An overview of vis-nir-swir field spectroscopy - Spectral International

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In the visible region, this is caused by electronic transitions such as Crystal Field
Effects (atomic energy level transitions), Charge Transfer (inter-element
electronic transitions), Conduction Band Transitions (electron transfer over
spatially close energy levels) and Color Center Phenomenon (lattice defect
induced energy levels). . Much of this simply involves release of energy when an
electron changes energy levels in an atom.
Absorption features in the SWIR region are a function of the composition of the
mineral. They are a manifestation of energy absorption within the crystal lattice
from vibrational state transitions. Because these vibrational states correspond to
distinct energy levels, the absorption features occur at well-defined wavelength
positions. The energy levels that define these wavelengths are a function of the
size of the ionic radii of the cations bonded to different molecules. The bonds will
vibrate at different wavelengths as a function of the length of the bond. Because
the bond lengths between a specific atom and molecule will be consistent, it is
possible to predict compositions and compositional changes in minerals being
analyzed by the wavelengths and wavelength shifts. (Hunt, 1977)
The transitions between energy levels and compositional differences are
manifested by absorption features at defined wavelengths. The common
absorption feature positions are listed in Table I.
TABLE I – MAJOR ABSORPTION FEATURES
POSITION MECHANISM MINERAL GROUP
~1.4 µm OH and WATER CLAYS, SULFATES
HYDROXIDES,
ZEOLITES
~1.56 µm NH4 NH4 SPECIES
~1.,8 µm OH SULFATES
~1.9 µm WATER SMECTITE
2.02, 2.12 µm NH4 NH4 SPECIES
~2.2 µm AL-OH CLAYS,.
SULFATES, MICAS
~2.29 Fe-OH Fe-CLAYS
~2.31 Mg-OH Mg-CLAYS,
ORGANICS
~2.324 Mg-OH CHLORITES
-2
~2.35 +/- µm CO3
CARBONATES
~2.35+ Fe-OH Fe-CHLORITES
The absorption features occur within a reflectance spectrum, with wavelength
positions and distinctive profiles that can be used to identify mineral and organic
phases. These are shown in Figure 2 below.
Each mineral has a distinctive spectral signature, composed of several
absorption features, which is a function of composition, crystallinity,
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