Spectroscopy of Rocks and Minerals, and Principles of Spectroscopy - Roger N. Clark.pdf - Spektroskopia molekularna - hinatka3991

USGS Spectroscopy Lab: Spectroscopy of Rocks and Minerals, and Principles of Spectroscopy

Spectroscopy of Rocks and Minerals, and

Principles of Spectroscopy

Roger N. Clark

U.S. Geological Survey, MS 964

Box 25046 Federal Center

Denver, CO 80225-0046

(303) 236-1332

(303) 236-1371 FAX

http://speclab.cr.usgs.gov

rclark@speclab.cr.usgs.gov

Derived from Chapter 1 in:

Manual of Remote Sensing

John Wiley and Sons, Inc

A. Rencz, Editor

New York

1999

This book chapter was produced by personel of the US Government

therefore it can not be copyrighted and is in the public domain

This Web Page last revised June 25, 1999

Contents

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USGS Spectroscopy Lab: Spectroscopy of Rocks and Minerals, and Principles of Spectroscopy

1. Introduction

1.1 About This Chapter.

1.2 Absorption and Scattering .

1.3 Spectroscopy Terms.

1.4 Imaging Spectroscopy.

1.5 Atmospheric Transmittance: Windows for Remote Sensing.

2. THE REFLECTION AND ABSORPTION PROCESSES

2.1 Reflection and Absorption. .

2.2 Emittance.

2.3 Summary.

3. CAUSES OF ABSORPTION

3.1 Electronic Processes.

3.1.1 Crystal Field Effects .

3.1.2 Charge Transfer Absorptions. .

3.1.3 Conduction Bands. .

3.1.4 Color Centers. .

3.2 Vibrational Processes.

3.2.1 Water and Hydroxyl.

3.2.2 Carbonates.

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USGS Spectroscopy Lab: Spectroscopy of Rocks and Minerals, and Principles of Spectroscopy

3.2.3 Other Minerals.

4. SPECTRA OF MISCELLANEOUS MINERALS AND MATERIALS

4.1 Organics.

4.2 Ices.

4.3 Vegetation.

5. THE SENSITIVITY OF ABSORPTION BANDS TO CRYSTAL STRUCTURE AND CHEMISTRY

5.1 Example: Pyroxenes.

5.2 OH Examples.

5.3 Al in Muscovite

5.4 Discussion.

6. THE SCATTERING PROCESS.

6.1 Mixtures.

6.2 Grain Size Effects.

6.3 The Continuum and Band Depth.

6.4 Continuum-Removed Spectral Feature Comparison.

6.5 Other Spectral Variability and Rules.

6.5.1 Viewing Geometry. .

6.5.2 Ratioing Spectra.

6.5.3 Iron Oxide, Hydroxide, Sulfate Complexity. .

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USGS Spectroscopy Lab: Spectroscopy of Rocks and Minerals, and Principles of Spectroscopy

7. QUANTITATIVE UNDERSTANDING: RADIATIVE TRANSFER THEORY.

7.1 "Hapke Theory."

8. SPECTRAL LIBRARIES.

9. CONCLUSIONS AND DISCUSSION.

Acknowledgements

10. References.

1. Introduction

1.1 About This Chapter. Spectroscopy is the study of light as a function of wavelength that has been emitted, reflected or scattered from a solid, liquid, or gas. In this

chapter I will primarily discuss the spectroscopy of minerals, but the principles apply to any material. No single chapter can cover this topic adequately, and one could

argue, not even a single book. Thus, in some ways, this chapter may fall short of expectations, depending on the reader. In this chapter, I have tried to provide an

overview of what is already known, some of which may be covered better in other reviews. I have also tried to present some of the practical lessons of spectroscopy,

some of which have been in use by spectroscopists as common knowledge, but have not necessarily been previously published in detail. See Adams (1975), Hunt

(1977), Farmer (1974), Hunt (1982); Clark and Roush (1984), Clark et al. (1990a), Gaffey et al . (1993), Salisbury (1993), and references in those papers for more

details.

Back to Contents

1.2 Absorption and Scattering . 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 may also originate from a surface, a process called emission. All natural surfaces emit photons

when they are above absolute zero. Emitted photons are subject to the same physical laws of reflection, refraction, and absorption to which incident photons are bound.

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 or emitted 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.

Back to Contents

1.3 Spectroscopy Terms. There are 4 general parameters that describe the capability of a spectrometer: 1) spectral range, 2) spectral bandwidth, 3) spectral sampling,

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USGS Spectroscopy Lab: Spectroscopy of Rocks and Minerals, and Principles of Spectroscopy

and 4) signal-to-noise ratio (S/N). Spectral range is important to cover enough diagnostic spectral absorptions to solve a desired problem. There are general spectral

ranges that are in common use, each to first order controlled by detector technology: a) ultraviolet (UV): 0.001 to 0.4 µm, b) visible: 0.4 to 0.7 µm, c) near-infrared

(NIR): 0.7 to 3.0 µm, d) the mid-infrared (MIR): 3.0 to 30 µm, and d) the far infrared (FIR): 30 µm to 1 mm (e.g. see The Photonics Design and Applications

Handbook, 1996 and The Handbook of Chemistry and Physics, any recent year). The ~0.4 to 1.0-µm wavelength range is sometimes referred to in the remote sensing

literature as the VNIR (visible-near-infrared) and the 1.0 to 2.5-µm range is sometimes referred to as the SWIR (short-wave infrared). It should be noted that these

terms are not recognized standard terms in other fields except remote sensing, and because the NIR in VNIR conflicts with the accepted NIR range, the VNIR and

SWIR terms probably should be avoided. The mid-infrared covers thermally emitted energy, which for the Earth starts at about 2.5 to 3 µm, peaking near 10 µm,

decreasing beyond the peak, with a shape controlled by grey-body emission.

Spectral bandwidth is the width of an individual spectral channel in the spectrometer. The narrower the spectral bandwidth, the narrower the absorption feature the

spectrometer will accurately measure, if enough adjacent spectral samples are obtained. Some systems have a few broad channels, not contiguously spaced and, thus,

are not considered spectrometers (Figure 1a). Examples include the Landsat Thematic Mapper (TM) system and the MODerate Resolution Imaging Spectroradiometer

(MODIS), which can't resolve narrow absorption features. Others, like the NASA JPL Airborne Visual and Infra-Red Imaging Spectrometer (AVIRIS) system have

many narrow bandwidths, contiguously spaced (Figure 1b). Figure 1 shows spectra for the mineral alunite that could be obtained by some example broadband and

spectrometer systems. Note the loss in subtle spectral detail in the lower resolution systems compared to the laboratory spectrum. Bandwidths and sampling greater

than 25 nm rapidly lose the ability to resolve important mineral absorption features. All the spectra in Figure 1b are sampled at half Nyquist (critical sampling) except

the Near Infrared Mapping Spectrometer (NIMS), which is at Nyquist sampling. Note, however, that the fine details of the absorption features are lost at the ~25 nm

bandpass of NIMS. For example, the shoulder in the 2.2-µm absorption band is lost at 25-nm bandpass. The Visual and Infrared Mapping Spectrometer (VIMS) and

NIMS systems measure out to 5 µm, thus can see absorption bands not obtainable by the other systems.

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Spectroscopy of Rocks and Minerals, and Principles of Spectroscopy - Roger N. Clark.pdf

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