Near infrared signature of opaline silica at Mars-relevant pressure and temperature

Opal is a mineral of great interest for tracing the aqueous Mars' history. Detection of opal on Mars is based on the near infrared (NIR) absorption bands related to the presence of water and hydroxyl. Because pressure and temperature can affect the amount and configuration of water in hydrated minerals, the associated absorption bands vary according to the environmental conditions at the surface of Mars. In this study, the effects of Mars' relevant surface pressure and temperature on opal's NIR signature was investigated. By exposing opal samples to pressures varying between 1 and 8 mbar, and temperatures between -51.1 and -96.5 degrees C, significant changes in opal samples' NIR features were observed. It was demonstrated that opal releases molecular water at low pressure, inducing changes on all NIR bands, as observed in previous studies. However, such dehydration was not systematic, as it was observed for only six out thirteen opal samples. When exposed to low temperature, water molecules in opal froze, inducing significant variation in shape and position of the bands at 5200 cm(-1) and 7000 cm(-1) (1.9 mu m and 1.4 mu m respectively). Low temperature experiments demonstrate that opal, and particularly opal-CT, can exhibit a water ice-like spectral signature. Such experimental data were compared with silica signature detected on Mars by CRISM and it was evident that martian opaline silica has a spectral signature specific of water ice, notably a shoulder near 5100 cm(-1) (1.96 mu m) and a wider 7000 cm(-1) (1.4 mu m) feature. We show that opal can retain water under martian conditions, an interesting property for sample return missions. (C) 2021 Elsevier B.V. All rights reserved.