Rocky road to confusion: Raman tricks geochemists

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  • Published: Sep 1, 2011
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Rocky road to confusion: Raman tricks geochemists

Piqued too soon?

Raman spectroscopy has become an increasingly useful tool in the geosciences partly because it is non-destructive and requires minimal sample preparation, but also because it can be carried out on site with various irreplaceable geological samples. Unfortunately, Raman can confuse, especially when substances such as haematite and disordered carbonaceous materials are present in the same sample.

In March, SpectroscopyNOW reported how Raman had led to confusion regarding evidence for microbial fossils in ancient rock. Now, Craig Marshall and Alison Olcott Marshall of the Department of Geology, University of Kansas, Lawrence, USA, the team involved in the revelations have issued a general warning to the geological sciences community regarding the use of the powerful, yet occasionally ambiguous, analytical technique of Raman spectroscopy. For instance, they point out that the geological literature is rather confused when it comes to distinguishing between the haematite vibrational mode at about 1320 wavenumbers and the D band at 1340 wavenumbers of the disordered sp2 carbon atoms in carbonaceous materials.

This issue is further compounded, they say, because geologists often collect two separate spectra, one what they refer to as the mineral finger print region (200 to 800 wavenumbers) and a second spectrum in the carbon first-order region (1000 to 1800 wavenumbers), instead of simply carrying out a complete scan of the whole spectrum. This, the team says often leads to the misclassification of the haematite mode as the carbonaceous D band. They suggest that "best practice" would be for geologists to collect spectra between 200 and 1800 wavenumbers so that they might better distinguish between haematite and disordered carbonaceous material.

What a difference a ray makes

Brown-black carbonaceous material is often present alongside the equally brown-black haematite in rock samples, silica-rich sedimentary rock and cherts. Previously, researchers had claimed that black-brown microstructures found in cherts of about 3.5 billion-years-old represented the oldest evidence of fossil bacteria. However, Marshall and colleagues demonstrated that these structures were nothing more interesting than haematite and were not carbonaceous material of organic origin at all. The earlier work first highlighted the need to be cautious in the interpretation of restricted spectroscopic evidence.

Given that the two materials - haematite and carbonaceous materials - both appear as opaque, black materials but have very different origins, it is important to have a simple analytical way to distinguish the two. Following the Marshall suggestion of scanning the full available Raman spectrum precludes such classification errors. Given that Raman is becoming more widely used by non-specialists and the instrumentation increasingly automated and database driven the warning is rather timely. After all, instrument vendors themselves have misclassified these two categories of material just as have researchers in the published literature, while many of the popular Raman fingerprint databases do not even cite the haematite band.

The number of papers giving Raman spectra increased from around 1500 in the early 1990s to twice that by the turn of the millennium, by 2009 there were almost 6000 papers. Without casting aspersions on the authors of any of those papers, it nevertheless remains unlikely that the number of experts in the niche has quadrupled in two decades.


The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Adapted from Marshall et al/Elsevier Spictrochim Acta. Raman spectroscopy has become an increasingly useful tool in the geosciences partly because it is non-destructive and requires minimal sample preparation, but also because it can be carried out on site with various irreplaceable geological samples. Unfortunately, Raman can confuse, especially when substances such as haematite and disordered carbonaceous materials are present in the same sample.
Mixed Raman message

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