The processes that control the evolution and maturation of amorphous opal (opal-A) to opal-CT to quartz chert in deep sea sediments are not yet fully constrainted. Specifically, to what extent do in situ lithologies and geothermal temperatures contribute to the transformation of silica to more stable thermodynamic states? We use sediments from two ODP cores 795 and 799 retrieved from the Sea of Japan where these mineralogical transitions are documented to have occurred. We isolated the silica phases with a robust cleaning and physical separation procedure. To analyze the mineral state of the silica (i.e., whether it is amorphous opal, opal-CT, or chert) we use XRD, SEM/EDS, XRF, XPS, and FTIR. Here we present images and intepret authigenic mineral overgrowths and substitutions in those sedimentary environments where the abundance of amorphous opal relative to clay is poor, suggesting that that silica is converted to clay rather than opal-CT or chert. In environments where the initial abundance of of opal-A is rich, we observe the amorphous opal matures to opal-CT and chert (this happens in these sediments of ages ∼4 Myr and older).
