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Accueil du site → Doctorat → États-Unis → 1985 → EVALUATION OF THE PHYSICAL PROPERTIES OF NATURALLY OCCURRING IRON(III) OXYHYDROXIDES ON ROCK SURFACES IN ARID AND SEMI-ARID REGIONS USING VISIBLE AND NEAR INFRARED REFLECTANCE SPECTROSCOPY

University of Washington (1985)

EVALUATION OF THE PHYSICAL PROPERTIES OF NATURALLY OCCURRING IRON(III) OXYHYDROXIDES ON ROCK SURFACES IN ARID AND SEMI-ARID REGIONS USING VISIBLE AND NEAR INFRARED REFLECTANCE SPECTROSCOPY

CURTISS, BRIAN

Titre : EVALUATION OF THE PHYSICAL PROPERTIES OF NATURALLY OCCURRING IRON(III) OXYHYDROXIDES ON ROCK SURFACES IN ARID AND SEMI-ARID REGIONS USING VISIBLE AND NEAR INFRARED REFLECTANCE SPECTROSCOPY

Auteur : CURTISS, BRIAN

Université de soutenance  : University of Washington

Grade : Doctor of Philosophy (PhD) 1985

Résumé
Diffuse reflectance spectra (0.35 um to 2.5 um) of various naturally occurring iron(III) oxyhydroxides are presented, and the observed variance of the spectral features is explained in terms of the compositional, morphological and crystallographic properties of the samples. This information has then been used to interpret spectra of rock surfaces, and/or multispectral images, from the island of Hawaii and the Goldfield-Cuprite region of Nevada. The iron in these minerals is octahedrally coordinated with oxygen and/or hydroxyl ligands. The wavelength and intensity of spectral features, resulting from electronic transitions in the crystal, depend on ligand identity, the symmetry of the site, and the degree of long range order of the crystal lattice. In a lattice with true octahedral symmetry, iron(III) crystal field absorption bands are spin and parity forbidden ; it is only because of departures from true octahedral symmetry and exchange coupling of adjacent octahedra that these bands are seen. Since these are natural samples, departures from the ideal crystal structure occur resulting in changes in the intensities, positions, and widths of electronic absorption features. Because of the similarity of the spectral signatures of weathering and alteration-produced iron(III) oxyhydroxides, an algorithm for the mapping of hydrothermally altered areas in a multispectral image, which is based only on the spectral signature of alteration produced iron(III) oxyhydroxides, can result in false identifications. The redistribution of iron which is commonly associated with hydrothermal alteration results in high spatial frequency variation of the concentration of iron(III) oxyhydroxides. Surface weathering processes result in iron(III) oxyhydroxides concentrations with very little high spatial frequency variation. A technique of image analysis has been developed for the mapping of hydrothermally altered areas that takes advantage of this difference in spatial variation of iron(III) oxyhydroxide concentration between weathering and hydrothermal alteration processes. Only those areas of the image which have spectral signatures with a large contribution from iron(III) oxyhydroxides with high spatial frequency variation are mapped. Using this technique areas of hydrothermal alteration in the Cuprite hydrothermal District of Nevada have been mapped using aircraft multispectral scanner data.

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