Attenuated total reflection infrared (ATR-IR) spectroscopy of antigorite, chrysotile, and lizardite

Abstract

Attenuated total reflection infrared (ATR-IR) spectroscopy allows measurements to be made directly from the surface of one-sided, diamond polished thin sections of geological samples. This method greatly reduces the sample preparation time when compared to other IR spectroscopy methods and opens the possibility of using infrared spectroscopy to study thin-section scale microstructures. ATR-IR spectroscopy of antigorite, chrysotile, and lizardite in samples from the Mt. Shiraga serpentinite body, central Shikoku, SW Japan, reveals clear spectral differences in the 650-1250 cm-1 region associated with the vibration of the Si-O bonds in SiO4 tetrahedra and in the 3300-3750 cm-1 region associated with the vibration of the O-H bond in MgO2(OH)4 octahedra. A data-processing algorithm developed in this study allows the absorbance intensity and wavenumber of a particular absorbance peak to be used to create serpentine mineral phase maps based on the highest intensity Si-O absorbance bands for antigorite, chrysotile, and lizardite. Our methodology can be used to map serpentinite microstructures in thin sections illustrating the potential of ATR-IR as a relatively un-explored analytical tool in petrological studies. A combination of ATR-IR and electron microprobe data shows that for antigorite the wavenumber of the O-H absorbance band is correlated with the Fe content. Metamorphic reactions of serpentine minerals play a key role in the hydrodynamics of the earth's lithosphere, and the new information on serpentine mineral hydroxyl group behavior obtained by applying the technique outlined in this study are of great potential interest to researchers in a wide range of different fields.