About Reflectance Spectroscopy

Note: this document currently contains many subscripts and superscripts for things like H2O (water) but there are now html standard formatting commands for subscripts and superscripts yet. So, based on context, you must figure out for yourself if a number or letter should be a subscript or superscript. In most cases, subscripts are indicated. A ^ is used to signify a superscript (Fe^2+ is Fe superscript 2+).

Note: Figures are not here yet, but will be added soon.

Introduction

Reflectance spectroscopy is the study of light as a function of wavelength that has been reflected or scattered from a solid, liquid, or gas. In this chapter I will discuss the reflectance spectroscopy of minerals, but the principles apply to any material.

As photons enter a mineral, some are reflected from grain surfaces, some pass through the grain, and some are absorbed. Those photons that are reflected from grain surfaces or refracted through a particle are said to be scattered. Scattered photons may encounter another grain or be scattered away from the surface so they may be detected and measured.

Photons are absorbed in minerals by several processes. The variety of absorption processes and their wavelength dependence allows us to derive information about the chemistry of a mineral from its reflected light. The human eye is a crude reflectance spectrometer: we can look at a surface and see color. Our eyes and brain are processing the wavelength-dependent scattering of visible-light photons to reveal something about what we are observing, like the red color of hematite or the green color of olivine. A modern spectrometer, however, can measure finer details over a broader wavelength range and with greater precision. Thus, a spectrometer can measure absorptions due to more processes than can be seen with the eye.

The Absorption Process

When photons enter an absorbing medium, they are absorbed according to Beers Law:

                         -kx
                 I = Io e                                        (eqn 1)

Causes of Absorption: Electronic Processes

Isolated atoms and ions have discrete energy states. Absorption of photons of a specific wavelength causes a change from one energy state to a higher one. Emission of a photon occurs as a result of a change in an energy state to a lower one. When a photon is absorbed it is usually not remitted at the same wavelength. For example, it can cause heating of the material, resulting in grey-body emission at longer wavelengths.

In a solid, electrons may be shared between individual atoms. The energy level of shared electrons may become smeared over a range of values called "energy bands." However, bound electrons will still have quantized energy states.

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. For all transition elements, unfilled d orbitals have identical energies in an isolated ion, but the energy levels split when the atom is located in a crystal field. This splitting of the orbital energy states enables an electron to be moved from a lower level into a higher one by absorption of a photon having an energy matching the energy difference between the states. The energy levels are determined by the valence state of the atom (e.g. Fe^2+, Fe^3+), its coordination number, and the symmetry of the site it occupies. The levels are also influenced by the type of ligands formed , the extent of distortion of the site, and the value of the metal-ligand interatomic distance (e.g. see Burns, 1970). The crystal field varies with crystal structure from mineral to mineral, thus the amount of splitting varies and the same ion (like Fe^2+) produces obviously different absorptions, making specific mineral identification possible from spectroscopy (Figure 1(NOTE: will be scanned and included soon)).

The unfilled shells of rare earth ions involve deep-lying electrons which are well shielded from crystal fields so the energy levels remain largely unchanged. Thus, absorption bands due to rare earth elements are not diagnostic of mineralogy but to the presence of the ions in the mineral (Figure 2(NOTE: will be scanned and included soon)).

Absorption bands can also be caused by charge transfer, or inter-element transition where the absorption of a photon causes an electron to move between ions or between ions and ligands. The transition can also occur between the same metal in different valence states, such as between Fe^2+ and Fe^3+. In general, absorption bands caused by charge transfer are diagnostic of mineralogy. Their strengths are typically hundreds to thousands of times stronger than crystal field transitions. The band centers usually occur in the ultraviolet with the wings of the absorption extending into the visible, which are the main cause of the red color of iron oxides (Figure 1c(NOTE: will be scanned and included soon)).

In some minerals, there are two energy levels in which electrons may reside: a higher level called the "conduction band," where electrons move freely throughout the lattice, and a lower energy region called the "valence band," where electrons are attached to individual atoms. The difference between the energy levels is called the band gap. The band gap is typically small or non-existent in metals, and very large in dielectrics. In semiconductors, the band gap corresponds to the energy of visible or near-infrared photons and the spectrum in these cases is approximately a step function. The yellow color of sulfur is caused by such a band gap. The minerals cinnabar (HgS) and rutile (TiO2) have spectra showing the band gap in the visible (Figure 3(NOTE: will be scanned and included soon)).

A few minerals show color by "color centers." A color center is caused by irradiation (e.g. by solar UV radiation) of an imperfect crystal. Energy levels are produced because of the defects and electrons can become bound to them. The movement of an electron into the defect requires photon energy. The yellow, purple and blue colors of fluorite are caused by color centers.

More detailed discussions of electronic processes can be found in the review paper by Hunt (1977) and the book by Burns (1970).

Causes of Absorption: Vibrational Processes

The bonds in a molecule or crystal lattice are like springs with attached weights: the whole system can vibrate. The frequency of vibration depends on the strength of each spring and their masses. For a molecule with N atoms, there are 3N-6 normal modes of vibrations called fundamentals. Each vibration can also occur at roughly multiples of the original fundamental frequency. The additional vibrations are called overtones when involving multiples of a single fundamental, and combinations when involving different types of vibrations.

A vibrational absorption will be seen in the infrared spectrum only if the molecule responsible shows a dipole moment (it is said to be infrared active). A symmetric molecule, like N2 is not normally infrared active unless it is distorted (for example under high pressure). Vibrations from two or more modes can occur at the same frequency, and because they can't be distinguished, are said to be degenerate. An isolated molecule with degenerate modes may show the modes at slightly different frequencies in a crystal because of the non-symmetric influences of the crystal field.

Traditionally, the frequencies of fundamental vibrations are labeled with the greek letter nu and a subscript. If a molecule has vibrations nu1, nu2, nu3, then it can have overtones and combinations at approximately 2nu1, 3nu1, 2nu2, nu1+nu2, and so on. Each higher overtone or combination is typically 30 to 100 times weaker than the last. Consequently, the spectrum of a mineral can be quite complex. In reflectance spectroscopy, these weak absorptions can be measured easily and diagnostic information routinely gained from 2nd and 3rd overtones and combinations.

Water and OH (hydroxyl) produce particularly diagnostic absorptions in minerals. The water molecule (H2O) has N=3, so there are 3N-6=3 fundamental vibrations. In the isolated molecule (vapor phase) they occur at 2.738 microns (nu1, symmetric OH stretch), 6.270 microns (nu2, H-O-H bend), and 2.663 microns (nu3, asymmetric OH stretch). In liquid water the frequencies shift due to hydrogen bonding: nu1=3.106 microns, nu2=6.079 microns, and nu3=2.903 microns.

The overtones of water are seen in reflectance spectra of H2O-bearing minerals. The first overtones of the OH stretches occur at about 1.4 microns and the combinations of the H-O-H bend with the OH stretches are found near 1.9 microns. Thus, a mineral whose spectrum has a 1.9-microns absorption band contains water, but a spectrum that has a 1.4-microns band but no 1.9-microns band indicates that only hydroxyl is present.

The hydroxyl ion has only one stretching mode and its wavelength position is dependent on the ion to which it is attached. In spectra of OH-bearing minerals, the absorption is typically near 2.7 to 2.8 microns, but can occur anywhere in the range from about 2.67 microns to 3.45 microns. The OH commonly occurs in multiple crystallographic sites of a specific mineral and is typically attached to metal ions. Thus there may be more than one OH feature. The metal-OH bend occurs near 10 microns (usually superimposed on the stronger Si-O fundamental in silicates). The combination metal-OH bend plus OH stretch occurs near 2.2 to 2.3 microns and is very diagnostic of mineralogy.

Carbonates also show diagnostic vibrational absorption bands. The observed absorptions are due to the planar CO3^-2 ion. There are four vibrational modes in the free CO3^-2 ion: the symmetric stretch, nu1: 1063 cm^-1 (9.407 microns); the out-of-plane bend, nu2: 879 cm^-1 (11.4 microns); the asymmetric stretch, nu3: 1415 cm^-1 (7.067 microns); and the in-plane bend, nu4: 680-1 (14.7 microns). The nu1 band is not infrared active in minerals. There are actually six modes in the CO3^-2 ion, but 2 are degenerate with the nu3 and nu4 modes. In carbonate minerals, the nu3 and nu4 bands often appear as a doublet. The doubling has been explained in terms of the lifting of the degeneracy (e.g. see White, 1974) due to mineral structure and anion site.

Phosphates, borates, arsenates, and vanadates also have diagnostic vibrational spectra.

Typical spectra of minerals with vibrational bands are shown in Figure 4(NOTE: will be scanned and included soon). See Hunt and Salisbury (1970, 1971), Hunt el al. (1971a, 1971b, 1972, 1973), Hunt (1977, 1979), Gaffey (1986, 1987), Clark et al, (1990), King and Clark (1989) and Farmer (1974) for more details. A summary of absorption band positions is shown in Figure 5(NOTE: will be scanned and included soon).

The Sensitivity of Absorption Bands to Crystal Structure and Chemistry

Reflectance spectroscopy shows a wealth of information about mineralogy. Why, then, is spectroscopy not a more widely used technique? In many cases spectroscopy is overly sensitive to subtle changes in crystal structure or chemistry. This has resulted in confusion in the past. More recently, this sensitivity has been recognized as a powerful means of studying the structure and composition of minerals. Additional problems occur with reflectance spectra due to scattering and will be discussed below.

Because spectroscopy is sensitive to so many processes, the spectra can be very complex and there is still much to learn. However, it is because of this sensitivity that spectroscopy has great potential as a diagnostic tool. Here, a few examples of the possibilities will be shown.

As shown in Figure 1b(NOTE: will be scanned and included soon), the iron bands near 1 and 2 microns shift with pyroxene composition. This series has been calibrated by Adams (1974). The olivine 1-microns band also shifts with composition (Figure 1a(NOTE: will be scanned and included soon)), although more subtly than with pyroxenes, and the shift has been calibrated by King and Ridley (1987).

The sharper OH-related absorption bands allow ever smaller band shifts to be measured. These bands can be so sensitive that it is possible to distinguish between the isochemical end-members of the Mg-rich serpentine group (chrysotile, antigorite, and lizardite; King and Clark, 1989, and Figure 6(NOTE: will be scanned and included soon)). The Fe:Fe+Mg ratio can be estimated from reflectance spectra of minerals with brucite-like structure (Clark et al, 1990, and Figure 6(NOTE: will be scanned and included soon)). The structure of the 2.2-microns Al-OH band has been shown to be diagnostic of disorder of kaolinite-dickite mixtures (Crowley and Vergo, 1988) and the degree of kaolinite crystallinity (Clark et al, 1990) and is illustrated in Figure 4c(NOTE: will be scanned and included soon).

The strong and sharp OH features have proven particularly diagnostic of clay mineralogy, perhaps better than with X-ray diffraction (XRD) analysis (like any method, spectroscopy has advantages in some areas and XRD in others). For example, it appears easy to distinguish kaolinite from halloysite with spectroscopy (e.g. Clark et al. 1990), as shown in Figure 4c(NOTE: will be scanned and included soon). Montmorillonite is easily distinguished from illite (e.g. Clark et al 1990) whereas XRD analysis combines them into the general term smectites.

Reflectance spectroscopy can be used without sample preparation, and it is non-destructive. This makes mapping of minerals from aircraft possible, including detailed clay mineralogy (e.g. Clark et al, 1992, 1993a).

Spectroscopy, on the other hand, is insensitive to some mineralogy in the visible and near-infrared wavelength region. For example, quartz has no diagnostic spectral features in the visible and near-infrared; in fact it is used as optical components in many telescopes and prisms. Quartz must be detected at its fundamental Si-O stretching region near 10 microns.

The Scattering Process

Scattering is the process that makes reflectance spectroscopy possible: photons enter a surface, are scattered one or more times, and while some are absorbed, others are scattered from the surface so we may see and detect them. Scattering can also be thought of as scrambling information. The information is made more complex, and because scattering is a non-linear process, recovery of quantitative information is difficult.

Consider the simple Beers Law in equation 1. In transmission, light passes through a slab of material. There is little or no scattering (none in the ideal case; but there are always internal reflections from the surfaces of the medium). Analysis is relatively simple. In reflectance, however, the optical path of photons is a random walk. At each grain the photons encounter, a certain percentage are absorbed. If the grain is bright, like a quartz grain at visible wavelengths, most photons are scattered and the random walk process can go on for hundreds of encounters. If the grains are dark, like magnetite, the majority of photons will be absorbed at each encounter and essentially all photons will be absorbed in only a few encounters. The random walk process, scattering and the mean depth of photon penetration are discussed in Clark and Roush (1984). This process also enhances weak features not normally seen in transmittance, further increasing reflectance spectroscopy as a diagnostic tool.

In a mixture of light and dark grains (e.g. quartz and magnetite) the photons have such a high probably of encountering a dark grain that a few percent of dark grains can drastically reduce the reflectance, much more than their weight fraction. A general rule with mixtures is that at any given wavelength, the darker component will tend to dominate the reflectance. The effect is illustrated in Figure 7(NOTE: will be scanned and included soon) with spectra of samples having various proportions of charcoal grains mixed with montmorillonite.

The amount of light scattered and absorbed by a grain is dependent on grain size. A larger grain has a larger internal path where photons may be absorbed according to Beers Law. It is the reflection from the surfaces and internal imperfections that control scattering. In a smaller grain there are proportionally more surface reflections compared to internal photon path length, or in other words, the surface-to-volume ratio is a function of grain size. As the grain size increases, the reflectance decreases, as shown in the spectra for pyroxene in Figure 8(NOTE: will be scanned and included soon).

Absorptions in a spectrum have two components: continuum and individual features. The continuum is the "background absorption" onto which other absorption features are superimposed. It may be due to the wing of a larger absorption feature. For example, in the pyroxene spectra in Figure 8(NOTE: will be scanned and included soon), the weak feature at 2.3 microns is due to a trace amount of tremolite in the sample and the absorption is superimposed on the larger 2-microns pyroxene band. The broader pyroxene absorption is the continuum to the narrow 2.3-microns feature. The pyroxene 1.0-microns band is superimposed on the wing of a stronger absorption centered in the ultraviolet.

The depth of an absorption band, D, is usually defined relative to the continuum, Rc:

D = Rb / Rc (eqn 2)

where Rb is the reflectance at the band bottom, and Rc is the reflectance of the continuum at the same wavelength as Rb.

The depth of an absorption is related to the abundance of the absorber and the grain size of the mineral. Consider a particulate surface with two minerals, one whose spectrum has an absorption band. As the abundance of the second mineral is increased, the band depth, D, of the absorption will decrease. Next consider the reflectance spectrum of a pure powdered mineral. As the grain size is increased from a small value, the absorption band depth, D will first increase, reach a maximum, and then decrease. This can be seen with the pyroxene spectra in Figure 8(NOTE: will be scanned and included soon). If the particle size were made larger and larger, the reflectance spectrum would eventually consist only of first surface reflection. The reflectance can never go to zero because of this reflection, unless the index of refraction of the material is 1.0.

Conclusions and Discussion

Reflectance spectroscopy is a rapidly growing science that can be used to derive significant information about mineralogy and with little or no sample preparation. It may be used in applications when other methods would be too time consuming. For example, imaging spectrometers are already acquiring millions of spatially gridded spectra over an area from which mineralogical maps are being made. It is possible to set up real-time monitoring of processes using spectroscopy, such as monitoring the mineralogy of drill cores at the drilling site. Research is still needed to better understand the subtle changes in absorption features before reflectance spectroscopy will reach its full potential. Good spectral databases documenting all the absorption features are also needed before reflectance spectroscopy can be as widely used a tool as XRD. Spectral databases are now becoming available (e.g. Clark et al, 1993b), and research continues on the spectral properties of minerals, but it will take about a decade before general software tools are available to allow reflectance spectroscopy to challenge other analytical methods. For certain classes of minerals, however, spectroscopy is already an excellent tool. Among these classes are clay mineralogy, OH-bearing minerals, olivines and pyroxenes.

Space limits the contents of any review article covering such a broad topic. Other review articles are Adams (1975), Hunt (1977), Gaffey et al, (1993) and Salisbury (1993).

Adams, J.B., Interpretation of visible and near-infrared diffuse reflectance spectra of pyroxenes and other rock-forming minerals, in Infrared and Raman Spectroscopy of Lunar and Terrestrial Minerals, Academic Press, New York, 94-116, 1975

Adams, J.B., Visible and Near-Infrared Diffuse Reflectance Spectra of Pyroxenes as Applied to Remote Sensing of Solid Objects in the Solar System, J. Geophys Res. 79, 4829-4836, 1974.

Chandrasekhar, S., Radiative Transfer, Dover Publ. Inc., New York, NY, 393p, 1960.

Clark, R.N., Spectral Properties of Mixtures of Montmorillonite and Dark Carbon Grains: Implications for Remote Sensing Minerals Containing Chemically and Physically Adsorbed Water, J. Geophys. Res. 88, 10635-10644, 1983.

Clark, R.N., and Roush, T.L., Reflectance spectroscopy: Quantitative analysis techniques for remote sensing applications, J. Geophys. Res., 89, 6329-6340, 1984.

Clark, R.N., T.V.V. King, M. Klejwa, G. Swayze, and N. Vergo, High Spectral Resolution Reflectance Spectroscopy of Minerals, J. Geophys Res. 95, 12653-12680, 1990.

Clark, R.N., G.A. Swayze, and A. Gallagher, Mapping Minerals with Imaging Spectroscopy, U.S. Geological Survey, Office of Mineral Resources Bulletin 2039, pp. 141-150, 1993a.

Clark, R.N., G.A. Swayze, A. Gallagher, T.V.V. King, and W.M. Calvin, The U. S. Geological Survey, Digital Spectral Library: Version 1: 0.2 to 3.0 \(*mm, U.S. Geological Survey, Open File Report 93-592, 1326 pages, 1993b.

Crowley, J.K. and Vergo, N., Near-infrared reflectance spectra of mixtures of kaolin group minerals: use in clay studies, Clays and Clay Min., 36, 310-316, 1988.

Farmer, V.C., The layer silicates, in The Infra-Red Spectra of Minerals, (V.C. Farmer, ed.) Mineralogical Society, London, 331-364, 1974.

Gaffey, S.J., Spectral reflectance of carbonate minerals in the visible and near infrared (0.35-2.55 \(*mm): Calcite, aragonite and dolomite, Am. Mineral. 71, 151-162, 1986.

Gaffey, S.J., Spectral reflectance of carbonate minerals in the visible and near infrared (0.35-2.55 \(*mm): Anhydrous carbonate minerals, J. Geophys. Res. 92, 1429-1440, 1987.

Gaffey, S.J., L.A. McFadden, D. Nash, and C.M. Pieters, Ultraviolet, Visible, and Near-infrared Reflectance Spectroscopy: Laboratory spectra of Geologic Materials, in Remote Geochemical Analysis: Elemental and Mineralogical Composition (C. M. Pieters, and P.A.J. Englert, eds.), Cambridge University Press, Cambridge, 43-78, 1993.

Hapke, B., Bidirectional reflectance spectroscopy 1. Theory, J. Geophys. Res. 86, 3039-3054, 1981.

Hunt, G.R., Spectral signatures of particulate minerals, in the visible and near-infrared, Geophysics 42, 501-513, 1977.

Hunt, G.R., Near-infrared (1.3-2.4 \(*mm) spectra of alteration minerals-Potential for use in remote sensing, Geophysics 44, 1974-1986, 1979.

Hunt, G.R., and Salisbury, J.W., Visible and near infrared spectra of minerals and rocks. I. Silicate minerals, Mod. Geology 1, 283-300, 1970.

Hunt, G.R., and Salisbury, J.W., Visible and near infrared spectra of minerals and rocks. II. Carbonates, Mod. Geology 2, 23-30, 1971.

Hunt, G.R., Salisbury, J.W. and Lenhoff, C.J., Visible and near infrared spectra of minerals and rocks. III. Oxides and hydroxides, Mod. Geology 2, 195-205, 1971a.

Hunt, G.R., Salisbury, J.W. and Lenhoff, C.J., Visible and near infrared spectra of minerals and rocks. IV. Sulphides and sulphates, Mod. Geology 3, 1-14, 1971b.

Hunt, G.R., Salisbury, J.W. and Lenhoff, C.J., Visible and near infrared spectra of minerals and rocks. V. Halides, arsenates, vanadates, and borates, Mod. Geology 3, 121-132, 1972.

Hunt, G.R., Salisbury, J.W. and Lenhoff, C.J., Visible and near infrared spectra of minerals and rocks. VI. Additional silicates, Mod. Geology 4, 85-106, 1973.

Hunt, G.R., Modifications of integrating sphere accessory to allow spectroscopic measurements of horizontal surfaces from above, Applied Optics, 19, 1746-1747, 1980.

King, T.V.V. and Clark, R.N., Spectral characteristics of serpentines and chlorites using high resolution reflectance spectroscopy, J. Geophys. Res. 94, 13997-14008, 1989.

King, T.V.V. and W.I. Ridley, Relation of the Spectroscopic Reflectance of Olivine to Mineral Chemistry and Some Remote Sensing Implications, J. Geophys. Res. 92, 11457-11469, 1987.

Morris, R.V., Lauer, H.V., Lawson, C.A., Gibson, E.K. Jr., Nace, G.A., and Stewart, C. Spectral and other physiochemical properties of submicron powders of hematite (\(*a-Fe\d2\uO\d3\u), maghemite (\(*g-Fe\d2\uO\d3\u), maghemite (Fe\d3\uO\d4\u), goethite (\(*a-FeOOH), and lepidochrosite (\(*g-FeOOH), J. Geophys. Res. 90, 3126-3144, 1985.

Salisbury, J.W., Mid-infrared spectroscopy: Laboratory data, in Remote Geochemical Analysis: Elemental and Mineralogical Composition (C. M. Pieters, and P.A.J. Englert, eds.), Cambridge University Press, Cambridge, 79-98, 1993.

Sherman, D.M. Crystal Chemistry, electronic structures and spectra of Fe sites in clay minerals, in Spectroscopic Characterization of Minerals and their Surfaces L.M. Coyne, S.W.S. McKeever, and D.F. Drake, eds.) pp. 284-309. American Chemical Society, Washington DC, 1990.

U.S. Geological Survey, a bureau of the U.S. Department of the Interior

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