signature of opal and chalcedony as a proxy for their structure and formation conditions

Opal-A, opal-CT and chalcedony develop in various geological environments mainly through continental weathering and hydrothermal processes. Although some spectroscopic criteria already differentiate the structural varieties of silica, no criterion distinguishes the formation process of opaline silica. The originality of this study is based on a unique collection of 38 hydrous silica samples of different structures formed in various geological contexts and lithologies. This large and diverse sampling of silica emphasizes that several new spectroscopic criteria distinguish opal-A, opal-CT and chalcedony: the apparent maximum of the absorption at 5200 cm(-1), the area of the absorption of silanol groups as well as the area ratio of the 5200 and 4500 cm(-1) bands. Moreover, we observed that the shape of the molecular-water band, quantified by a new criterion developed here (concavity-ratio criterion, CRC), differentiates opals formed by weathering (CRC < 0.74) from opals precipitated by hydrothermal processes (CRC > 0.82), regardless of their structure (-A or -CT). This new method quickly assesses the geological conditions of opal formation (on Earth or other terrestrial planets) for which data are unclear or missing.