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: \(\begin{gathered} 2{\text{F}}{{{\text{e}}}_{{{\text{host}}}}} + {{\left( {{\text{M}}{{{\text{g}}}_{{1 - n}}}{\text{F}}{{{\text{e}}}_{n}}} \right)}_{2}}{\text{Si}}{{{\text{O}}}_{4}} = 2n{{\left[ {{\text{FeO}}} \right]}_{{{\text{host}}}}} + {{\left[ {n{\text{Si}}{{{\text{O}}}_{2}} + 2n{\text{F}}{{{\text{e}}}^{0}} + \left( {1 - n} \right){\text{M}}{{{\text{g}}}_{{\text{2}}}}{\text{Si}}{{{\text{O}}}_{4}} + 2n{{v}^{{2 - }}} + 2n{{v}^{{2 + }}}} \right]}_{{{\text{ol}}}}}, \\ 2{\text{F}}{{{\text{e}}}_{{{\text{host}}}}} + {{\left( {{\text{M}}{{{\text{g}}}_{{1 - n}}}{\text{F}}{{{\text{e}}}_{n}}} \right)}_{2}}{\text{Si}}{{{\text{O}}}_{{\text{4}}}} = 2n{{\left[ {{\text{FeO}}} \right]}_{{{\text{host}}}}} + {{\left[ {n{\text{MgSi}}{{{\text{O}}}_{3}} + n{\text{F}}{{{\text{e}}}^{0}} + \left( {1 - n} \right){\text{M}}{{{\text{g}}}_{{\text{2}}}}{\text{Si}}{{{\text{O}}}_{4}} + n{{v}^{{2 - }}} + n{{v}^{{2 + }}}} \right]}_{{{\text{ol}}}}}. \\ \end{gathered} \) 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 \({\text{ + }}{{v}^{{2 + }}}\)) 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.