USING RAMAN SPECTROSCOPY TO INVESTIGATE CRYSTALLINITY VARIATIONS, MINERALOGICAL DIVERSITY, AND SP2 CARBONACEOUS MATERIAL WITHIN OOLITIC HEMATITE

Oolitic hematite, a distinctive marine sedimentary rock composed of chemically precipitated hematite (a-Fe₂O₃) spheres, presents significant interest due to its complex mineralogy and formation processes. This study builds on a preliminary investigation by our group^[1] which investigated hand samples from the Lower Silurian Red Mountain Formation that revealed a diverse mineralogy, including rhodochrosite, siderite, magnesite, and apatite, variations in hematite crystallinity, and the presence of sp² carbonaceous material. Our Raman spectroscopic analysis of thin sections confirms the presence of quartz, calcite, low Mg calcite and two distinct hematite populations within oolitic hematite: moderate crystallinity and poor crystallinity. The moderate crystallinity hematite is characterized by reduced intensity and line-broadening in the Raman spectrum, whereas the poor crystallinity hematite exhibits fewer and broader phonon modes, with a notably diminished 2LO mode at 1320 cm^-1. The disordered sp² carbonaceous material is identified by the presence of the D band at 1355 cm^-1, which becomes Raman active due to disorder in the sp² carbon network, and the G band at 1604 cm^-1. Our findings are significant as they represent the first detection of organic macromolecular materials within oolitic hematite, the first identification of two distinct hematite populations within oolitic hematite, and the first detection of low-crystallinity hematite and disordered carbonaceous material co-located in oolitic hematite. We postulate that the presence of organic macromolecular materials suggests that these organics may hinder crystal lattice alignment during hematite precipitation, leading to poor crystallinity. Thus, the co-location of disordered sp² carbonaceous material with low crystallinity hematite may indicate microbial involvement in the formation process, potentially serving as a biosignature. These investigations have important implications for the study of ancient sedimentary environments and the search for biosignatures in similar deposits on Earth and Mars.

[1] Dowling, A., Rufledt, C., Brande, S., and Marshall C. (2019) Raman spectroscopy of oolitic hematite: Evidence of biosignatures in iron oxide rich rocks. In GSA Annual Meeting in Phoenix. Arizona.