Preservation of biosignatures in opal probed by infrared nanospectroscopy

Opal (SiO2·nH2O) is a common alteration mineral formed at the subsurface of planets that have experienced liquid water. Occasionally, opals found on Earth exhibit no discernible trace of biological remnants, but display discrete infrared signature of organic matter (OM), possibly originating from life (i.e. biosignature). This may suggest another type of interaction between OM and abiotic opal, distinct from silica fossilisation. To address this, we investigated two samples using μFTIR and AFM-IR: 35 Myr pink opal from Quincy, France, and 550–1700 Myr black opal from Volyn, Ukraine. The FTIR results reveal characteristic stretching vibration of aliphatic (CH2 and CH3) in the 2800–3000 cm−1 range in both opals. The AFM-IR spectra highlight organic signatures at 1450 cm−1 (CHx), 1620 cm−1 (C=C of aromatics) and 1720 cm−1 (C=O of carbonyls). Moreover, the micron-scale IR maps, acquired by AFM-IR, reveal that the organics are clustered and located in the microporosity. Given these findings, we propose a new hypothesis regarding the occurrence of OM in opal which we attribute to a two-stage entrapment mechanism during opal formation.