Graham R. Hunt

SPECTRAL SIGNATURES OF PARTICULATE MINERALS IN THE VISIBLE AND NEAR INFRARED

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 refe...

Near-infrared (1.3-2.4 mu m) spectra of alteration minerals; potential for use in remote sensing

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 mu m. 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 1.4 mu m which are unavailable for remote-sensing activities because of atmospheric obscuration. For remote-sensing purposes, the features in the 2.2-mu m 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 2.4 mu m. Detection of unique features near 1.76 mu m 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 0.1-mu m wide filters in the 2.2-mu m 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.

Mars: Components of infrared spectra and the composition of the dust cloud

Abstract Infrared spectra of Mars are made up of three separate components, each of which may dominate the spectrum under different Martian meteorological and observational conditions. By means of laboratory examples we show that both the shape and spectral contrast of the spectral curves change dramatically, depending on which component is dominant. Each experimental condition has been experienced during either the Mariner 69 or 71 observations. Comparing the preliminary Mariner 71 radiance data with laboratory transmission spectra, we suggest that the clay mineral montmorillonite could be the major component of the Martian dust cloud.

The use of near-infrared spectroscopy to determine the degree of serpentinization of ultramafic rocks

Visible and near‐infrared (0.35 to 2.5 μm) bidirectional reflection spectra were recorded for a suite of particulate samples from mineralogically well‐characterized serpenatinized ultramafic rocks. The reflection spectra typically exhibit well‐defined minima due to electronic and vibrational processes in the individual mineral constituents. The contrast of near‐infrared spectral features of primary magnesian silicate minerals and secondary hydrous‐serpentine group minerals can be used to indicate the degree of serpentinization of the rock, provided less than about 1 percent of finely divided magnetite is present. The effect of magnetite, apparent in rocks with more than 50 percent serpentine, is to reduce the overall reflectance and the contrast of spectral bands. Near‐infrared spectrometry is potentially a rapid and reliable technique for detecting the highly serpentinized rocks which constitute target areas for asbestos exploration.

Martian Surface Materials: Effect of Particle Size on Spectral Behavior

The presence of abundant limonite on Mars has long been the subject of controversy. Some advocates of abundant limonite also suggest that the albedo differences between Martian light and dark areas are caused by different sizes of particles in those areas. We show that the relative albedo is reversed from the blue to the red for samples of limonite with particles of different sizes. Observations of Mars reveal no blue-red albedo reversal between the light and dark areas. Consequently, the hypothesis of particle size control of albedo is incompatible with the presence of abundant limonite on Mars.

Lunar Surface Features: Mid-Infrared Spectral Observations

The moon has been observed and spectrally scanned at midinfrared wavelengths, in particular through the 16- to 24-micron atmospheric window. The data indicate that there are differences in mineral composition among several features of the lunar surface.

A statistical analysis of the reflectance of igneous rocks from 0.2 to 2.65 microns

Abstract Reflectance spectra from 0.2 to 2.65 microns were obtained for several igneous rocks and minerals. The reflectance of all the rock samples, when crushed, increases with decreasing particle size. A statistical analysis established the correlation between reflectance and the wavelength of energy, the generalized composition, and the particle size of the rock samples. Most of the absorption bands present in the spectra correspond to some form of water in the sample, or are common to unrelated rocks and minerals, and, therefore, are not diagnostic of composition. A correlation analysis between the generalized composition variable and the ratios of reflectance at two different wavelengths has indicated correlation coefficients as high as 0.88 and may prove useful in selecting the more characterizing regions of the spectrum for remote sensing experiments.

Lunar Eclipse: Infrared Images and an Anomaly of Possible Internal Origin

Infrared images of the lunar eclipse of 13 April 1968 were obtained and compared with infrared images of the 19 December 1964 eclipse. A similarity of apparent strength and distribution of most thermal anomalies on the maria is evident from inspection of these images, indicating that these features are not ephemeral. One new linear thermal anomaly was discovered, which is thermally enhanced during the lunar afternoon. Its close relation to a lunar crustal fracture line and other features of probable internal origin suggests that this anomaly may be of internal origin.

Infrared Emission Spectra: Enhancement of Diagnostic Features by the Lunar Environment

Information diagnostic of general rock type is available as a well-defined emission maximum (related to the Christiansen frequency), rather than as contrast-depleted minima, in the infrared spectra of particulate solids. The amount of spectral information varies directly with the sharpness of the thermal gradient at the sample surface. Lunar thermal conditions optimize this gradient, making the moon an excellent target for remote sensing.

Assessment of Landsat filters for rock type discrimination, based on intrinsic information in laboratory spectra

Ratio values of the reflected energy available through different Landsat band‐pass filters were formed using laboratory spectra of 284 particulate rock and soil specimens, and the potential for using these data to discriminate among rock types was examined. Based exclusively on the intrinsic spectral information in the multispectral scanner (MSS) bands, thereby excluding textural, geomorphic, and vegerational effects that may occur in the field, it was concluded that the ratios can generally be used to discriminate different lithologic units. Further, only mafic and ultramafic rocks, and alteration involving ferric oxide staining, typically display sufficiently characteristic spectral behavior to allow specific identification. Certain unique features, such as those produced by chrominum in some chloritic quartzites can be used for discrimination of a few rare rock types.