Opaline Silica and Amorphous Silicates in Extraterrestrial Materials: Mineralogical Evidence, Formation Pathways, and Aqueous Histories

Hydrated silica, multication amorphous silicates, and shock-produced silica glasses occur in diverse extraterrestrial materials. Although these phases may exhibit overlapping diffraction signatures, they differ fundamentally in composition, hydration state, and formation history. This review examines how opal-A, opal-CT, multication amorphous silicates, and shock-produced silica glass can be distinguished using composition, diffraction, vibrational spectroscopy, water speciation, and petrographic context, and surveys reported occurrences from chondrites and interplanetary dust to returned asteroid samples and Martian and lunar materials. The best-documented meteoritic case is nanoscale amorphous SiO2 interpreted as opal-A within ferrihydrite-bearing iddingsite veins in Nakhla, although the hydration of the silica itself has not been measured. Opal reported from the polymict ureilite EET 83309 remains a candidate supported only by conference reports. Ryugu and Bennu are dominated by phyllosilicates; no opal has been reported, but the published analyses were not designed to detect isolated nanoscale silica. At Bills Bay in Jezero crater, in situ Raman spectroscopy, supported by elemental measurements, provides strong evidence for opal-A and yields a calibration-dependent estimate of its hydration state. An evidence-graded classification separates measured from inferred hydration and direct observations from model-dependent inferences about the pH, redox state, and water-to-rock ratio. Future priorities are hydration measurements on the same nanoscale domains that are imaged, and volume-calibrated searches in curated samples.