Aqueous Alteration Studies on Mukundpura (MK) Carbonaceous Chondrite using FTIR, TGA and Raman spectroscopy and its CM classification

Abstract

FTIR measurement on MK immediately after its fall, shows a unique doublet around 10 micrometer, significantly different from many ordinary CM2 chondrites, where only a singlet around 10 micrometer is observed. Also, a very faint 11.2 micrometer feature in MK indicates the absence of anhydrous silicates, olivine and thus complete serpentinization of anhydrous silicates due to severe aqueous alteration on parent body. Raman studies show a low peak metamorphic temperature around 0 {\deg}C and consistent with the absence of peak corresponding to tochilinite in FTIR spectrum, which forms at higher temperature. The first thermogravimetric measurement was carried out within 24 hrs. of MK fall, showing 10% weight loss in 400-770 {\deg}C range and is consistent with TGA on another MK fragment of same batch after 30 months, confirming no environmental impact on the water bound to hydrated clay. This large weight loss also rules out any post aqueous alteration thermal event suffered by MK and signify the presence of hydrated clay. The measured ratio of MgO/FeO is about 0.56, and sulfur weight is 3.4 %. Recently, based on only aqueous alteration Potin et al. (Potin et al. 2020), classified MK as CM1, implying MK should be utmost altered CM, whereas Ray et al. (Ray et al 2018), as equivalent to Paris like (CM2.7), least aqueous altered. However, if we combine the observed aqueous alteration, MgO/FeO weight ratio, and sulfur weight % together will provide a more comprehensive understanding for MK, and thus, we classify it as CM2.3.insted CM1 or CM(2.7-2.9).