The utility of multispectral remote sensing techniques for discriminating among materials is based on the differences that exist among their spectral properties. As distinct from spectral variations that occur as a consequence of target condition and environmental factors, intrinsic spectral features that appear in the form of bands and slopes in the visible and near infrared (.325 to 2.5 μm) bidirectional reflection spectra of minerals (and, consequently, rocks) are caused by a variety of electronic and vibrational processes. These processes, such as crystal field effects, charge‐transfer, color centers, transitions to the conduction band, and overtone and combination tone vibrational transitions are discussed and illustrated with reference to specific minerals. Spectral data collected from a large selection of minerals are used to generate a “spectral signature” diagram that summarizes the optimum intrinsic information available from the spectra of particulate minerals. The diagram provides a ready reference for the interpretation of visible and near infrared features that typically appear in remotely sensed data. In the visible‐near infrared region, the most commonly observed features in naturally occurring materials are due to the presence of iron in some form or other, or to the presence of water or OH groups.
Visible and near-infrared (0.35 to 2.5 •m) bidirectional reflection spectra were recorded for a suite of well-characterized hydrothermally altered rock samples. The spectra typically display well-defined bands caused by both electronic and vibrational processes in the individual mineral constituents. Electronic transitions in the iron-bearing constituent minerals produce diagnostic minima near 0.43, 0.65, 0.85, and 0.93 •m. Vibrational transitions in clay and waterbearing mineral constituents typically produce characteristic single or nmltiple features over limited spectral ranges near 1.4, 1.75, 1.9, 2.2, and 2.35 /•m. The most abundant feature-producing minerals present in these rocks are hematite, goethite, and alunite, while others frequently present are jarosite, kaolinite, potassium micas, pyrophyllite, montmorillonite, diaspore. and gypsum.This study shows that visible-near infrared sl)ectrometry is a reliable and rapid technique for detecting and identifying clay minerals and alunite in rocks. Because these minerals are important constituents of altered rocks, the feasibility of using the visible and near infrared for detecting altered rocks by remote-sensing techniques is indicated. The spectral region near 2.2 t•m is particularly important for this purpose.
Use of maximums in the infrared emission of particulate surfaces for compositional remote sensing is a recent development that has not been thoroughly investigated. We show that experimental conditions, such as particle size and packing, background temperature, atmospheric pressure, and uniformity of sample heating, all affect the wavelength of the emission maximum and the contrast of the spectrum. These effects emphasize the necessity for laboratory study of emission spectra under conditions which simulate the remote environment to provide appropriate information for correctly deciphering remote sensing data. When experimental conditions are held constant, simulating those on the moon, emission maximums are shown to be diagnostic of rock type.
Several inorganic materials are used to illustrate that bands in infrared emission spectra of solids change shape as the particle size of the sample is altered, and that the way in which a particular band changes depends upon the value of its absorption coefficient. The relationship found to exist between corresponding band shape changes in emission and reflection spectra indicates that Kirchoff's law holds to a first‐order approximation for particulate, or mat, surfaces. The implications of band shape changes are discussed in relation to remote compositional sensing, and it is pointed out as a consequence that the most obvious spectral features used for the identification of samples with polished surfaces are not necessarily the most useful for determining the composition of a target with a rough or mat surface.
Reflection spectra of particulate samples of minerals that commonly occur in hydrothermally altered rocks and soils were recorded to display their features at their natural spectral bandwidths in the near-infrared from 1.3 to 2.4 pm. Atmospheric transmission spectra were recorded over limited wavelength segments in the same region to demonstrate the availability of some of the diagnostic mineral bands that occur close to regions of intense absorption. Changes occur in the appearance of all these spectra caused by instrumental factors such as less than adequate spectral resolution and response time The features in the mineral spectra are sufficiently characteristic to be used for analytical purposes, especially including those near I .4 pm which are unavailable for remote-sensing activities because of atmospheric obscuration. For remote-sensing purposes, the features in the 2.2-km region are emphasized as particularly valuable because they are common to alteration minerals and allow discrimination from nonalteration minerals which provide features-only as-close as?.-Cpm. Detection of unique features near 1.76 pm that occur only in alunite and gypsum is possible through the atmosphere and so provides diagnostic potential. The location and shape of mineral spectral features are retained unaltered in the spectra of rocks, and intensity with which they appear is governed by the accessibility of the particular mineral in the rock to the interacting radiation. For remote-sensing purposes, it appears that at least two 0. I-pm wide filters in the 2.2-pm region would be necessary to unambiguously identify the presence or absence of alteration minerals, and that judicious selection of the exact location of filters could provide finer discrimination.
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