Spectra of altered rocks in the visible and near infrared

Abstract

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.

An assessment of hydrothermal alteration in the Santiaguito lava dome complex, Guatemala: implications for dome collapse hazards

“…Clays and hydrous sulfate minerals (such as kaolinite and alunite) show distinct spectral features in the SWIR region, which spans 1,000-2,500 nm in wavelength. Spectral features in these minerals are related to vibrations of structural hydroxyl groups and molecular water (Hunt and Ashley 1979). The Advanced Spaceborne Thermal Emission and Reflection Radiometer Plagioclase is abbreviated to plag in mineral peak labels (ASTER) is a multispectral imaging instrument aboard NASA's Terra satellite, which also carries the Moderate Resolution Imaging Spectroradiometer (MODIS).…”

Mapping geological stages of climate-dependent iron and clay weathering alteration on lithologically uniform sedimentary units using Thematic Mapper imagery (Tertiary Duero Basin, Spain)

“…These hardened rocks show spectra with typically asymmetrical absorption features at 1400, 1900 and 2200 nm with thresholds at 1350, 1380 and 1930 nm and a shoulder between 1150 and 1170 nm (figure 4) (Hunt and Ashley 1979). Less important, a shoulder on 2400 nm can be seen.…”

“…Satellite imagery in the visible and near-infrared (0.4-2.5 fim) has been conven tionally used to map hydrothermal alteration related to mineral deposits (Hunt and Ashley 1979, Prost 1980, Podwysocki et al 1983, Townsend 1987, Fraser 1991.…”

“…Limonitic alteration was one of the first features attracting the attention of geological remote sensing (Rowan et al 1976, Abrams et al 1977, Hunt and Ashley 1979, Segal 1982, Townsend 1987. Weathering processes produce the same minerals as hydrothermal alteration processes (Buckingham and Sommer 1983), and mask the spectral response of underlying rocks with coatings and internal mineralogical transformations (Lyon 1997).…”

A standard design for collecting vegetation reference spectra: Implementation and implications for data sharing

“…Numerous references discuss the relationship between physical/biophysical variables and EMR, including: the way in which minerals absorb photons and their associated wavelength dependency (for example, see Hunt and Salisbury, 1970;Hunt et al, 1971a and b;Rowan et al, 1977;Hunt and Ashley, 1979;Hunt, 1977 and1979;Goetz and Rowan, 1981); the spectral reflectance of vegetation (Collins, 1978;Horler et al, 1983;Milton and Mouat, 1989;Boochs et al, 1990;Elvidge, 1990;King et al, 1995;Campbell, 1996;Dawson and Curran, 1998;Datt, 1999 and2000); and, characteristics of soils (Baumgardner et al, 1985;Irons et al, 1989).…”