Raman Spectroscopy as a Tool to Measure Silanol as Evidence of Water–Rock Interactions for Astrobiological Exploration

Because a range of silica minerals can precipitate from water, the analysis of silica mineral phases is important for astrobiological exploration. In this context, poorly crystalline opaline minerals that contain intracrystalline water are commonly accepted indicators of the presence of water in the geological past. However, opaline minerals are not the only silica phases that are evidence of past interaction with water. Water may become incorporated within crystalline quartz as silanol (Si-OH)-hydroxyl groups present as structural defects within a crystal lattice. Raman spectroscopy is a highly reliable method for detecting mineral composition, and it can also detect silanol. By analyzing Raman spectra from various silica gemstones and rocks, we found that 52 out of 71 quartz samples contain silanol. However, silanol was not universally present across all samples. Microcrystalline quartz and samples in which silica phases had replaced other minerals tended to display the highest levels of silanol, whereas macrocrystalline quartz exhibited the lowest values, as indicated by the Sil_prop parameter. In addition, we observed instances where quartz-hosted silanol and carbonaceous materials were codetected, which suggests the potential for Raman to be used to detect both carbonaceous organic matter and water, and therefore potential indications of both life and habitability.