Spectroscopy of Crystalline Silicates

Laboratory Astrophysics group of the AIU Jena


Astrophysical Background:
Before the Infrared Space Observatory (ISO) satellite opened the mid- and far-infrared range for high-resolution spectroscopy, it was generally assumed that cosmic dust silicates were of amorphous structure.
Spectra gained by the Short Wavelength Spectrometer (SWS) of the ISO have provided striking evidence for the presence of crystalline silicates in comets, circumstellar envelopes around young stars and, most of all, evolved stars and planetary nebulae.


Optical properties of silicates
Since optical properties of astrophysically relevant crystalline silicates are lacking in the literature we set up a spectroscopic programm to present optical properties of crystalline silicates.

There are two approaches to characterize the extinction properties of these analogues:

The classical method is the determination of the optical constants of the bulk material and the use of these data as input for small-particle scattering calculations such as Mie calculations. This method is only suitable for materials which can be prepared as bulk material.

The other method is based on transmission measurements on small-particle systems. In this case, the importance of size, shape, and clustering effects is difficult to estimate as long as no measurements on isolated particles with a narrow size and shape distribution are performed.

Astrophysical relevant minerals
We focuse on minerals that are expected to be high temperature condensates in stellar outflows and which also occur in stellar accretion disks. This can be concluded from thermodynamic model calculations. Furthermore, such minerals are found in chondritic meteorites as CAI (calzium alumium inclusion) that are assumed to be original condensates of the solar nebulae.
Hydrous silicates are also found in meteorites and have to be regarded in exploring the thermal history of the solar nebulae.
The most abundant dust forming elements O, Si, Mg, Fe condense in a mixture of pyroxene and olivine. Infrared spectra of crystalline silicate dust will be dominated by absorption and emission features of these two mineral groups.

High temperature condensates:
garnet                                                 (Mg, Fe, Ca)3Al2O4
perovskite                                           CaTiO3
melilithe, gehlenite, akermannite      (Ca, Na)2(Al, Mg)(Si, Al)2O7
feldspar (anorthite)                             Ca[Al2Si2O8]
spinell                                                (Mg, Fe)(Al, Cr)2O4

Hydrous silicates:
serpentine                                          Mg6[(OH)8Si4O10]
talk                                                     Mg3[(OH)2/Si4O10]

Most abundant minerals in cosmic dust
clinopyroxene:
diopside, hedenbergite                     Ca(Fe, Mg)(SiO3)2
orthopyroxene:
enstatite, hypersthene                       (Mg,Fe)SiO3
olivine: (Colaboration project with Prof. W. Assmus, Kristall. und Materialentwicklungslabor, Universität Frankfurt am Main)
forsterite, fayalite, olivine                (Mg, Fe)2SiO4

Measured transmission and reflectance spectra as well as derived optical data that are published are collected in our optical constants database.
We want to present:
Mass Absorption Coefficient (MAC) spectra in the wavenumber range 2 ... 1 mm.
Reflectance spectra 200 nm ... 1 mm.
Optical constants (derived by KKR analysis or oscillator fit from R) 200 nm ... 1 mm.

Laboratory study
For spectroscopy we use both natural and synthesized single crystals. All samples are characterised by XRD and EDX analysis to detect impurities and verify their chemical composition.
Mass Absorption Coefficients (MAC) are determined from transmission measurements performed on powdered minerals. The bulk samples are ground in an agate mill and sedimented to separate a charge of particles smaller than 1 mikrometer. From this charge KBr and PE pellets are prepared and transmission spectra obtained.
Reflectance spectra are obtained from polished bulk samples. Optical anisotrop minerals (all non-cubic crystal systems) are oriented by the X-ray Laue method and embedded in epoxyd resin. Reflectance measurements is then performed using polarised light.
We endeavour to present data that cover the wavelength range from 200 nm to 1000 Mikrometer (if sufficient big samples are available).

Low temperature measurements
In astrophysical enviroments (molecular clouds or the outer ranges of accretion disks) dust temperature is relatively low (T < 100 K). Opacity of silicate particles at this temperature might be different from that measured at room temperature. Therefore, measurements at low temperatures are of heavy importance!
First study in this field has been done for some amorphous and crystalline silicates as well as for important oxides. Spectra of these samples which were cooled to about 10 K show a sharpening and strengthening of the vibrational bands in the MIR range with decreasing temperature. In the far infrared range a decrease in absorption with falling temperature was found (Henning, Mutschke 1997).
We plan to extend low temperature measurements to a broader number of interesting silicates and oxides.

Comparison with astronomical data
Measured data are compared to observed spectra of astronomical dust sources.
The measured absorption spectra are multiplied by a planck curve that relates to the assumed dust temperature.
Optical constants are used in model calculations.

A) Spectra of MAC of amorphous and crystalline SiO2 multiplied with a planck function of 100 K.

B) The following plot is an example that data of laboratory investigated crystalline silicates fit the observed spectra of astrophysical objects in the IR  wavelength range sufficiently well.
(from Jäger et al. 1998)

Analytical methods



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Last modified: Dirk Fabian, February 99