The shape and composition of interstellar silicate grains ”

In the paper entitled ”The shape and composition of interstellar silicate grains” (A&A, 462, 667-676 (2007)), Min et al. explore non-spherical grain shape and composition in modeling the interstellar 10 and 20 μm extinction features. This progression towards more physically realistic models is vitally important to enabling valid comparisons between dust observations and laboratory measurements. Min et al. proceed to compare their model results with GEMS (glass with embedded metals and sulfides) from interplanetary dust particles (IDPs) and to discuss the nature and origin of GEMS. Specifically, they evaluate the hypothesis of Bradley (1994) that GEMS are remnant interstellar (IS) amorphous silicates. From a comparison of the mineralogy, chemical compositions, and infrared (IR) spectral properties of GEMS with their modeling results, Min et al. conclude that ”the composition of interstellar medium (ISM) silicates is not consistent with that of GEMS” and that ”GEMS are, in general, not unprocessed leftovers from the diffuse ISM”. These original conclusions were based, however, on erroneous GEMS data. In the accompanying Erratum, Min et al. provide corrections, evaluate the impact on the paper and retain their original conclusions that GEMS chemical properties are inconsistent with the bulk ISM and that most GEMS formed in the early solar system. We respectfully disagree. With the corrections, two additional fundamental chemical properties of GEMS (Mg/(Mg+Fe) and O/Si), as well as a key optical property, the 10 μm infrared silicate feature1, are consistent with those of the IS amorphous silicates as predicted by the Min et al. model, which itself relies on assumptions. The additional properties further solidify the remarkable similarity between the exotic properties of GEMS and IS amorphous silicates (Bradley 1994; Flynn 1994; Goodman & Whittet 1995; Martin 1995; Bradley et al. 1999). We agree that the match is imperfect, but given the statistical limitations of comparing less than a microgram of GEMS with the enormous mass of silicates in the ISM, this is not surprising.