Deformation microstructure, metallic iron and inclusions of hollow negative crystals in olivine from Seimchan pallasite: evidence of solid-phase reduction of Fe2+

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 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: 2Fehost+ (Mg1−nFen)2SiO4 = 2n[FeO]host + [nSiO2 + 2nFe0 + (1 − n)Mg2SiO4 + 2nv2− + 2nv2+ ]ol, 2Fehost+ (Mg1−nFen)2SiO4 = 2n[FeO]host + [nMgSiO3 + nFe0 + (1 − n)Mg2SiO4 + nv2− + nv2+ ]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 (v2- and + v2+) in dislocation zones. Voids in channels and in negative crystals are products of the annihilation of anionic and cationic structural vacancies having opposite charges. Phase association formed in this solid-phase transformation of olivine corresponds to the either OSI (olivine → SiO2 + 2Fe0) or OPI (olivine → pyroxene + Fe0) buffer equilibrium. The results can be used for reconstruction of the thermal and shock histories of different types of pallasites.