Laboratory Spectra of Geologic Materials
from Part I - Theory of Remote Compositional Analysis Techniques and Laboratory Measurements
Published online by Cambridge University Press: 15 November 2019
Visible/near-infrared (VNIR) reflectance spectra are used in laboratory, field, and airborne studies to characterize geologic materials. This chapter covers the region 0.3–5 µm and describes the species responsible for the absorption of radiation at specific wavelengths that create spectral features used to identify minerals, rocks, and other geologic materials. Fe contributes greatly to VNIR spectral signatures, producing features near 1 and 2 µm for Fe2+ in spectra of pyroxene and glass, while a broad, strong band from ~0.9 to 1.3 µm is characteristic of Fe2+ in olivine, carbonate, and many sulfates; a weak band near 1.2 µm is due to Fe2+ in feldspar; and bands near 0.6 and 0.9 µm arise from Fe3+ in ferric oxides/hydroxides. Water bands occur near 0.96, 1.15, 1.4, 1.9, and 2.9 µm, depending on the mineral structure, while structural OH bands occur near 1.4, 2.1–2.5, and 2.7 µm. Additional features are observed for carbonates, nitrates, sulfates, phosphates, chlorides, and perchlorates. The spectral signatures of geologic samples are also affected by how photons interact with particles in the sample. Factors such as grain size, coatings and mixtures influence the reflectance, transmittance, and absorption of photons at grain boundaries and contribute to the VNIR spectral properties of geologic materials.
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