Introduction
Silicatic particulates are an ubiquitous component of cosmic dust.
The interplanetary space is populated by a wide size spectrum of
small particles that have been produzed by impacts, collisions and disintegration
of small bodies. As a rule, solar system silicate dust, e.g. cometary grains
and dust around main sequence stars contains crystalline silicates.
In contrast to this, most of the observed silicate bands in spectra
of circumstellar or interstellar IR sources have very broad and almost
structureless profiles.
But recently, infrared spectra gained by the Infrared Space Observatory
(ISO) satellite gave evidence for the presence of crystalline silicates
around young and evolved stars and planetary nebulae.
Dust in molecular clouds
Molecular clouds that are the places of star
formation are assumed to be the coldest environments in space. Silicate
particles that consists of a mixture of amorphous olivine and pyroxene
are most probably covered with carbonaceous or organic material. The outermost
layer of the particles consists of ice.
Silicate particles in this environment are
assumed to be strictly amorphous due to the amorphization by cosmic rays.
Infrared spectra of compact molecular sources show the presence of a strong,
broad diffuse band centered about 10 micrometer and a weaker , broad diffuse
band centered near 18 - 19 micrometer. No crystalline silicate bands are
observed.
Interstellar Dust
Interstellar dust is multiform because it
consists of particles from a wide range of sources (e. g. formed in stellar
atmospheres, novae or supernovae). In the interstellar space it is subjected
to the action of high energy rays. It is assumed that this will lead to
amorphization of the material. This is also true for silicates.
Dust in accretion disks
The silicates in outer regions of accretion
disks are very similar to those in the interstellar medium (ISM). Olivine
and pyroxene are the dominant silicates with olivine having a larger mass
fraction than orthopyroxene.
In the warmer regions of accretions diskas
as well as in the warmer regions of molecular cloud cores, some or all
of the grain spezies are chemically transfomed or sublimate. Silicates
change into the crystalline state due to the effect of thermally annealing.
Spiraling inward towards the protostar the
grain spezies start to vaporize. Model calculations of solar type stars
indicate that silicate vaporation takes place at 1300 ... 1450 K. Thios
temperature is reached in the protoplanetary disk at about 0.6 ... 0.3
AU [Gail 1998].
Dust in stellar outflows of oxygen-rich
giants
High temperature condensation:
The silicate condensation sequence starts
with the formation of Al2O3 which, upon cooling,
reacts with gaseous SiO, Ca and Mg to form spinel (MgAl2O4),
melilithe and then diopside (CaMgSi2O6). Only a small
fraction of the Mg and Si is involved in this part of the condensation
sequence.
Olivin and pyroxen condensation:
Most of the silicon nucleates and condenses
as forsterite. Possibly the excess SiO converts forsterite into enstatite.
Gaseous Fe may react with enstatite grains to form fayalite, rather than
condense as metallic iron [Tielens 1998].
Furthermore, the chaotically condensed silicates
might be subdued to thermally annealing to transform into the crystalline
state.
Generally, in the outflows and ejecta of evolved
stars the largest number of different dust spezies among the cosmic dust
populations has been found [Dorschner 1999].
Finally, dust condensed in stellar atmospheres
becomes part of the interstellar medium.
Meteorites and interplanetary
and cometary dust
Interplanetary dust particles (IDPs) have been produced by impacts,
collisions and disintegration of small bodies.
But also refractrory grains frozen in cometary nuclei, which are
released by the sublimation of the ices through the solar radiation contribute
to the IDP population. Fortunately, IDPs can be collected by planes and
studied in the laboratory. In such grains hints for presolar components
have been found.
Primitive material can also be found as inclusions in chondritic
meteorites. These calcium aluminium inclusions (CAI) are strongly suspected
of being very primitive, they have been correlated with the first, i.e.
highest temperature, condensation products predicted to form from a hot
and cooling solar nebulae.
As a rule, even primitive solar system solids IDPs, CAI in chondrites)
and the stardust grains isolated form meteorites are crystalline. Furthermore,
even infrared observations of comet hale bopp proved that silicate particles
of comets that are released by ice-evaporation into the interplanetary
space are at least partially crystalline [Croivisier 1999].
This fact is surprising because it was generally assumed that comets
formed from rather unprocessed material, i. e. silicates woud be still
in amorphous state.
There are long range mixing processes in the solar nebulae neccesary
to explain the transport of crystalline silicates from approx. 1 AU to
beyond 40 AU!