Vesta and Spectroscopy of Electronic transitions

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Vesta and Spectroscopy of Electronic transitions

• Lecture 1

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Vesta

• The fourth asteroid discovered (hence 4 Vesta)
• First target of Dawn
• Probable source of HED meteorites

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What is reflectance

• One of many definitions Ratio of the observed
  amount of light, (observed amount and light
  later) to a perfect diffuse reflector identically
  illuminated and observed.
• A perfect diffuse reflector does not reflect like
  a mirror, but rather incident light has an equal
  probability of scattering in any direction

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What is reflectance
Q1
Q2
Q1 Q2
Perfect mirror, intensity of light incident
equals intensity of light exiting
Mirror
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What is reflectance
Perfect Diffuse Reflector Intensity of
reflected light equal in all directions (diffuse
part) Sum of reflected light equals incident
light (perfect part) Can be approximated using
packed powders, various plastics, MgO smoke was
an early standard
Q1
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What is reflectance
Reflectance is the ratio of observed light from a
surface of interest to a perfect diffuse
reflector.
Reflectance Dim/Brite
Brite!
Dim..
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What is a spectrum?

• Variation in a quantity as a function of
  wavelength or frequency
• Wavelength is related to frequency by
• cln
• Spectral reflectance is the reflectance
  measured in a narrow band of wavelength as a
  function of wavelength or frequency

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Spectral Ranges (Aerospace Industry)
UV 200-400nm VIS 400-800nm NIR (near
infrared) 800-1000nm (1-micron) SWIR(short wave
infrared) 1-2.5 microns MWIR (midwave infrared)
3-5 microns LWIR (long wave infrared) 8-14 microns
Named spectral range are historical (e.g.
UV,visible), or dictated by atmospheric windows
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Spectral Ranges (Astronomy)
UVBGRI J H K
Named spectral ranges are historical (e.g.
UV,visible), Or dictated by atmospheric windows
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Spectral Ranges (Astronomy)
J H K L M N Q
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Spectral Ranges (Planetary Science)
UV 100-400nm VIS 400-800nm NIR (near
infrared) 800-1000nm sometimes 0.8-5
microns Mid-infrared 3-10 microns, 7-14
microns Thermal infrared 4-50 microns Real
mans infrared 10-20 microns
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Spectral Ranges (Planetary slang)
UV Vis 400-1000 Solar reflectance region
.4-2.5 microns Three micron region2.5-4 Ten
micron region 8-14 Real mans infrared 10-20
microns
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Reflectance Spectrum of Vesta

• Obtained telescopically
• Near-IR absorptions near 1 and 2 microns
• Troughs are where things are happening. Peaks
  are where things are not happening (vis and near
  IR only)

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Spectra of Common Rock-forming minerals
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Spectra of Common Rock-forming minerals

• Only pyroxene exhibits strong absorptions with
  similar widths at 1 and 2 microns, hence
• Vestas surface contains pyroxene

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Origin of near-IR spectra features in igneous
rock-forming minerals

• Spectra of olivines of widely varying
  compositions provides an important clue
• Iron-free olivine (forsterite) lacks an
  absorption feature and is bright
• Suggests absorption is due to iron

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Origin of near-IR spectra features in igneous
rock-forming minerals

• Reflectance of pyroxene correlates with iron
  abundance at all wavelengths
• Absorption is due to iron

(Absorption coefficient is inversely and
nonlinearly proportional to reflectance)
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Origin of near-IR spectra features in igneous
rock-forming minerals

• Roger Burns interpreted near-IR absorptions in
  terms of quantum mechanical considerations in
  crystal field theory
• Absorption is due to photons exciting an electron
  in unfilled d-shells in transition metals.
• The excited state is enabled by asymmetry in the
  sites occupied by iron in pyroxene

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Origin of near-IR spectra features in igneous
rock-forming minerals

• Molecular orbitals are highly structured and
  interact with the structure of minerals

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Origin of near-IR spectra features in igneous
rock-forming minerals

• Molecular orbitals are highly structured and
  interact with the structure of minerals
• In olivine and pyroxene, Fe resides in distorted
  octahedral sites that enable splitting of the
  energy of the ion.
• The lobes of the orbital in closer proximity to
  negatively charged anions are inhibited from
  occupation.

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Causes of absorption features

Electron wants to be in the portion of the
orbital farthest from the negatively charged
anions
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Causes of absorption features

A photon of appropriate energy can provide the
electron with sufficient energy to overcome the
electrostatic repulsion
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Causes of absorption features

The required energy is dependent upon the size
and shape of the site, hence provides mineralogic
information by affecting the wavelength of
absorption
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Causes of absorption features

Crystal field theory can predict the approximate
position of absorptions. Current theory cannot
predict width, or intensity There is hope,
studies of chemical dynamics may enable a theory
to be generated
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Causes of absorption features
FNAQ Why doesnt the site promptly re-emit a
photon and re-establish equilibrum low energy
state? The presence of the excited electron in
proximity to the anion causes instantaneous
stress on the site, which responds by distorting,
carrying away energy through vibration. The site
achieves temporary higher symmetry and the
electron decays to the equilibrium position
with no or little energy penalty. It is possible
that the mineral does re-emit a long wavelength
photon (fluorescence), would be an interesting
experiment. You would have to illuminate with a
short laser pulse and watch what happens with a
fast spectral detector.

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Pyroxene and its spectral character

• Major rock forming mineral Single-chain
  silicate (Ca, Fe, Mg)2Si2O6 (see papike)
• Ubiquitous in planetary igneous rocks
• Composition Defined by Fe, Mg Ca.

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Pyroxene and its spectral character

• Composition Defined by Fe, Mg Ca.
• Remember major structural dichotomy (ortho vs.
  clino)

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• Low-Ca pyroxenes have orthorhombic symmetry ?
  orthopyroxenes
• High-Ca pyroxenes have monoclinic symmetry ?
  clinopyroxenes
• Beta not equal to 90

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Pyroxene and its spectral character

• Quadrilateral pyroxenes have one and two micron
  bands

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Pyroxene and its spectral character

• Cpx shows two types, A and B, B looks like opx,
  A, vaguely like olivine.

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Pyroxene and its spectral character
Type A pyroxenes are barely pyroxenes, v.v.Ca rich
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Pyroxene and its spectral character
Adams plot of 1 and 2 micron minima positions,
shows correlation between 1 and 2 micron Bands,
suggests related origin. Filled are opx, open
are cpx
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Semi-Quantitative Band Analysis

• To understand the systematics of pyroxene band
  centers, Cloutis and Gaffey 1997 synthesized
  existing data for pyroxene band centers
• Distribution reflects terrestrial distribution in
  rocks

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Semi-Quantitative Band Analysis

• Band positions were measured and assigned to each
  composition

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Semi-Quantitative Band Analysis

• These were then hand-contoured to produce a
  mapping from band position to composition

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Pyroxene and its spectral character
Ca seems to dominate band positions in pxn,
however,there is little correlation of band
position with Ca in orthopyroxenes.
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Pyroxene and its spectral character
In contrast, there is strong correlation of band
position with iron in clino and ortho-pyroxenes
if they are treated separately
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Pyroxene and its spectral character
In contrast, there is strong correlation of band
position with iron in clino- and ortho-pyroxenes
if they are treated separately
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A New Quantitative Pyroxene Composition
Model Effects on composition on absorption
features
1?m feature
2?m feature
Calcium
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A New Quantitative Pyroxene Composition
Model Effects on composition on absorption
features
1?m feature
2?m feature
Iron
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A New Quantitative Pyroxene Composition
Model Effects on composition on absorption
features
1?m feature
2?m feature
Iron
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A New Quantitative Pyroxene Composition
Model Effects on composition on absorption
features
1?m feature
2?m feature
Magnesium
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Key Concepts

• The electronic spectrum of a mineral is directly
  linked to its composition owing the the energy
  environment dictated by its structure, and the
  abundance of the absorbing ion (Fe mostly)
• Visual inspection can reveal basic composition if
  spectral features are present
• Detailed inspection can refine the inferred
  composition

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