Hydrated silica is found in a variety of martian deposits within aqueously altered mineral suites. Its common occurrence is attributed to its ease of formation in different weathering environments. Because of its presence in disparate units, hydrated silica makes a good tracer mineral to compare otherwise dissimilar martian deposits and relate their relative degrees of aqueous alteration. This work combines near-infrared and thermal-infrared spectroscopy to determine the relative degree of crystallinity and bulk SiO_2 abundance of surfaces containing hydrated silica. A range of crystalline structures are present, from non-crystalline (hydrated glass) to weakly crystalline (opal) to crystalline (quartz), implying a range in the maturity of these silica deposits. However, bulk SiO_2 contents show less diversity, with most martian hydrated silica deposits having SiO_2 abundances similar to Surface Type 2 (basaltic andesite or weathered basaltic composition)—a widespread and common surface composition that suggests limited interaction with water. We also find that hydrated silica crystallinity—as a proxy for degree of alteration—is correlated with the geochemistry of the deposit as inferred by its associated minerals: highly crystalline hydrated silica is found with Fe/Mg-phyllosilicates, moderately crystalline hydrated silica is associated with Al-phyllosilicates, and poorly crystalline phases are associated with sulfates. This corroborates previous predictions of the waning of surficial water from the Noachian → Hesperian and demonstrates the usefulness of hydrated silica as a stand-alone mineral for predicting the degree of alteration of ancient mineral suites
