RAMAN SPECTROSCOPY OF GEOLOGICAL VARIETIES OF HEMATITE OF VARYING CRYSTALLINITY AND MORPHOLOGY

Hematite, α-Fe2O3, is the most thermodynamically stable geological iron oxide mineral that forms under a variety of low and high temperature regimes on the Earth’s surface and within the crust. It is a common rock-forming mineral found in sedimentary, metamorphic, igneous rocks, as well as meteorites, and is ubiquitous on Mars. By and large, hematite occurs in a variety of forms, varying in crystallinity, morphology, and texture, which include: specular, micaceous, massive, kidney ore (botryoidal), oolitic, rainbow, and rare bulk single-crystals. It is well known that phonons arising from the first Brillouin Zone center, in the Raman spectrum of synthetic hematite, have been observed to vary in position and width due to the variation of crystallinity. Despite the plethora of literature in the geosciences on the application of Raman spectroscopy to investigate iron oxide minerals, thus far, only one study has observed effects on the Raman spectra of two different geologically formed hematite samples (Minitti et al., 2005). These researchers noted differences between the Raman spectra acquired on single-crystal hematite, hematite of an unknown origin, and synthetic hematite powder (Minitti et al., 2005). The major spectral differences that they observed were change in band position, breadth, and relative height. However, thus far, no researchers have investigated the effects on the Raman spectra of the whole variety of geologically formed hematite. To this end, in this work, we undertake a systematic approach to elucidate the effects of crystallinity, morphology, and texture of a variety of geologically sourced hematite samples on their Raman spectra, and hence to develop a Raman parameter to distinguish between hematite varieties, and additionally temperature regimes of formation. Here we show that biaxial plots between the ratio of the E_g/A_1g modes (245/225 cm^-1) and 1LO/2LO 660/1320 cm^-1, and external modes (lattice modes) E_gT/E_gL (245/294 cm^-1) and 1LO/2LO 660/1320 cm^-1, can distinguish between specular, micaceous, massive, kidney ore (botryoidal), oolitic, rainbow, and rare bulk single-crystal hematite varieties.

Minitti, M. E., Lane, M. D., & Bishop, J. L. (2005). A new hematite formation mechanism for Mars. Meteoritics & Planetary Science, 40(1), 55-69.