Context.Shock-induced changes in ordinary chondrite meteorites related to impacts or planetary collisions are known to be capable of altering their optical properties. Thus, one can hypothesize that a significant portion of the ordinary chondrite material may be hidden within the observed dark C/X asteroid population.Aims.The exact pressure-temperature conditions of the shock-induced darkening are not well constrained. Thus, we experimentally investigate the gradual changes in the chondrite material optical properties as a function of the shock pressure.Methods.A spherical shock experiment with Chelyabinsk LL5 was performed in order to study the changes in its optical properties. The spherical shock experiment geometry allows for a gradual increase of shock pressure from ~15 GPa at a rim toward hundreds of gigapascals in the center.Results.Four distinct zones were observed with an increasing shock load. The optical changes are minimal up to ~50 GPa. In the region of ~50–60 GPa, shock darkening occurs due to the troilite melt infusion into silicates. This process abruptly ceases at pressures of ~60 GPa due to an onset of silicate melting. At pressures higher than ~150 GPa, recrystallization occurs and is associated with a second-stage shock darkening due to fine troilite-metal eutectic grains. The shock darkening affects the ultraviolet, visible, and near-infrared region while changes to the MIR spectrum are minimal.Conclusions.Shock darkening is caused by two distinct mechanisms with characteristic pressure regions, which are separated by an interval where the darkening ceases. This implies a reduced amount of shock-darkened material produced during the asteroid collisions.

A comparative analysis of structural and phase transformations in the Sikhote-Alin meteorite with different shock prehistory has been done: shards from the impact crater, conditionally unstressed individual fragments and samples after model experiments on spherically converging shock waves loading. This meteorite belongs to the class of coarse octahedrites, and Fe-Ni-Co alloy with P,S and N impurities. Techniques of optical and scanning electron microscopy with energy dispersive X-ray spectrometry (EDS) and electron backscatter diffraction (EBSD) analysis units have been used for provided information on temperature and deformation changes in extraterrestrial matter in case of impact of space and terrestrial conditions, as well as in case of shock-wave loading. The formation of specific structural elements of deformation in shards of meteoritical substance from an impact crater was observed: curved Neumann lines, fibrous structure, regions of localized flow in the alpha-phase kamacite Fe (Ni, Co) and traces of brittle localized destruction of rod-shaped phosphides (Fe,Ni)(3)P. Analysis of the behavior of an extraterrestrial substance during model explosive loading gave an opportunity to match microstructural changes occurring in the meteorite substance and factors accompanying shock-wave action. Localized flows and traces of ductile fracture of thin roaldite plates (FeNi)(4)N have been observed. Intensive generation of new twins in the near-surface zone of samples due to localization of deformation and increase of local pressure up to 20 GPa was detected. Systematization of structural and phase changes in an inhomogeneous substance of Sikhote-Alin meteorite depending on the intensity of the shock wave effects has been suggested.

The aim of this work is to investigate the spectral properties of the stony and stony-iron meteorites after shock-wave loading experiments, and after annealing. This kind of treatments provides the material which is analogous to the space-weathered materials. Investigation of the spectral properties on such materials allows understanding of the external structure and mineral composition of atmosphereless bodies by means of spectral properties. It was shown that the spectral properties of the meteorites clearly depend on treatment.

We have studied microstructural deformation-induced changes and phase transformations in the material of the Sikhote-Alin iron meteorite (IIAB) after loading by spherically converging shock waves. The results obtained by the method of electron backscatter diffraction, as well as the data of local chemical analysis unambiguously indicate the presence of regions experiencing polymorphic α → ε and ε → α transitions in the loaded sample.

Cooling rates of meteoritic iron–nickel alloys were estimated using a processing of morphological features of their structure. All data have been acquired with optical and scanning electron microscopy. Two types of cooling rates were obtained. Slow cooling during the formation of the meteoroid parent body determined the first type. The second type is a result of melt crystallization after a shock event.