Deformation Microstructure, Metallic Iron, and Inclusions of Hollow Negative Crystals in Olivine from the Seymchan Pallasite: Evidence of Fe2+ Solid-State Reduction

Olivine grains from the Seymchan pallasite were studied using optical microscopy, Raman spectroscopy, and scanning electron microscopy (SEM). Olivine is characterized by the presence of hollow straight channels <1 µm wide and inclusions of hollow negative crystals of prismatic habit 1–2 µm thick. The channels are oriented parallel to [001] of olivine and developed along [001] screw dislocations. The elongation axes of negative crystals are also oriented parallel to [001]. In the channels, hollow segments alternate with segments filled with metallic iron. Negative crystals are crystallographically faceted voids in olivine; the largest of them contain inclusions of metallic iron. The rectilinear configuration and crystallographic orientation of the channels correspond to the characteristics of [001] screw dislocations, which allows us to consider [001] dislocations as channel precursors. The data obtained demonstrate for the first time the evolution of [001] dislocations in olivine as a result of the shock-induced reduction of divalent iron during the interaction of olivine with the host FeNi metal. A model is proposed for the transformation of dislocations with the formation of channels and hollow negative crystals in Seymchan olivine in accordance with one of the reactions: 2Fe_host + ( Mg_1 - nFe_n)_2SiO_4 = 2n[ FeO]_host + [ nSiO_2 + 2nFe^0 + ( 1 - n)Mg_2SiO_4 + 2nv^2 - + 2nv^2 + ]_ol, 2Fe_host + ( Mg_1 - nFe_n)_2SiO_4 = 2n[ FeO]_host + [ nMgSiO_3 + nFe^0 + ( 1 - n)Mg_2SiO_4 + nv^2 - + nv^2 + ]_ol. According to the model, at T > 1000°C the reduction process is accompanied by an increase in the concentration of Fe0 and associated vacancies ( v^2 - and + v^2 + ) in dislocation zones. Voids in channels and negative crystals are the products of the annihilation of anionic and cationic structural vacancies having opposite charges. Phase association formed in this solid-state transformation of olivine corresponds to either OSI (olivine → SiO2 + 2Fe0) or OPI (olivine → pyroxene + Fe0) buffer equilibrium. The results can be used for the reconstruction of the thermal and shock histories of different types of pallasites.