The value of 50 mu m was used as an approximate maximum penetration depth to include all mineral species which may contribute to a reflectance signal. Jarosite and alunite occur only in altered areas, whereas hematite and goethite occur in both altered and unaltered areas. Incorporation of aluminum into hematite occurs only in unaltered areas. The spectral shift to longer wavelengths which accompanies aluminum substitution should be useful in delineating mineralized zones.--Modified journal abstract.
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Buckingham et al. (1983) studied this question.