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)