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.

Stoichiometric troilite (FeS) is a common phase in differentiated and undifferentiated meteorites. It is the endmember of the iron sulfide system. Troilite is important for investigating shock metamorphism in meteorites and studying spectral properties and space weathering of planetary bodies. Thus, obtaining coarse‐grained meteoritic troilite in quantities is beneficial for these fields. The previous synthesis of troilite was achieved by pyrite or pyrrhotite heating treatments or chemical syntheses. However, most of these works lacked a visual characterization of the step by step process and the final product, the production of large quantities, and they were not readily advertised to planetary scientists or the meteoritical research community. Here, we illustrate a two‐step heat treatment of pyrite to synthesize troilite. Pyrite powder was decomposed to pyrrhotite at 1023–1073 K for 4–6 h in Ar; the run product was then retrieved and reheated for 1 h at 1498–1598 K in N₂ (gas). The minerals were analyzed with a scanning electron microscope, X‐ray diffraction (XRD) at room temperature, and in situ high‐temperature XRD. The primary observation of synthesis from pyrrhotite to troilite is the shift of a major diffraction peak from ~43.2°2θ to ~43.8°2θ. Troilite spectra matched an XRD analysis of natural meteoritic troilite. Slight contamination of Fe was observed during cooling to troilite, and alumina crucibles locally reacted with troilite. The habitus and size of troilite crystals allowed us to store it as large grains rather than powder; 27 g of pyrite yielded 17 g of stochiometric troilite.

Shock metamorphism in ordinary chondrites allows for reconstructing impact events between asteroids in the main asteroid belt. Shock-darkening of ordinary chondrites occurs at the onset of complete shock melting of the rock (>70 GPa) or injection of sulfide and metal melt into the cracks within solid silicates (∼50 GPa). Darkening of ordinary chondrites masks diagnostic silicate features observed in the reflectance spectrum of S-complex asteroids so they appear similar to C/X-complex asteroids. In this work, we investigate the shock pressure and associated metamorphism pattern in rubble-pile asteroids at impact velocities of 4–10 km s⁻¹. We use the iSALE shock physics code and implement two-dimensional models with simplified properties in order to quantify the influence of the following parameters on shock-darkening efficiency: impact velocity, porosity within the asteroid, impactor size, and ejection efficiency. We observe that, in rubble-pile asteroids, the velocity and size of the impactor are the constraining parameters in recording high-grade shock metamorphism. Yet, the recorded fraction of higher shock stages remains low (<0.2). Varying the porosity of the boulders from 10% to 30% does not significantly affect the distribution of pressure and fraction of shock-darkened material. The pressure distribution in rubble-pile asteroids is very similar to that of monolithic asteroids with the same porosity. Thus, producing significant volumes of high-degree shocked ordinary chondrites requires strong collision events (impact velocities above 8 km s⁻¹ and/or large sizes of impactors). A large amount of asteroid material escapes during an impact event (up to 90%); however, only a small portion of the escaping material is shock-darkened (6%).