Measuring

1
Measuring Calibrating TIR Spectra, Spectral
Features of Minerals and Rocks in the Thermal
Infrared

• Lecture VEH3
• 09-04-03

2
Topics

• Review sources of TIR spectral features
• Measuring TIR spectra
• Issues involved in calibrating TIR spectra
• Approaches to emissivity calibration (lab)
• Mineral spectra
• Rock spectra

3
Review - Sources of TIR Spectral Features

• Recall that many VNIR absorptions dependent on
  presence of transition metal
• Vibrational processes dependent on atomic masses,
  bonding environment, molecular geometry
• Lighter elements vibrate at higher frequency than
  heavier ones in a given structure
• Higher bond strengths increase frequencies

4
Review - Sources of TIR Spectral Features

• Modes in solids are
• stretch (symmetric, asymmetric)
• bend (in-plane, out-of-plane)
• metal-oxygen, chain, and lattice vibrations
• Stretching modes occur at higher frequencies than
  bending modes, M-O, chain, and lattice modes

5
Vibrational Modes - H2O
l2 6.1 µm HOH bend
l1 3.1 µm symmetric OH stretch
Commonly observed as a single, broad band
l3 2.9 µm asymmetric OH stretch
6
Measuring TIR Spectra

• Spectrometers come in many flavors the purpose
  of all of them is to separate incident light into
  discrete wavelengths in such a manner that we can
  measure the wavelength/frequency dependent energy
  and use this information to learn something about
  a material
• the source (a blackbody emitter)
• an optical system
• the detector (thermal, pyroelectric, or quantum)

7
Types of Spectrometers

• Monochromators (Dispersive)
• a system of slits, mirrors, and
  prisms/gratings/wedges that disperse radiation
  into separate wavelength
• measures one wavelength at a time virtually
  obsolete
• Interferometers (FTIR)
• two mirrors, one moving and one static with a
  beamsplitter in between radiation undergoes a
  constructive/destructive interference process by
  traveling to and from each mirror and recombining
  at the beamsplitter superimposed on each beam is
  an interference pattern an FFT converts the
  interferogram into a spectrum
• all wavelengths are measured simultaneously
• Michelson is most common type

8
Interferometer
9
Interferometer, cont.

• Advantages of FTIR over monochromator
• Rapid acquisition of data at improved SNR
  (Fellgetts advantage)
• Simultaneous vs. sequential collection
• Higher throughput
• Excellent wavenumber repeatability
• Constant bandwidth across spectrum
• Detector type and beamsplitter material will
  dictate wavelength range
• DTGS vs. MCT
• KBr vs. CsI

10
Types of Measurements

• Transmission
• Measures transmitted energy, absorption
  coefficient dominates
• Reflectance
• Measures reflected energy, n and k important
  spectral features similar to, but not
  transmission
• Can be measured several ways

11
Types of Measurements

• Emission
• Measures energy emitted by sample (sample is
  source) see also reflectance
• E 1 - R Kirchhoffs Law
• Only hemispherical reflectance (or other
  instrument modification) is truly comparable to
  emission
• Raman
• Also information about vibrational properties,
  but via different process involving visible laser
  excitation
• Recall that Raman-active modes not necessarily
  IR-active

12
Bi-directional
Hemispherical
Transmittance ()
Reflectance ()
Emissivity
Counts
a-Quartz
13
Issues of Calibrating IR Emission Spectra

• Why emission?
• Sources of emission in lab measurement
• Detector
• Reflection from beamsplitter onto source (minor)
• Instrument cavity/components radiate energy
  without passing through interferometer (not a
  spectral effect)

14
Approaches to e Calibration

• Two-temperature method Christensen and Harrison,
  1993
• Energy from sample and blackbody measured at two
  temperatures (4 measurements) cancels
  contributions from reflected and instrument
  energy as well as instrument response function
• Difficult to get sample and bb at same T - errors
  in accuracy and reproducibility
• Difficult to know surface T of sample accurately
• Background energies need to remain constant
• Measuring four times for each sample is annoying

15
Approaches to e Calibration

• One-temperature method Ruff et al., 1997
• Two bbs of differing T are measured (once), and
  sample is measured at one T (determined from
  spectrum) reflected energy is determined from
  knowledge of environmental conditions
• Response function
• Instrument energy
• Sample T
• Reflected energy
• Sample reflectivity
• Environment emissivity
• Environment energy

16
Approaches to e Calibration

• Errors in derivation of any of the temperatures
  associated with blackbodies, instrument, sample,
  or environment produce errors in the calibrated
  emissivity spectrum
• Usually, but not always, observed as a negative
  slope
• Derivation of Tsamp requires assumption that e1
  somewhere along the spectrum (usually the CF),
  but not all geologic materials have this property

17
Mineral Spectra

• Spectrum/spectra
• What is a mineral?
• Naturally occurring, inorganic solid
• Definite chemistry (within limits)
• Definite structure (arrangement of atoms)
• Each mineral differs from all other minerals by
  either chemistry, structure, or both

18
Mineral Spectra

• Atoms in minerals will be different, have
  different bonding environments, and will
  therefore have unique vibrational modes (and
  unique vibrational spectra)
• Minerals are composed of cations and anions, and
  grouped according to their dominant elements

19
Mineral Spectra

• silicates (SiO4-4)
• quartz, SiO2 pyroxene, (Mg,Fe)Si2O6
  plagioclase, NaAlSi3O8
• carbonates (CO3-2)
• calcite, CaCO3
• oxides (O), hydroxides (OH-)
• hematite, Fe2O3 goethite, FeO(OH)
• halides (Cl-, Br-, F-, I-), sulfates (SO4-2),
  etc.
• halite, NaCl gypsum, CaSO42H2O
• native elements (Au, Ag, C, Cu)

20
(No Transcript)
21
Silicates

• Fundamental vibrational modes are domianted by
  Si-O stretching and bending modes (Al can sub for
  Si)
• Stretch (1200 - 800 cm-1, or 8-12 µm region)
• Bend (600 - 200 cm-1, 15 - 50 µm)
• Grouped by degree of polymerization of SiO44
  anion

22
Silicate Polymerization

• Isolated
• Olivine
• Chain
• Pyroxene, amphibole
• Sheet
• Micas
• Framework
• Quartz, feldspar
• Others
• Double isolated (epidote), ring (beryl,
  tourmaline)

23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
Silicates
27
Silicates - Solid Solution

• Solid solution series minerals have traceable
  spectral characteristics too
• olivine Mg/Fe
• pyroxene opx/cpx Mg/Fe
• feldspars

28
Pyroxenes
29
Silicates - CF

• Minerals display shift of Christiansen feature
  position to longer wavelengths with decreasing
  polymerization
• Christiansen frequency is located in a wavelength
  region where scattering is minimized because n
  changes rapidly, approaching the refractive index
  of the medium around the sample (i.e., 1)
• occurs on the short wavelength side of the
  fundamental vibrational mode, where absorption is
  relatively low and energy emitted is at a maximum
  (reflectance is at a minimum)

30
Silicates - CF

• Christiansen frequency is tied to an optical
  constant, but environmental factors (to be
  discussed later) can change the position of
  absorption band
• Refer instead to the Christiansen feature (CF),
  which is the emission maximum (reflectance
  minimum)
• Generally located on the short wavelength side of
  the primary absorption feature (8-12 µm
  stretching mode)
• Term reststrahlen band
• CF has been used to characterize mineral
  composition Conel, 1969 and igneous rock
  composition papers by Salisbury and colleagues

31
Silicates
32
Carbonates

• Fundamental vibrational modes dominated by C-O
  stretching and bending modes
• Generally in the 1600-1400, 900-850, and 400-300
  cm-1 regions (6-7, 11.5, and 25-30 µm)
• Carbonates are a solid solution series, so
  features shift as a function of composition

33
(No Transcript)
34
Oxides/Hydroxides

• Fundamental absorptions dominated by metal-O
  modes
• Typically in the lt800 cm-1 (gt12 µm) region

35
(No Transcript)
36
Halides and Sulfates

• Absorptions dominated by primary element-O modes
• Halides (halite, sylvite, fluorite) have VERY
  broad features in the IR
• Minerals are isometric and have strong ionic
  bonds that cause primary vibrations to be
  dominated by lattice vibrations as a whole rather
  than (e.g.,) Na-Cl modes
• Sulfate (gypsum, anhydrite) absorptions common
  around 1100-1200, 700-200 cm-1 (8-10, 15-50 µm)

37
Native Elements

• Should they have IR spectral features?
• Why/not?

38
Water Hydroxyl (not mineral groups, but
whatever)

• Fundamental modes of H2O at 2.9 and 6.1 µm only
  the bending mode is visible in TIR
• Visibility of 6.1 µm band in TIR depends on
  particle size (more on this later)
• OH- minerals (without H2O) display a 2.7 µm band,
  but no 2.9 or 6.1 µm band

39
Rock Spectra

• Simple linear combination of component mineral
  spectra, in proportion to abundance
• Christiansen feature position is no longer tied
  to a single optical constant
• CF migrates to longer wavelengths from felsic to
  ultramafic compositions - WHY?
• CF is correlated with bulk chemistry Lyon, 1964
  Salisbury and Walter, 1989 Walter and Salisbury,
  1989

40
CF Correlations