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

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SPECTRAL INFORMATION
Field Spectrometers, whether they are based on an integrating sphere or fiber
optic cable, allow the collection and presentation of spectral data. Although the
primary objective of this operation is the identification of the minerals found in
different alteration systems, there is other information that can be obtained from
the data and which is of great significance in the interpretation of the spectral
information. These include chemical substitution, crystallinity, effects of water,
paragenesis and temperature.
CHEMISTRY
The ability to discern compositional differences in mineral groups that have solid
solution substitution is very useful. Some examples are the carbonates,
amphiboles, chlorites, illite/micas, tourmalines, smectite clays, alunites, all of
which exhibit wavelength shifts with cation substitution (Figure 7A, 7B, and 7C).
A B
Figure 7 – The spectral plots in [A] are for alunites of different compositions from Ca to Na
to K to NH4. Note that the diagnostic feature ranges from 1495nm for Ca to 1460nm for
NH4 . In Figure 7B, the diagnostic wavelength shift occurs in the 2330 nm range for Mgchlorites
shifting to 2350+ for Fe-chlorites. The carbonate group is shown in [C]. These
mineral show a significant shift with different cation compositions ranging from (top to
bottom) aragonite (Ca), Dolomite (Mg, Ca), calcite (Ca), rhodocrosite (Mg), stronianite (Sr),
magnesite (Mg), cerrusite (Pb), siderite (Fe), ankerite (fe, Mg), malachite (Cu), and azurite
(Cu).
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C