Illite sample - sometimes spectra of illite and smectite look similar. Here you can see
the difference with a distinctive doublet absorption feature between 2300-2500nm
that doesn't appear with smectite.
Kaolinite spectra identified using EZ-ID software. Scan taken in a soil pit in
Wisconsin.
Clay mineralogy in soil
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Clay minerals are a major component of most soils. They are layer
silicates typically formed by weathering of other silicate minerals.
Clays have a large surface area and can hold an electrical charge –
usually a negative charge that attracts and holds positively charged
ions from nutrients. This cation exchange capacity (CEC) has an
effect on soil structure stability, nutrient availability, soil pH and soil
reaction to fertilizers and other additives. In addition, various clays
have the ability to attract water molecules and swell which can impact
soil drainage. With the
PSR+ high resolution field spectroradiometer
and EZ-ID mineral identification software a researcher can identify
and analyze the clay properties of soil in the field or lab for a more
accurate assessment of soil characteristics.

Some of the most common clays are kaolinite, smectite, and illite.
Soils with a higher clay content tend to have a moderate to high CEC
and possess a greater capacity to retain nutrients against leaching.
High CEC soils have a higher water holding capacity and greater
capacity to resist changes to soil pH. Soils with more kaolinite and
illite tend to be less fertile than soils with a higher smectite content.  

The PSR+ running EZ-ID software with the USGS and optional
SPECMin spectral libraries can help identify different clay minerals in
soil samples. The PSR+ collects high resolution/low noise scans that
can be analyzed and matched against known samples in the EZ-ID
libraries by measuring and matching major absorption features.

For clays like illite, kaolinite and smectite (montorillonite),
absorptions near 2200 nm are due to combination vibrations and
overtones of Al-OH, H2O, OH and CO3.

For kaolinite the absorption doublet at 1400nm is due to overtones.
An O-H stretch combination and an Al-OH bend are shown near
2200nm. Smectite, a group that includes Montmorillonite, has strong
characteristics with absorptions at 1400, 1900 and 2200nm. Smectite
has an octahedral layer causing the first overtone of a structural O-H
stretching that appears at the 1400nm band. Bound water in the
lattices of Smectite will show bands at 1400 and 1900nm due to
vibrations. The combination bands that are caused by vibrations of
bound water come about at somewhat shorter wavelengths at 1400
and 1900nm.

Smectite and Illite can sometimes show very similar absorptions if
one does not know what to look for. Illite shows absorptions at 1400,
1900 and 2200nm just like Smectite. Illite can be distinguished from
Smectite by having additional absorptions at 2340 and 2445nm.

Soil analysis using a PSR+ full range UV/VIS/NIR spectroradiometer
with a 350-2500nm spectral range is fast, non-destructive, affordable,
and doesn’t involve hazardous chemicals. With our handheld sample
contact probe, soil spectra can be collected in the field from
outcrops or a soil pit. EZ-ID provides a best match based on
examining the target spectra and the library spectras. A researcher
can add or remove regions of interest to focus on only the prominent
features in the spectra for a better identification in clay mixtures.

The PSR + can be combined with a wide range of accessories
including a range of FOV options such as direct attaching 4°, 8°, or
14° lenses, 25° fiber optic, diffuser, or integrating sphere. Fiber
mount options include 1, 2, 3, 4, 5, 8, and 10° lenses. It can be used
with a sample contact probe, leaf clip, or benchtop probe with sample
compactor and can store 1,000 spectra on the instrument without a
computer. Communications interfaces include USB and Class 1
Bluetooth.
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