Chemical identification of comet 81P/Wild 2 dust after interacting with molten silica aerogel

Flight aerogel in Stardust allocation C2092,2,80,47,6 contains percent level concentrations of Na, Mg, Al, S, Cl, K, Ca, Cr, Mn, Fe, and Ni that have a distinctive Fe- and Cl-normalized distribution pattern, which is similar to this pattern for ppb level chemical impurities in pristine aerogel. The elements in this aerogel background were assimilated in non-vesicular and vesicular glass with the numerous nanometer Fe-Ni-S compound inclusions. After correction for the background values, the chemical data show that this piece of comet Wild 2 dust was probably an aggregate or small (<500 nm) amorphous ferromagnesiosilica grains with many tiny Fe,Ni-sulfide inclusions Plus small Ca-poor pyroxene grains. This distinctive Fe- and Cl-normalized element distribution pattern is found in several Stardust allocations. It appears to be a common feature in glasses of quenched aerogel melts but its exact nature is yet to be established.