Identification by Raman spectroscopy of Mg-Fe content of olivine samples after impact at 6 km s−1 onto aluminium foil and aerogel: In the laboratory and in Wild-2 cometary samples

Olivine, (Mg, Fe)2[SiO4], is a common mineral in extraterrestrial materials, whose Mg-Fe content varies from the end-members Forsterite (Mg2SiO4: denoted 'Fo') to Fayalite (Fe2SiO4: denoted 'Fa'), together with minor quantities of Ca, Cr, Mn and Ni. Olivine is readily identified by Raman spectroscopy, and the Mg-Fe content can be obtained by precise measurements of the position of the two strongest Raman peaks. Here we show that this is not only true for pristine and highly crystalline olivine, but also for grains which have undergone high pressure shock processing during hypervelocity impact. We demonstrate that there are subtle changes to the Raman spectra in grains impacted at 6.1 km s−1 onto aluminium foil and into low density aerogel. We quantify these changes, and also show that if no correction is made for the impact effects, the Fe:Mg molar ratio of the olivine can be significantly misinterpreted. This study was stimulated by NASA's Stardust mission to comet 81P/Wild-2, since freshly ejected cometary dust particles were collected (via impact) onto aluminium foil and into aerogel cells at 6.1 km s−1 and these samples are being investigated with Raman spectroscopy. We identify the residue in one Stardust impact crater on aluminium foil as arising from an olivine with a composition of Fo97-100.