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Lab Reflectance Spectroscopy
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SpectraLab Reflectance Spectroscopy

Spectroscopy is a tool that detects chemical bonds in molecules through either absorption or emission features in the spectrum of the material.  Reflectance spectroscopy is the study of light as a function of wavelength that has been reflected from a solid, liquid, or gas (13). Thus, even though a solid crystalline structure can affect the shape of spectral features, non-crystalline materials, like glass, still display absorption features in their spectra.   Reflectance spectroscopy can be used on minerals, among other materials (11). 

How Does it Work?

As photons enter a mineral, some are reflected from grain surfaces, some pass through the grain, and some are absorbed (11).  Those reflected or refracted photons are said to be scattered (11).  Scattered photons can either meet another grain or be scattered away from the surface for detection and measuring (11).  Photons can be absorbed in minerals in a variety of ways.  The absorption process and the wavelength dependence enables scientists to derive information about the chemistry of a mineral based on the reflected light.  Even the human eye can be considered a crude reflectance spectrometer (11).  This is because when the eye looks at a surface it detects color.  For instance the red color of hematite can be observed because human eyes and the brain process the wavelength-dependent scattering of visible-light photons (11).  However, this red mineral may not be hematite, and therefore modern spectrometers are useful for measuring finer details over a broader wavelength range with much greater precision.  This enables a spectometer to measure absorptions due to more processes than can be observed through the human eye and can therefore truly determine the identity of the mineral (11).

Absorption Process

Photons enter an absorbing medium and are absorbed according to Beers Law:

I = Io e-kx
I = observed intensity
Io = original light intensity
k = absorption coefficient, units of 1/cm
x = distance traveled through the medium, units of cm
The equation above holds for a single wavelength.  However, at other wavelengths, the absorption coefficient is different and the observed intensity varies. The absorption coefficient, k, as it relates to wavelength is extremely important in describing the interaction of photons with material (13).

Absorption of photons of a specific wavelength causes a change from one energy state to a higher one.  Emission of a photon occurs when energy changes from a higher state to a lower one (13).  This is due to the fact that individual atoms and ions have discrete energy states, or quantum mechanics.   However, when a photon is absorbed it does not always emit at the same wavelength.  This is because absorption can cause heating of the material which in turn causes grey-body emission at longer wavelengths (13).  In a solid, electrons are not only a part of one individual atom, but they may be shared between individual atoms.  The energy value of these shared electrons becomes a range called "energy bands."  Yet, bound electrons will still have quantized energy states (13).

Reflectance Spectroscopy of Minerals

Electronic Process

The most common electronic process revealed in the spectra of minerals is due to unfilled electron shells of transition elements and iron is the most common transition element in minerals. (11)  However, for every transition element, unfilled d orbitals have identical energies in an isolated ion.  Yet, when the atom is located in a crystal field, the energy levels of the d orbitals split, according to Figure 1 (below).  This splitting is responsible for the movement of electrons from a lower energy to a higher energy level when a photon with enough energy is absorbed.  The spacing of the d orbitals is determined by the valence state of the atom, its coordination number, and its symmetry (13).  Furthermore, the type of ligand formed, the extent of distortion of the site, and the value of the metal-ligand distance also contribute to the difference between the states.  Because the crystal field varies with crystal structure from mineral to mineral the amount of splitting varies (11).  This variation in splitting produces different absorptions which ultimately leads to the possibility for mineral identification from spectroscopy (5).

Crytsl field splitting
Encyclopedia Britannica, Crystal Field Splitting, http://www.britannica.com
Figure 1. Crystal field splitting.  In an octahedral complex, the d oribitals of the central metal ion divide into two sets of different energies.  The separation ins energy is the crystal-field splitting energy, delta. 

(A) When delta is large, it is energetically favorable for electrons to remain in the lower set of orbitals, even though electrons must occupy the same orbital.

(B)  When delta is small, it is energetically favorable for electrons to occupy both sets in order to create as many parallell electron spins as possible.

Vibrational Process

The bonds in a molecule or crystal lattice are like springs with attached weights.  Therefore, the whole system can vibrate.  Vibrational energy depends on the strength of the bond and the mass of the attached atoms (13).  For a molecule with N atoms, there are 3N-6 normal modes of vibrations called fundamentals.  However, if the molecule is linear, this equation changes to 3N-5.  In addition to fundamental vibrations, each vibration can also occur at rough multiples of the original fundamental frequency.  These supplemental vibrations are called overtones.  A vibrational absorption will e seen in the IR spectrum only if the molecule has an oscillating dipole.  If the molecules has a dipole moment it can be labeled IR active.  Any symmetric molecule, like N2 or O2, is not normally IR active.  Vibrations from two or more modes can occur at the same frequency and if this occurs they are degenerate because they cannot be distinguished on the spectra (13).  However, in a crystal these degenerate modes may differ slightly because of the non-symmetric influences of the crystal field (13).  Traditionally the frequencies of fundamental vibrations are labeled with a greek letter nu and a subscript (13).  Each higher overtone or combintaion is typically 30 to 100 times weaker than the last. 

Because of the existence of overtones and combinations, the spectrum of a mineral can be complicated.  For this reason, diagnostic information about mineralsis typically gained from 2nd and 3rd overtones and combinations in reflectance spectroscopy (13).  Water and hydroxyl also produce particularly useful absorptions in minerals.  The water molecule has N=3, so there are 3 fundamental vibrations (3(3) - 6).  These three fundamental vibrations are due to the symmetric stretch, the bend and the asymmetric stretch.  In water vapor they occur at 2.738 microns, 6.270 microns, and 2.663 microns, respectively (13).  In liquid water these frequencies shift slightly due to hydrogen bonding. Additionally, overtones of water do appear in reflectance spectra of water-bearing minerals (13).  The existence of overtones can help to differentiate between the presence of water or the presence of hydroxyl (13).  Carbonates also show diagnostic vibrational absoprtion bands.  Because carbonate is N=4, there are six vibrational modes.  However, two of these modes are degenerate and two of these often appear as a doublet (13).  Therefore, in carbonate there are four distinct vibrational modes.  The four modes include the symmetric stretch, the out-of-plane bend, the asymmetric stretch, and the in-plane bend. 

Monazite Spectrum
Monazite Mineral
Monazite Spectrum, http://minerals.caltech.edu/COLOR_Causes/Metal_Ion/monazite2349.gif
Figure 2. Spectrum of monazite mineral.  Rare earth elements are abundant in this mineral and have brown to orange-brown colors.  The narrow peaks on the spectrum are due to Ce, Pr, and Nd.
Monazite Mineral, http://minerals.caltech.edu/COLOR_causes/Metal_Ion/monazite2349.jpg
Figure 3. Monazite mineral.  Notice the color of the mineral.  Brown and orange-brown tones are visible.

Lab reflectance spectroscopy was used in the analysis of the World Trade Center after it fell in September of 2001 (14).  In this example, analysis of the WTC ultraviolet to near-infrared reflectance spectroscopy was used (0.35 to 2.5 microns).  This technique has different sensitivities to materials than X-Ray Diffraction (14).  In this range, the spectroscopic technique is sensitive to hydroxyl and water-bearing materials, organic compounds, carbonates (like those found in marble), water-bearing sulfates (like gypsum used in wallboard) and iron-containing compounds (like hematite: iron rust used to color bricks red) (14).  Using this technique enabled scientists to determine what types of materials were present at and around the site of the WTC.  The table below presents a map of the sample locations and provides examples of spectra obtain from reflectance spectroscopy (14).  These spectra provide evidence for the presence of gypsum and chrysotile asbestos in the samples from the WTC site.

WTC spec 2
Asbestos Spectra
WTC mao

USGS, Chrysotile Spectrum, http://pubs.usgs.gov/of/2001/ofr-01-0429/spectra/chrysotile.spectrum.large.gif

Figure 4. Spectra of each dust sample were averaged and corrected to absolute reflectance using a National Institute for Standrds and Technology traceable spectral correction.  The first spectrum presents a 0.35 - 2.5 micron spectral region that is dominated by vibrational absorptions caused by gypsum from pulverized wallboard.  The second spectrum represents chrysotile asbestos.  The region is dominated by strong 1.385 and 2.323 micron vibrational absorptions.  Overtones of OH are also labeled.
USGS, World Trade Center Site Map, http://pubs.usgs.gov/of/2001/ofr-01-0429/locmap10.29.01.large.gif

Figure 5. Reflectance spectra of 33 dust samples collected on Sept. 17, 18, and 19th from areas within a 1 km radius circle of the WTC collapse site.  Samples were measured in a laboratory using HEPA fumehood with an Analytical Spectral Devices Full Range Spectrometer over the range from 0.35 microns.  A halogen lamp was used for illumination and a spectralon panel for reference.

Sensitivity and Application of Reflectance Spectroscopy

Reflectance spectroscopy provides a wealth of information about mineralogy, as well as other topics.  However, spectroscopy is not as widely used as expected (11).  This is because it is extremely sensitive, perhaps overly sensitive, to subtle changes in crystal structure or chemistry.  Though sensitivity can be a positive thing, it can also lead to confusion.  However, scientists are starting to recognize the power of this technology for studying the structure and composition of minerals (11).   Consequently,  scientists are looking to learn more about the complexity and sensitivity of spectroscopy.  When this technology can be better understood it has the potential to be used as an incredible diagnostic tool.  Some of the possible applications include diagnostics of clay mineralogy, pyroxene composition determination, and  mineral mapping (11).

Reflectance spectroscopy can be used without sample preparation and is non-destructive (13).  This makes mapping of minerals from aircrafts possible.  The benefits of reflectance spectroscopy have made it possible to expand and develop AVIRIS (13).  Reflectance spectroscopy is a rapidly growing science that can be used to derive significant information about the environment with little or no sample preparation (13).  This method can be used when other methods would be too time consuming.  This research can provide spatially gridded spectra over an area, set up real-time monitoring and create databases of spectral properties of materials (13).

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