Origin and formation of iron silicide phases in the aerogel of the Stardust mission

Abstract

Suessite along with hapkeite and more Fe-rich iron-silicides up to Fe7Si2 formed near the entrance of aerogel track #44. These phases are ~100 nm, quenched-melt spheres, but the post-impact cooling regime was such that melt vitrification produced a polycrystalline mixture of Fe silicides and kamacite. The compositional similarities of the impact-produced Fe-Si phases and the Fe-Ni-S phases scattered throughout the aerogel capture medium strongly supports the idea that Fe silicides resulted from a reaction between molten Fe-Ni-S phases and aerogel at very high heating and cooling rates. Temperatures of around 1500 °C are inferred from the observed compositions had the silicide spheres formed at thermodynamic equilibrium, which seems unlikely. When the conditions were kinetically controlled, they could have been similar to those leading to the formation of solids with predictable deep metastable eutectic compositions in laboratory condensation experiments.