Transcript Mineral Spectroscopy - University of Colorado Boulder

Mineral Spectroscopy
Visible
Infrared
Mössbauer
Raman
NMR
Properties of Light
• Light is conducted through
materials on the valence electrons.
• Light travels more slowly in
materials.
• Electrically conducting materials
are opaque
Electromagnetic Spectrum
Visible Light: 7700 - 3900Å
Properties of Light
• Light is conducted through
materials on the valence
electrons.
• Light travels more slowly in
materials.
• Electrically conducting
materials are opaque.
Behavior of Light in Materials
• Absorption (light is absorbed by materials)
• Color (absorption is a function of wavelength)
• Pleochroism (absorption is a function of direction)
• Refraction (light travels slowly in some materials)
• Dispersion (velocity is a function of wavelength)
• Birefringence (velocity is a function of direction)
Absorption
• Light is attenuated on entering any
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material.
The attenuation is a function of distance.
I is intensity at some point t
I0 is initial intensity.
k is absorption coefficient in cm-1.
Lambert’s Law:
I / I0  e
 kt
Color
• Absorption may be a function of wavelength.
• Materials may appear colored in transmitted light
Color
• Absorption in the visible mainly due to electron
transitions in d-orbitals or f-orbitals.
• Color in minerals primarily due to presence of
transition metals or rare earth elements.
Visible and NIR spectra
of Ametrine (quartz)
Courtesy: George Rossman
Ringwoodite is Blue
• (g-Mg1.63Fe0.22
H0.4 Si0.95O4)
• ~10 % of Fe
present as ferric
(Mössbauer)
Pleochroism
• Pleochroism is the variation of absorption with
direction in a crystal.
• Pleochroism is observed as a color change on
rotation in plane-polarized light (not crossed
polars).
• Pleochroism only occurs in non-cubic crystals.
• Pleochroism indicates the presence of transition
metals (esp Fe, also Mn, Cr, V, etc).
• Biotite, tourmaline, amphibole.
Refraction and Reflection
• When light strikes a polished surface of
a material it is split into two rays.
• One is reflected and the other refracted
Infrared spectroscopy
• Near IR 5000 - 13000cm-1
– orbital transitions
• Mid-IR 2500 - 5000cm-1
– N-H and O-H bond vibrations
• Far IR 500 - 2500 cm-1
– Cation-Oxygen bond vibrations
– Structural phonons.
FTIR Spectrometer
Mid IR spectroscopy
Mid IR spectroscopy
Raman Spectroscopy
• Looks at wavelength shifts in scattered
light.
• Shifts are in atomic vibrational part of
spectrum
• 0 - 5000cm-1. (same as mid to far IR)
• Excitation is usually by a
monochromatic source in the visible
region (commonly a laser).
Raman Spectroscopy
Mössbauer Spectroscopy
Resonant Gamma Ray spectroscopy
Uses 57Fe gamma decay at 14.4 MeV
Source is 57Co
Source is accelerated mechanically to
produce ultra-fine relativistic energy shifts
• Absorption as a function of source velocity
• Looks at electric field effects at nucleus
due to d-orbital occupancy and
perturbations from local coordination
effects
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Mössbauer spectroscopy
Mössbauer spectroscopy
Mössbauer spectroscopy
NMR Spectroscopy
• Nuclear Magnetic Resonance
• Similar to Mössbauer spectroscopy but
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many more nuclides
Radio frequency emission spectroscopy
due to magnetic transitions in nucleus.
Solid samples are spun in a strong
magnetic field (Magic Angle Spinning)
A RF field applied and turned off.
Sample emits RF
NMR Spectroscopy
NMR Spectroscopy
NMR Spectroscopy