Raman spectroscopy as a tool for the non-destructive identification of organic minerals in the geological record

Organic minerals can be considered as a forgotten group of organic components in the geological record. They are salts of organic acids originating directly in plant metabolic processes, pure crystalline terpenes formed from precursor conifer decomposition products and polycyclic aromatic hydrocarbons originating in the accumulation of fractions generated by natural high temperature distillation from specific precursors. The presence of some of these minerals in Quaternary or Tertiary continental sedimentary environments in soils and peats is known, as are rare occurrences in lignite, shales and other rocks from older geological formations. Raman spectral signatures have been identified for mellite (the hydrated aluminium salt of benzene hexacarboxylic acid), the terpenoids fichtelite (norabietane) and hartite (α-phyllocladane), as well as for idrialite, a complex polycyclic aromatic hydrocarbon mineral. The survival of organic minerals such as oxalates, terpenoid hydrocarbons and aromatic minerals in the geological record is important for an appreciation of the diversity of molecular compounds that need to be assessed in the search for life detection signatures in terrestrial and extraterrestrial scenarios.