Raman Spectroscopy of Geological Varieties of Hematite of Varying Crystallinity and Morphology

Hematite, α-Fe₂O₃, is a common rock-forming mineral found in sedimentary, metamorphic, igneous rocks, as well as meteorites, and is ubiquitous on Mars. It is the most thermodynamically stable geological iron oxide mineral that forms under a variety of low- and high-temperature regimes, low- and high-pressure regimes, Eh and pH conditions. Hematite occurs in a variety of forms, varying in crystallinity, morphology, and texture, which include micaceous, massive, kidney ore (botryoidal), oolitic, rainbow, and rare bulk single-crystals. Despite the plethora of literature in the geosciences on the application of Raman spectroscopy to investigate iron oxide minerals, thus far, there is a paucity in the literature on the effects on the Raman spectra of these geologically formed different textural hematite varieties. Here, we investigated the effects on the Raman spectra of the whole variety of geologically formed hematite. Our results clearly demonstrate that biaxial plots between the ratio of the E_g/A_1g modes (410/225 cm⁻¹) and E_g/E_gL modes (410/294 cm⁻¹) can distinguish between micaceous, massive, kidney ore (botryoidal), oolitic, rainbow, and rare bulk single-crystal hematite varieties, with reasonably high correlation coefficients.