Spectra of altered rocks in the visible and near infrared

Visible and near-infrared (0.35 to 2.5 mu 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 mu m. Vibrational transitions in clay and water-bearing mineral constituents typically produce characteristic single or multiple features over limited spectral ranges near 1.4, 1.75, 1.9, 2.2, and 2.35 mu 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 spectrometry 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 mu m is particularly important for this purpose.