Chelyabinsk : a rock with many different (stony) faces: An infrared study

TL;DR

This study measures mid-infrared spectra of Chelyabinsk meteorite lithologies to identify spectral features and how shock processes alter these features, aiding remote sensing identification.

Contribution

It provides detailed spectral data for different lithologies of the Chelyabinsk meteorite and analyzes how shock affects their infrared spectral features.

Findings

Spectral features vary with lithology and shock level.

Shock darkening causes loss of characteristic spectral features.

Infrared spectral data can distinguish lithologies and shock effects.

Abstract

In order to provide spectral ground truth data for remote sensing applications, we have measured midinfrared spectra (2 to 18 micron) of three typical, well defined lithologies from the Chelyabinsk meteorite. These lithologies are classified as (a) moderately shocked, light lithology, (b) shock darkened lithology, and (c) impact melt lithology. Analyses were made from bulk material in four size fractions (0 to 25 micron, 25 to 63 micron, 63 to 125 micron, and 125 to 250 micron), and from additional thin sections. Characteristic infrared features in the powdered bulk material of the moderately shocked, light lithology, dominated by olivine, pyroxene and feldspathic glass, are a Christiansen feature (CF) between 8.5 and 8.8 micron; a transparency feature (TF) in the finest size fraction at about 13 micron, and strong reststrahlen bands (RB) at about 9.1 micron, 9.5 micron, 10.3 micron, 10.8 micron, 11.2 to 11.3 micron, 12 micron, and between 16 and 17 micron. The ranges of spectral features for the micro FTIR spots show a wider range than those obtained in diffuse reflectance, but are generally similar. With increasing influence of impact shock from pristine LL5 (or LL6) material (which have a low or moderate degree of shock) to the shock-darkened lithology and the impact melt lithology as endmembers, we observe the fading or disappearing of spectral features. Most prominent is the loss of a twin peak feature between 10.8 and 11.3 micron, which turns into a single peak. In addition, in the pure impact melt endmember lithology features at about 9.6 micron and about 9.1 micron are also lost. These losses are most likely correlated with decreasing amounts of crystal structure as the degree of shock melting increases.