Hematite and carbonaceous materials in geological samples: A cautionary tale

Over the last few decades Raman spectroscopy has been increasingly applied as an analytical tool in geoscience research. Raman spectroscopy is a powerful tool for geologists as it is non-destructive, requires little to no sample preparation, and can be undertaken in situ on various irreplaceable geological samples. Also, this technique is useful in the identification of minerals and geo-organic material. However, despite this ease of application, there are some facets of Raman spectroscopy data that can lead to erroneous interpretations. For instance, there is much confusion in the geological literature distinguishing the difference between the hematite vibrational mode at ca. 1320 cm−1 and the disordered sp2 carbonaceous material D band at 1340 cm−1. Furthermore, geologists will often collect 2 spectra, one in the mineral finger print region (200–800 cm−1) and then a spectrum in the carbon first-order region (1000–1800 cm−1), rather than performing a full-region scan. This allows the misidentification of the hematite mode at 1320 cm−1 as the D band from disordered carbonaceous material. Here we show that it is best practice for geologists to collect spectra between 200 and 1800 cm−1 to better distinguish between hematite and disordered carbonaceous material, materials that often co-occur in geological samples.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Over the last few decades Raman spectroscopy has been increasingly applied in geoscience research. ► The geological literature contains confusion between hematitic and carbon modes (ca. 1320 cm−1). ► Geologists will obtain 2 spectra, the mineral finger print region and the carbon first-order region. ► Thus, the presence of a hematite band at 1320 cm−1 can be interpreted as a false positive for carbon. ► It is best practice to collect spectra between 200 and 1800 cm−1 to distinguish between hematite and carbon.