Infrared spectroscopic taxonomy for carbonaceous chondrites from speciation of hydrous components

Abstract

Abstract— Mid‐infrared absorption spectra for all types of carbonaceous chondrites were obtained in this study to establish a versatile method for spectroscopic classification of carbonaceous chondrites. Infrared spectra were measured using a conventional KBr pellet method and diamond press method. Spectra of hydrous carbonaceous chondrites exhibit intense O‐H stretching vibrations. CI chondrites are identifiable by a characteristic sharp absorption band appearing at 3685 cm−1, which is mainly attributable to serpentine. X‐ray diffraction analysis showed the presence of serpentine. However, Yamato (Y‐) 82162 (C1) does not have the band at 3685 cm−1 because of its thermal metamorphism. CM and CR chondrites have an intense absorption band at approximately 3600 cm−1. This absorption tends to appear in CM chondrites more strongly than CR chondrites because the intensity ratios of an OH stretching mode at 3520 cm−1 compared to 3400 cm−1 for CM chondrites are in the range of 0.95–1.04, which is systematically higher than those of CR chondrites (0.86–0.88). Therefore, the two types of chondrites are distinguishable by their respective infrared spectra. The spectrum feature of the Tagish Lake meteorite is attributable to neither CI nor CM chondrites. CO chondrites are characterized by weak and broad absorption at 3400 cm−1. CV chondrites have weak or negligible absorption of water. CK chondrites also have no water‐induced absorption. CH and CB chondrites have a sharp absorption at 3692 cm−1 indicating the presence of chrysotile, which is also supported by observations of X‐ray diffraction and TEM. The combination of spectroscopic classification and the diamond press method allows classification of carbonaceous chondrites of very valuable samples with small quantities. As one example, carbonaceous chondrite clasts in brecciated meteorites were classified using our technique. Infrared spectra for a fragment of carbonaceous clasts (<1 μg) separated from Willard (b) and Tsukuba were measured. The 3685 cm−1 band found in CI chondrites was clearly detected in the clasts, indicating that they are CI‐like clasts.