Low Temperature and Far Infrared to Sub-mm Spectra of Phyllosilicates
S. Zeidler1, H.Mutschke1, Th. Posch2
1 Astrophysikalisches Institut und Universitäts-Sternwarte, Friedrich-Schiller-Universität Jena, Schillergäßchen 2-3, D-07745 Jena
2 Institut für Astronomie der Universität Wien, Türkenschanzstraße 17, A-1180 Wien, Österrreich

3. Main results

1. Background
Phyllosilicate is the name of a special class of silicates with a layer or leaf structure which contains OH- or even
H2O (because of that it is often called hydrous silicate). Many of them (especially clay minerals like the chlorite
or smectite group) are weathering and alteration products of other silicates like olivines or pyroxenes.
Phyllosilicates are very popular as a possible water reservoir in space since one finds them in IDPs (Sandfordet al. 1982) but until now we have no evidences that they can be found in other planetary systems too.
However, the possibility for their existence outside our planetary system is very high and also the number ofquestions about their formation and development and of course their relationship to amorphous silicates.

Raw data manipulation: On the left: typicaltransmission spectra that we obtained for our1:100 ratio pellets (here talc, blue curve shifted),
sharp bands are easily detected up to awavelength of 60µm. However, the signal tonoise ratio is not high enough to observe very
weak bands especially at larger wavelength. For
this purpose we used the 1:10 ratio pellets.

According to that, we see our measurements as a kind of preparation
work for the HERSCHEL program which probably will start in springnext year (spectroscopy-range: 57-627µm)

Principal structure:Left: top view of one layer (rings: Oxygen atoms,full dot: Silicon atom); under each silicon atom
the fourth oxygen atom of one SiO44- Tetrahedron can be found.
Right: 2-layer-lattice (top) and 3-layer-lattice(below) structure; two different geometricalstructures can be seen: the Tetrahedron and theOctahedron. The Tetrahedra build up the SiO44layer while the Octahedra contain metalic
anions in the center with O, OH- as ligands andbuild up the anionic layer.

Although the 1:10 ratio pellets lead to ahigher signal intensity, often it was not easy
to characterise the bands precisely.
Interference fringes appear in the FIR due
to multiple reflection within the pellet. To
remove these fringes we decided to use a 5
point adjacent average smoothing
procedure, this means we replace one
point by the average of itself and the 4
most adjacent points. On the right side one
can see the result for a band of picrolite(77µm at room temperature (RT)).

What happens at low temperatures?
Due to the complex inner structure of many crystals one can not calculate the specific shape of thepotential which can be „seen“ by an atom, but we can approximate these local potentials via
anharmonic oszillators. The quantum mechanical calculation of energy states of an anharmonic
oszillator potential shows that with decreasing energy level the energetical distance between twostates increases. Therefore when the temperature and with that also the mean occupied energy level
decreases, more energy is required for a vibrational excitation. Consequently we obtain a shift of bands
to shorter wavelength with decreasing sample temperature and additionaly a change in shape tosharper and more intense bands, due to the decreasing energy state distribution at lower
temperatures.

New bands: on the left the absorption coefficientof talc between 48-120µm is displayed. We
discovered a previously unknown band at 98µm
(RT) which shows a relatively strong change inshape and position (shift to 95µm at 10K) atdecreasing temperature while the double band
further left hasn‘t changed so much its positionand shape when reaching 10K.

Left: measurement data of a weak band of picrolite (~26,8 µm at room temperature); while decrasing thetemperature, the band becomes narrower and more intense and switches to shorter wavelength
Right: scetch of a potential of an anharmonic oszillator with the quantum mechanical energy states (blue). The
distance between different energy states is decreasing when the energy level is increasing

Very long wavelength bands: chamosite
possesses a feature at 277µm (RT) which is theband at longest wavelength ever being observed
for silicates.

2. Experimental setup
Samples:
We used four different members from four different groups of Phyllosilicates for our measurements:• talc for talc-pyrophyllite-group• montmorillonite from the montmorillonite-group• chamosite as a member of the chlorite-group• picrolite for the serpentines

Hidden bands: in the two upper pictures chamosite and montmorillonite spectra between 50350µm can be seen. Obviously some bands show not only a change in position and shape but alsoa split of in several hidden bands while decreasing the temperature (attend the small picture in
the montmorillonite spectrum, it shows the 10K curve in a linear scale). A change of continuum
absorption in the montmorillonite spectra (at 110-350µm) by reaching 10K has been observed. Atroom temperature the decrease follows a function: y~λ-1.8 and at 10K: y~λ-3.04.

They all have a 3-layer-lattice structure, except picrolite with its2-layer-lattice structure

4. Comparison with astronomical data
Fourier Transform spectrometer Bruker 113 v
Si-Bolometric Detector at 1.5K
Spectral coverage: 25µm-400µm
Spectral resolution: 0.5-1cm-1

Continuous-flow liq. He Kryostat(Cryovac-Konti Spectro B)

Considered temperatures: 300K, 200K, 100K, 10K

Sample preparations
The samples are embedded in a Polyethylene matrix with amass ratio of 1:100 for measurements between 25-100µm and1:10 for measurements up to 400µm wavelength(corresponding to a column density of 1.51 and 15.1 mg/cm2respectively);
The grain size of our samples was appointed to be <5µm(achieved by grinding in a mortar/ball mill and sedimentation in
acetone).

In 1999 Malfait et al. compared a FIR spectrum of
the outer region of the protoplanetary disk of theyoung star HD 142527 with calculated spectra ofamorphous silicates, water ice and phyllosilicates (inparticular montmorillonite). They concluded that theshape of the spectrum (measured with the ISO SWSand LWS spectrometer) at 100µm can be ascribed tothe emission of montmorillonite particles in the
protoplanetary disk. Our results show that thefeature of montmorillonite at 100µm is too sharp
compared with that of the ISO spectrum. Also theother phyllosilicates can‘t explain the observed
spectrum. The broad band around 100µm could bereproduced by the emission of dust at 38K with afeatureless (grey) emissivity proportional to λ-1.

References• K.Malfait, C. Waelkens, J. Bouwman, A. de Koter, L. B.F.M. Waters; Astron. Astrophysics 345, 181 (1999)• H. Mutschke, S. Zeidler, Th. Posch, Th. Henning; Astron. Astrophysics (in prep.)