Characterizing the degree of aqueous alteration in a fresh sample of Mukundpura CM chondrite fall using ATR-FTIR and TGA

A. Dixit, R. P. Tripathi, Sudhanshu Kumar, Mohd. Azaj Ansari, K. Sreenivas

Version of Record online: 27 July 2022


“Fourier transform infrared (FTIR) measurements on immediately collected Mukundpura show the typical feature for phyllosilicates around 10 μm, corresponding to Si-O stretching mode in silicate, and its broadness signifies the amorphous or poorly crystalline silicates. The absence of the 11.2 μm feature (a characteristic of anhydrous silicate olivine) and the weight loss observed in thermogravimetric analysis (TGA) imply aggressive aqueous alteration, which resulted in phyllosilicate formation at the expense of primary anhydrous silicates. It is consistent with Mössbauer spectra, showing the presence of both Fe2+ and Fe3+ in phyllosilicates, but no characteristic peak for olivine is observed, suggesting the major fraction of primary silicates are aggressively altered due to the presence of water on the parent body, and now major lithology must be highly altered. TGA measurements were carried on it (i) within 24 h and (ii) after 30 months of its fall. In both cases, the weight loss was ∼10% in the 400–770 °C temperature range, confirming the absence of any environmental impact on the water bound to the hydrated clay in Mukundpura samples. Appreciable weight loss in 400–770 °C indicated the presence of hydrated clay that corroborated FTIR measurements and ruled out any thermal event suffered by its postaqueous alteration, consistent with amorphous or poorly crystalline silicate phase observed in FTIR. When we couple the results of the present study and already reported results by our group on the same Mukundpura fragment, it is inferred that our sample has suffered a very high degree of aqueous alteration on the parent body. The fingerprint ratios, which are extensively used to correlate or assign petrological subgroup, are FeO/SiO2, hydrous silicate/anhydrous silicates, and MgO/FeO, which are either considered alone or in combination, and for Mukundpura, the values for these ratios are 1.05, 7.2, and ∼0.60, respectively. These values indicate that the major lithology of Mukundpura fresh fragment must be assigned as CM2.1.”