Organic matter in opal: in situ investigation with non destructive techniques

Opal (amorphous silica, SiO₂.nH₂O) is a glassy and porous mineral formed by the aqueous alteration of silicate rocks through both weathering and hydrothermalism. Terrestrial and Martian observations, as well as fluid-rock alteration experiments, show that opal forms in various geological contexts (e.g., acidic volcanic traps, alluvial fans and deltas) and at different spatial scales (nanometers to kilometers). All these processes are related to planetary surface, involving near-atmospheric pressure and low temperature (0 to 200°C). By analogy with the Earth, such environments are also favorable to the presence and development of life forms.

Over geological time on Earth, several opal deposits formed in horizons containing ancient biological remains (e.g. wood, plant roots and seeds, vertebrate skeletons, etc.) that have been preserved by the silicification process. In the case of wood, several studies shown that this process does not seem to preserve significant amounts of OM, but rather act as a silica cast (Mustoe, 2023). Moreover, in these cases OM has never been visualized or qualified in-situ by detection methods such as spectroscopy but only observed after HF treatment (St. John, 1927) or roughly estimated by loss-on-ignition experiments (Mustoe, 2016).

In another hand, some opals (predominantly black) that do not exhibit evidences of macroscopic fossils, display discreet infrared signatures characteristic of organic matter (OM) (Banerjee & Wenzel, 1999; Herrmann et al., 2019). Such samples may indicate the existence of another type of interaction between OM and opal, distinct from the petrification of the dead organisms.

In order to document and characterize this OM-silica relationship, we conducted a non-destructive, in situ investigation using Micro-FTIR and AFM-IR on two samples from distinct localities where OM was suspected: pink opals from Quincy, France (found in 35 Ma-old lacustrine limestone) and black opal from Volyn, Ukraine (found in 1.5Ga to 550 Ma-old weathered magmatic rocks).

The FTIR and Micro-FTIR analyses indicate the presence of OM in both samples. The Quincy pink opal spectra show four well-defined peaks around 2855, 2880, 2930 and 2965 cm^-1 characteristic of the stretching vibration of ν_s(CH₂), ν_s(CH₃), ν_as(CH₂), and ν_as(CH₃). A spatial variations investigation show that the OM bond’s signature varies positively with the intensity of the pink color. The spectra of Volyn black opal show only two well-defined peaks around 2855 and 2925 cm^-1, characteristic of ν_s(CH₂) and ν_as(CH₂) respectively. These signatures are present in all black opal spectra with a similar relative intensity, indicating a diffuse and homogeneous distribution through the sample.

Micrometer-scale IR maps combined to topography images, both acquired by AFM-IR, reveal that the organics are clustered and localized preferentially in the micro- to nano-pores, at the pore-silica matrix interface in both opals. This spatial distribution suggests that the OM was trapped during the formation and deposition of the opals, rather than during a later event. This is supported the strong ability of silicic and polysilicic acids to bond with OM through the hydroxyl groups in solution, which is considered as a preliminary step in the petrifaction process of wood through templating (Leo & Barghoorn, 1976; Mustoe, 2023).

All these elements suggest that two processes are involved for the incorporation of OM in opal: 1) The precipitation of silica, leading to opal formation, may serve as a segregator of OM within the fluids through hydrogen bonding. 2) The mechanical deposition of silica nanograins and aggregated structure may serve as a local accumulator of OM.

Therefore, these observations make opal a promising candidate for preserving pristine organic matter, possibly related to life, over geological timescales on Earth and other planetary bodies, such as Mars, that experienced liquid water on their surface.

References:

Banerjee, A., & Wenzel, T. (1999). Black opal from Honduras. European Journal of Mineralogy, 11(2), 401‑408. https://doi.org/10.1127/ejm/11/2/0401

Herrmann, J. R., Maas, R., Rey, P. F., & Best, S. P. (2019). The nature and origin of pigments in black opal from Lightning Ridge, New South Wales, Australia. Australian Journal of Earth Sciences, 66(7), 1027‑1039. https://doi.org/10.1080/08120099.2019.1587643

Mustoe, G. E. (2016). Density and loss on ignition as indicators of the fossilization of silicified wood. IAWA Journal, 37(1), 98‑111. https://doi.org/10.1163/22941932-20160123

Mustoe, G. E. (2023). Silicification of Wood : An Overview. Minerals, 13(2), 206. https://doi.org/10.3390/min13020206