tokens: unexpected to condense from a gas cooling from high temperature (Grossman et al. 2012 ). The Fe content of these minerals is an increasing function of oxidation state. Even in an oxygen-rich condensing gas, these minerals are expected to be very Fe-poor, with fayalitic contents (=Fe/(Fe+Mg)) of no more than 4% (Grossman et al. 2012 ). Before the return of samples from comet Wild 2, it was thought that oxidation of crystalline silicates might have occurred within the parent bodies of meteorites, during aqueous alteration or thermal metamorphism. However, olivines returned from comet Wild 2 show a very broad and nearly flat distribution in fayalitic content from 0 to ∼40%, and a range of fayalitic content from 0% to 97% (Frank et al. 2014 ), so the oxidation appears to have taken place in a nebular environment, through an unknown mechanism. We note that all three collections of cometary materials have the potential to have suffered oxidation state change, or the appearance of oxidation state change through sample bias, between release from the parent body and analysis in the laboratory. Samples from comet Wild 2 were captured in the coma at 6.1 km s −1 , which may have led to reduction of Fe through smelting: the arrow in Figure 3 indicates that the original material may have been less reduced than observed in laboratory analyses. Cometary materials captured in the stratosphere may suffer from biases against metal-rich, high-density particles, because of their shorter residence time. This is consistent with the metal-poor composition of individual small CP-IDPs, in contrast to the relatively metal-rich composition of particles from giant cluster IDPs. However, none of these capture or sample bias effects go in a direction to reconcile cometary oxidation state with that of the ISM. 5.2. Summary A comparison of Fe L-edge absorption spectrum of continuum X-rays from Cygnus X-1 with extinction spectra derived from synchrotron X-ray microprobe analyses of mineral standards leads to the following conclusions: 1. Observations are consistent with the hypothesis that Fe is sequestered principally in metals, at least in this line of sight. The data are marginally consistent with Fe evenly divided between sulfide and metal, a composition that would be expected if Fe and S reside entirely in metal and sulfide, e.g., in a high-temperature condensation sequence. Less than 38.5% (2 σ ) of Fe resides in amorphous silicates. 2. The observations are inconsistent with the hypothesis of Ishii et al. ( 2018b ) that interstellar silicates consist of GEMS or their subcomponents as found in chondritic-porous IDPs. This conclusion is robust even if some Fe is oxidized between residence in the ISM and observation in the laboratory. 3. Fine-grained cometary material, as represented by FGM returned by the Stardust mission from comet 81P/Wild 2, with a WD01 size distribution predicts an extinction spectrum that is inconsistent with Chandra observations. 4. Our conclusions
ner_tags: O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Formula O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O B-CelestialObject I-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject I-CelestialObject O O O O B-CelestialObjectRegion O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O B-CelestialObject I-CelestialObject O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Mission I-Mission O B-CelestialObject I-CelestialObject I-CelestialObject O O B-Model I-Model I-Model O O O O O O O O B-Telescope O O O O