A new database of infrared mineral spectra for astrophysics by Anne M. Hofmeister Many thanks to Janet Bowey, Angela Speck, and Mike Barlow Star sapphire
A new database of infrared mineral spectra for astrophysics
by
Anne M. Hofmeister
Many thanks to Janet Bowey, Angela Speck, and Mike Barlow
Star sapphire
Philosophy
• Measure solids with diverse chemical compositions and structures
• Study known astrominerals and condensates in copious detail
• Obtain intrinsic, quantitative spectra (understand and eliminate sampling artifacts)
• Use cryogenic temperatures (future work)
Far-IR to Visible Spectrometer
IR microscope Bomem FTIR
Interpretation of observational data rests on the quality of laboratory IR measurements
Egg Nebula ( R. Thompson et al. NASA site)
Cell for thick films (t = 6 m)
Control: Subject:
Reflectance is the best approach, but fairly large samples are needed:
Specular reflection device
Let’s look at reflectivity data – do artifacts exist?
mirrors
sample
S-polarization
FTIR microscope
0
0.2
0.4
0.6
0.8
0
1
2
3
4
5
200 400 600 800 1000
Re
flect
ivity
Ab
sorp
tion
coe
fficien
t, 1/m
Kramers-Kronig
Rmeas
MgO
50 25 20 1014.2 12.5m16.7
Damped harmonic oscillator
A
0
2
4
6
8
200 400 600 800 10000
1
2
3
4
5
Opt
ical
Fun
ctio
ns
Wavenumbers, cm-1
k
n
MgO
*
Damped harmonic oscillator
Kramers-Kronig
Opaque spectral regions yield
reliable data for thin samples, but
I0 I0R I0R(1-
R)2(1-)2
I0(1-R) I0R(1-R)(1-)2
d
Imeas
I0(1-R)(1-) I0R(1-R)(1-)
I0(1-R)(1-)(1-R) = Imeas
0
0.02
0.04
0.06
0.08
0.1
0 2000 4000 6000 8000 10000 12000 14000 16000
Re
flect
ivity
Wavenumbers, cm-1
Rmeasured
H2O
5 2 1.25 1 0.625, m
damped har. osc.
0.8
frommeas.n
back-reflections affect transparent spectral regions
A high-pressure device provides essentially quantitative absorption data from powdered,
hard minerals
Olivine-enstatite-diopside rock from Earth’s interior
Diamond anvil cell used to make thin films(t = 0.1 to 3 m)
Absorption/transmission spectra depend on
Areal coverageSample thicknessIntensity of bands
(absorption strength increases with reflectivity)
0
1
2
3
4
5
0
20
40
60
80
100
200 300 400 500 600 700 800 900
Absorbance % Transmission
%T
A
total coverage
0
0.2
0.4
0.6
0.8
1
0
20
40
60
80
100
200 300 400 500 600 700 800 900
Ab
sorb
an
ce
% T
ran
smission
%T
A
50 %coverage
Wavenumbers, cm-1
0
1
2
3
7 9 11 13 15 17 19 21
Ab
sorb
anc
e
Wavelenght, m
Quartzt~0.1 mthinning
0
1
2
3
7 9 11 13 15 17 19 21
Ab
sorb
an
ce
Wavelenght, m
Quartzt>0.1 m
thickening by adding material
Various peaks “saturate” at different thicknesses, depending on individual band strengths
TO modes saturate before LO, which rounds the profile, making spectra of crystalline material appear amorphous
Cryostat for dispersion or reflection(fixed points: 77, 200, 273 or 298 K)
Measurements at temperature are needed to provide relevant peak
parameters
More work is needed: e.g. liquid helium temperatures with a variable T cryostat
0
0.5
1
1.5
2
2.5
100 200 300 400 500
Che
mic
al a
bsor
banc
e
Wavenumbers, cm -1
condensedwater ice(artifact)
77 K
298 K
thin films
dispersionsblack hibonite
synthetic hiboniteblack hibonite
NGC 6302
Room temperature measurements provide a first-order model of cold dust
in a nebula
0
200
400
600
800
1000
0
0.0004
0.0008
0.0012
0.0016
0.002
0.0024
0.0028
0.0032
0 100 200 300 400
ISO
Int
ensi
ty
Em
issions, W
/cm2/cm
NGC 6302
Wavenumber, cm -1
BB 47 K
hibonite47 K
3
5
7
10
1
1/2
2
1/4
BB 33 K
BB /3080 K
Focus on far-IR becausecold temperatures cut-offhigh frequency peaks:
200
300
400
500
600
700
800
900
1000
0
0.2
0.4
0.6
0.8
1
50 100 150 200 250 300 350 400 450
ISO
Int
ens
ity
Che
mic
al a
bso
rban
ce
hibonite
Al2O
3
NGC 6302
Wavenumber, cm -1
grossite
spinel(offset +0.65)
Calcium aluminates provide the best match,
Hibonite CaAl12O19 is presolar 200
300
400
500
600
700
800
900
1000
0
0.2
0.4
0.6
0.8
1
50 100 150 200 250 300 350 400 450
ISO
Int
ensi
ty
Che
mic
al A
bsor
ban
ce
gehlenite
akermanite
NGC 6302
Wavenumber, cm -1
melilite
but Ca, Mg, Al silicates match well, too:
200
300
400
500
600
700
800
900
1000
0
0.5
1
1.5
2
50 100 150 200 250 300 350 400 450
ISO
Inte
nsity
Chem
ical absorb
ance
calcite
NGC 6302
Wavenumber, cm -1
XG
G
F
G HG
XG
D
XEG
E
XD
F
S
F
GH
G
GH
H
C G
F
GG
G
H
XD
C
F
ice77 K
D
deh
The refractory end of the condensation sequence seems to be present in NGC 6302
C = corundum Al2O3
D = diopside CaMgSi2O6
E = enstatite MgSiO3
F = forsterite Mg2SiO4
G = grossite CaAl4O7
H = hibonite CaAl12O19
S = spinel MgAl2O4
X = melilite Ca2MgSi2O7 –
CaAl2SiO7
Could hydrosilicates be stable in space?
Water + forsterite = lizardite
Water + diopside = tremolite
(and we can get band strengths, too)
0
1
2
3
-2
0
2
4
6
500 1500 2500 3500
Ab
sorb
an
ce (
thin
film
s)
Ab
so
rba
nce
(thic
k film
)Wavenumber, cm-1
H2O
t = 6 m
Brucite
Mg-O-H
Mg(OH)2
O-H
Mg-(OH)
t = 1 m
0.8 < t < 1.2 mFF
Ffff
100 20 10 5 4 2.5, m
0
1
2
-2
-1
0
1
2
3
4
5
500 1500 2500 3500A
bso
rba
nce
(th
in f
ilms)
Ab
sro
ba
nce
(thic
k film
)
Wavenumbers, cm-1
LizarditeMg
3Si
2O
5(OH)
4
O-H
t = 5 m
t = 0.5 m
ff ff
M-O-H C-H
+ ==
0
0.1
0.2
0.3
0.4
0.5
40 60 80 100 120
-0.4
-0.2
0
0.2
0.4A
bso
rba
nce
Wavelength, m
*
*
*
*
*
*
*
Hydrosilicates t = 1 m
tremolite
sapphirine
saponite
chrysotile
montmorillonite
talc
400 200 100 80125Wavenumbers, cm-1
Low frequency region best identifies dust
Lizardite and saponite (or their dehydroxylates) may be present in NGC 6302
Lizardite froms via alteration of forsterite below 700 K. Saponite
via alteration of basalts.
Average band strengths (Hofmeister and Bowey in prep). type ν(cm ¹)⁻ brucite tremolite lizardite talc saponite mont. average
O-H stretch 3500 1.4 0.40 0.95 0.45 0.11 0.21 0.6Mg-O-H 1600 0.2 0.9 0.045 - 0.2 0.08 0.3overtones 2000 0.01 0.05 0.045 0.025 0.1 0.014 0.04Si-O stretch 1000 - 3.3 2.8 5.8 2.3 1.6 3.1Si-O-Si bend 670 - 0.6 1.2 1.9 0.5 ? 1.0O-Si-O bend 450 - 1.8 3.7 4.3 2.0 1.0 2.5Mg-O stretch 300 2.4 0.3 0.67 - - 1.0Ca translation 200 - 0.3 - ? 0.01 0.15Mg translation 200 - 0.1 0.05 0.08 0.05 0.01 0.06
True absorption coefficients (in 1/μm) are given for the dominant band in the various spectral regions.
allow estimation of concentrations even if mineral identification is unsure.
SiC has more features than the Si-C stretch expected for its simple structure
due to • stacking disorder (polytypism)
• impurities such as excess C or Si
• crystallinity (bulk vs. nano vs. amorphous)
Some minerals warrant detailed studies:
The “21 m” feature is SiC with excess C
Ueta et al. 2000
Speck and Hofmeister 2004
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
5 10 15 20 25 30
Ch
em
ical
Ab
sro
ban
ce
Wavelength,m
22 m20
nano
amorphous
nano bulk
SiC
9.26
8.4
12.2 m
10.85
bulk a
SiC in various forms has distinct spectra
(Speck and Hofmeister in prep.)
The 9 m features in AGBs are due to SiC with excess C(Speck and Hofmeister in prep.)
This substance has the diamond structure and is nano-crystalline
(Kimura and Kaito 2003)