Hydrated silica minerals are elusive products of aqueous alteration of silicate rocks on Earth. When observed in their geological contexts, these minerals allow tracing the silica cycle which in turn provides access to the history of liquid water. Among the many paleo-environments that their study yields access to, some have exobiological implications because silica minerals interact with the carbon cycle. They are known to enduringly host organic matter and are observed to host the oldest fossils on Earth. Silica minerals are therefore suspected for Mars to represent prime exobiological targets, a potential which has barely been tapped into. Silica minerals have only recently been found at Mars, and the origin of most deposits remain unexplained to date. Due to geologic processes and a global biosphere, non-biogenic formation pathways of silica on Earth are rare and remain poorly understood. By contrast, Mars is likely more suitable than Earth to exploit the potential of silica minerals to record the early paleo-geochemical states in which they formed. PALEOSILICA proposes a new holistic approach to silica minerals for Mars and Earth. We will bring a broad range of expertise and technical skills together, with experts in geology, mineralogy, planetology, remote sensing, geochemistry and astro-chemistry. We will study silica minerals from the microscopic scale of their internal structure, of their interaction with organic matter and searching for bio-signatures, to the geological scale of planetary evolution. This will be achieved by conducting: a) remote infrared hyperspectral data analyses of Earth and Mars; b) field studies for geological understanding; c) novel geochemical and thermodynamic modeling; and d) lab experiments on the precipitation of hydrated silica polymorphs with or without incorporation of planetary analog organic matter. This multidisciplinary approach will address 3 science objectives: 1) Exploit the known capability of silica minerals to record water-rich environments on Earth to gain knowledge on Early Mars geology; 2) Qualify the potential of past silica-rich mineral locales to have been habitable, then to have sequestered and preserved the long sought-after organic matter of Early Mars; 3) Refine knowledge of primitive Earth environments, because early Mars is considered the best available proxy to Hadean and Archean Earth, the knowledge gained at Mars will in turn tie back to our own planet. This project will yield a new framework on which to ascertain the merit of silica minerals for the study of ancient environments on Mars, and in turn on Early Earth. Of particular timeliness, our proposal will both contribute to the science exploration of Mars and benefit from it.