Chemical group identification in Ordinary Chondrites by Raman Spectroscopy

A large fraction of the meteorites collected worldwide is represented by ordinary chondrites. Most common classification techniques are based on bulk chemistry or on elements ratio of selected minerals and imply destructive or expensive and time-consuming analyses.Here we propose to use Raman spectroscopy for identifying the chemical group in ordinary chondrites. The chemical group can be determined by the ratio Mg/Fe in olivine and low-Ca pyroxene, which are common phases in chondrites. The ratio Mg/Fe is generally expressed as fayalite (Fa) and ferrosilite (Fs) content in olivine and low-Ca pyroxene respectively. The correlation between the position of selected peaks in the Raman spectrum of olivine and pyroxene and the relative content in Fa and Fs allows, therefore, the chemical classification of ordinary chondrites. The existing correlation has been here calibrated for ordinary chondrites, by investigating type 3 ordinary chondrites, which exhibit the largest range of olivine and pyroxene compositions among meteorites.Antarctic and North-Western Africa ordinary chondrites, provided by the Japanese National Institute of Polar Research and by the Royal Belgian Institute of Natural Science, were investigated in the form of polished thin section and polished chips. Raman spectroscopy was performed with a confocal Raman microscope LabRAM HR Evolution (HORIBA Scientific), equipped with a multichannel air-cooled CCD detector (spectral resolution <1 cm-1, lateral resolution 0.5 µm, axial resolution 2 µm), with a solid-state laser corresponding to green light (532 nm) at 1.25 mW excitation with 10% filter for preventing any sample damage, available at the Vrije Universiteit Brussel. The precise peak position was evaluated with an Octav/MatLAB script.Blind tests on samples with different chemical classification, weathering and shock stage confirm the potential of the techniques. However, the reliability of the method depends on the number of analysis, which should be statistically meaningful for the investigated petrologic type. Non-equilibrated chondrites can exhibit a large range of olivine and pyroxene composition and therefore induce difficulties in classification only by means of Fa and Fs content. Highly equilibrated chondrites have a narrow range of olivine and pyroxene composition, so that a small number of analyses suffice for their chemical classification.Due to the reduced sample preparation (no carbon coating and limited influence of the quality of polishing), the quick instrumental calibration (by checking a silicon wafer, no need of reference materials), and the fast data acquisition (tens of seconds-one minute per spectrum), Raman spectroscopy provides an alternative to microprobe for ordinary chondrite chemical classification. The chemical group can be, therefore, determined first by non-destructive Raman spectroscopy, allowing a preliminary fast screening and rough classification of most of ordinary chondrites in large collections.