As a matter of fact, pure magnesium silicates have to be considered as a marginal case to real silicates. Annealing performance of silicates would be affected by the presence of iron, aluminium and less abundant elements (Karner & Rietmeijer 1996; Karner et al. 1996). Experiments using smoke containing iron and aluminium were already done by Brucato et al. (1999). They found that the smoke evolved into pyroxene - in contrast to this study.
Additionally, crystallization due to hydration has not yet been fully understood. Crystalline layer-lattice silicates observed in the cold dust component of the shell surrounding the young star HD 142527 may be formed by hydrocryogenic and low-temperature aqueous alteration or processes that can be described as low-temperature or athermal annealing (Malfait et al. 1999; Molster et al. 1999b).
More generally, formation of crystalline silicates by the process of annealing cannot be considered independently of dust-gas reactions. Far below the condensation temperature, amorphous silicates may undergo a change into hydrous crystalline silicates by equilibration with (Rietmeijer 1995).
Annealing is also responsible for chemical fractionation. Impurities (e.g. Al, Ca) may be released from the lattice and reassemble at the surface or in separate inclusions (Gail & Sedlmayr 1998). This effect would affect the spectroscopic properties of the silicate particles. At the present time, silicates that contain iron, calcium and aluminium as well as hydrous silicates are investigated and will be discussed in a succeeding paper.
© European Southern Observatory (ESO) 2000
Online publication: December 15, 2000