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

Abstract

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.