Amorphous Magnesiosilicate Smokes Annealed in Vacuum: The Evolution of Magnesium Silicates in Circumstellar and Cometary Dust

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2002-03, Vol.156 (1), p.269-286
• Main Authors: Rietmeijer, Frans J.M., Hallenbeck, Susan L., Nuth, Joseph A., Karner, Jim M.
• Format: Article
• Language: English
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1006/icar.2001.6762

The mid-infrared spectral evolution of amorphous metastable eutectic magnesiosilica smokes, obtained by kinetically controlled gas-to-solid condensation of a Mg–SiO–O 2–H 2 vapor, proceeded in three distinct phases as a function of increasing time and temperature. This paper reports the mineralogical and chemical properties of these same samples. We found a previously unanticipated size dependence of the petrologic development of the initially amorphous magnesiosilica smokes that may also be at least partially responsible for the previously reported spectroscopic changes. Condensed grains less than approximately 20 nm in diameter remained amorphous throughout the thermal annealing experiment. Mineralogical changes occurred only after fusion of condensed magnesiosilica grains and chemical homogenization of large amorphous agglomerates and ring structures. Kinetically favored nucleation and growth produced the thermodynamically unstable nanocrystalline assemblage forsterite + tridymite. Further mineralogical development was stalled until continued fusing of agglomerates, rings, and some fraction of condensed grains had produced smooth amorphous magnesiosilica sheets of 42 and 20 wt% MgO. In rare sheets with more than ∼55 wt% MgO large forsterite crystals had grown, while enstatite had nucleated in low-MgO sheets still in the presence of forsterite and tridymite. The mineralogical evolution of the samples is critically dependent on the mass of the structural entities in the condensed sample and seems to be restricted to fused agglomerates and ring structures larger than about 20 nm in diameter and the sheet materials. We discuss the implications of our study for the interpretation of similar astrophysical dust analog studies and for astrophysical applications.