Spectrochimica Acta Part A 80 (2011) 133–137 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy j ourna l ho me page: www.elsevier.com/locate/saa Hematite and carbonaceous materials in geological samples: A cautionary tale Craig P. Marshall ∗ , Alison Olcott Marshall Department of Geology, University of Kansas, Lawrence, KS 66045, USA a r t i c l e i n f o Article history: Received 22 August 2010 Received in revised form 1 February 2011 Accepted 2 March 2011 Keywords: Raman spectroscopy Hematite Carbonaceous material Geoscience D band Hematite vibrational modes a b s t r a c t Over the last few decades Raman spectroscopy has been increasingly applied as an analytical tool in geo- science 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 inter- pretations. 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 sp 2 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. Published by Elsevier B.V. 1. Introduction Over the last few decades there has been a rapid growth in the utilization of Raman spectroscopic analyses in geoscience research. This technique has become a powerful analytical tool used by many geoscientists as it is non-destructive, provides chemical and struc- tural information on the micrometer scale (point analyses and imaging), little to no sample preparation, and can be undertaken in situ on thin sections, rock chips, drill core, hand samples and other geological materials. Additionally, Raman spectroscopy provides molecular–structural information on inorganic materials, particu- larly the covalently anionic groups present in minerals, and organic materials, ranging in size from simple small molecules to macro- molecules. One common application of Raman spectroscopy in the geo- sciences is the identification of optically similar material within a sample. For example, hematite and sp 2 carbonaceous material can co-occur within a sample and often appear as black opaque materials whose morphology can be hard or impossible to dis- tinguish [1]. Commonly brown-black carbonaceous material can co-occur with black-brown-reddish brown hematite in cherts, a fine-grained silica-rich microcrystalline or cryptocrystalline sedi- mentary rock. Recently, Marshall et al. [1] has demonstrated that black-brown-reddish brown microstructures in ∼3.5 Ga (Billion- year-old) cherts, identified as the oldest evidence of fossil bacteria ∗ Corresponding author. Tel.: +1 785 864 4974; fax: +1 785 864 5276. E-mail address: cpmarshall@ku.edu (C.P. Marshall). on Earth are composed of hematite and not carbonaceous material as previously reported [2]. Typically, Raman spectroscopy is used to examine the mineral fingerprint region (200–800 cm -1 ) and the carbon first order region (800–1800 cm -1 ) separately in order to identify these materials. However, these two spectral regions can- not be considered separately, due to the overlap of hematite and carbonaceous material vibrational modes around 1320–1350 cm -1 . In this paper, an examination of graphite, carbon-rich sediments and hematite minerals reveals the importance of collecting spec- tra between 200 and 1800 cm -1 to distinguish between hematite and carbonaceous materials and the interpretation of vibrational modes in this spectral region. 2. Materials and methods 2.1. Samples Raman spectra were collected on carbonaceous material and hematite within standard 30-m thick petrographic thin sections and isolated kerogen made from hand samples collected from the 3.5 Ga Apex Chert, and 3.4 Ga Strelley Pool Chert, Pilbara Craton, Western Australia. The thin section was prepared by cutting a thin sliver of rock from the hand sample with a diamond saw, then mounted on a glass slide and then ground smooth using progres- sively finer abrasive grit until the sample is only 30 m thick. Kerogen is the geochemical or operational term for the solvent and acid insoluble carbonaceous material in sedimentary rocks. The kerogen was prepared according to standard isolation meth- ods using hydrochloric and hydrofluoric acids [3]. Raman spectra 1386-1425/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.saa.2011.03.006