RAMAN SPECTRAL FEATURES OF CHLORITE: A NEW CALIBRATION

Chlorite is a common hydrous clay mineral found in most geological environments. Its chemical composition is dependent on temperature, pressure, and bulk rock composition, which makes it an excellent geothermometer. The determination of chlorite composition is conventionally performed through electron microprobe analysis which is both timely and costly. Here, we demonstrate the capability of Raman spectroscopy technique to determine chemical composition of chlorite. Although Raman spectroscopy is primarily a technique for identifying minerals, chemical composition also influences Raman spectral features (e.g., by shifting characteristic Raman peaks). Recent studies have demonstrated that Raman spectroscopy can pick up on subtle cationic substitution in minerals.

In this contribution, we have correlated specific Raman spectra features with chemical composition estimated by electron microprobe analysis on 20 chlorite samples representing different geological environments (e.g., hydrothermal, diagenetic, metamorphic) and compositions. We propose a peak fitting procedure by decomposing each band into sub-peaks that allows analyzing the difference between chemical composition and the sub-peak parameters: peak positions, normalized relative intensities and peak widths.

Our results suggest that specific Raman chlorite peaks slightly, but quantifiably, change as a function of their chemical composition. We find that iron composition (apfu) is linearly correlated with the peak positions of four sub-peaks, but there is currently no clear relationship between silicon, aluminum or magnesium and any spectral features. Our results allow us to extract several empirical rules that will help geoscientists to quickly identify and characterize chlorite composition based on in-situ and micrometric Raman spectroscopic measurements. The results of this research are significant for planetary exploration by providing a calibration for future Raman instruments.