Can Raman Spectroscopy replace microprobe in OC chemical classification?

Introduction: The classification of the hundreds of ordinary
chondrites returned from Antarctic expeditions is an expensive
and time-consuming process. The classification of ordinary
chondrites is generally based on two steps [1]: (a) measurement
of the Fe content in olivine and low-Ca pyroxene for identifying
the chemical group (H, L, or LL) and (b) petrographic observations
for determining the petrologic type (from 3 to 6). Chemical
analyses are commonly performed with electron microprobe or
quantitative energy-dispersive x-ray spectroscopy with scanning
electron microscopy, by comparison with standards. The Fe content
is estimated by the calculated mineral formula and expressed
as fayalite (Fa) and ferrosilite (Fs) component for olivine and pyroxene
respectively. These analytical techniques require specific
sample preparation and special operation condition. We show
here that the use of Raman spectroscopy for the chemical classification
of ordinary chondrites, following the already existing
calibrations [2][3], preliminary improved with new data [4],
might be an alternative.

Methods: Raman spectra on selected grains were measured
with a confocal Raman microscope LabRAM HR Evolution
(HORIBA Scientific), with a solid-state laser corresponding to
green light (532nm). Spectra were processed with MatLab and
specific scripts were used for quickly identifying the characteristic
peaks. The composition of the selected grains was checked
with electron microprobe. Data from the online database Rruff
[5] and newly collected measurements from Type 3 ordinary
chondrites of the collection of Royal Belgian Institute of Natural
Science were used. Type 3 meteorites were selected for providing
a wide range of olivine and pyroxene composition. The obtained
calibration was blind tested on other ordinary chondrites of different
chemical group and petrologic type. Results were compared
with independent classification performed at the NIPR with
the traditional technique.

Results: The existing correlation between the Raman shift of
selected peaks of olivine and pyroxene and the corresponding Fa
and Fs content respectively was calibrated in the common range
covered by ordinary chondrites. Possible causes of error are associated
to the occurrence of shocked olivine and to the complexity
of the pyroxene spectrum. However, a statistically meaningful
number of analyses dramatically reduce the possibilities of mistakes.
The use of Raman spectroscopy for the chemical classification
of ordinary chondrites seems a reliable, easy and convenient
method, although for problematic samples (in the case of difficult
interpretation) the use of microprobe is still recommended.