gems, quartz, chalcedony and opal, Opal-CT, Opal-A, Analytical, Near infrared, FTIR spectroscopy

Opal-A, opal-CT, and chalcedony form in diverse geological settings primarily via continental weathering and hydrothermal activity. While existing spectroscopic criteria can distinguish between silica structures, none previously identified the formation processes of opaline silica. This study introduces a novel approach using a comprehensive set of 38 hydrous silica samples of varying structures and geological origins. It establishes new spectroscopic markers—such as the apparent maximum absorption at 5200 cm⁻¹, the silanol group absorption area, and the area ratio between the 5200 and 4500 cm⁻¹ bands—to differentiate between opal-A, opal-CT, and chalcedony. Crucially, the study introduces a new parameter, the concavity-ratio criterion (CRC), to characterize the shape of the molecular-water absorption band, effectively distinguishing opals formed by weathering (CRC < 0.74) from those formed hydrothermally (CRC > 0.82), regardless of structural type. This innovative method offers a rapid means of inferring the geological formation environment of opals, both on Earth and other terrestrial planets.