Impact metamorphosed compositions of the Fe-Si-Ni-S system

The characterization of impact materials is discussed from the point of vie w of compositional change after an impact event. Various compositions of im pact-metamorphosed materials can be explained by vapor condition found as ( a) reduction phase with vaporized phases, (b) melted condition as mixed amo rphous grains of projectile and target materials, and (c) high-pressure con ditions as solid phases of target materials. The composition of projectile materials is largely changed under different impact conditions. In fact, Fe -Ni metal phases of a projectile cannot be obtained as the same phases afte r impact by mixing with target materials, which are found in fine-grained o r spherulitic particles in meteoritic impact craters or the Cretaceous-Tert iary (K/T) boundary. This is mainly because Fe-Ni phases formed by long ann ealing time within the closed system of asteroids cannot recrystallize as a single metallic phase after a short impact reaction. (C) 1999 Published by Elsevier Science S.A. All rights reserved.