EBSD Analysis of Iron‐Nickel Metal in L Type Ordinary Chondrites: 2. Formation of Net, Acicular, Duplex, and Pearlitic Plessite

Abstract

The microstructure of plessite, which is composed of kamacite (α) and taenite (γ), exhibits diverse formation mechanisms and is highly complex. In this study, four types of plessite, namely net, acicular, duplex, and pearlitic plessite, were investigated in L group chondrites using the Electron Backscatter Diffraction (EBSD) technique. This enabled comprehensive determination of their orientation relationships, evolution paths, and formation mechanisms. The findings from this study demonstrate that net plessite is formed due to shock reheating and is characterized by partial austenitization of kamacite with taenite filling between residual kamacite grains. Acicular plessite forms during cooling prior to reaching the α + γ region where silicate inclusions within taenite serve as intergranular nucleation sites for kamacite. Duplex plessite forms during the quenching event after shock reheating when the homogenous taenite becomes thermodynamically unstable, leading to either spinodal decomposition or nucleation and growth; meanwhile, duplex plessite is also formed through brittle fracture of kamacite. Pearlitic plessite forms upon reaching the eutectoid point where kamacite nucleates on grain boundaries and grows into surrounding taenite grains. The formation mechanism behind these four distinct forms of plessite provides valuable insights not only into the thermal evolution of L chondrites but also into other meteorites containing Fe‐Ni metal.