In-Situ Identification of Soil Clay Minerology using UV-VIS-NIR Spectrometers

Soil mineral detection is a critical metric in environmental, geological, and soil research today. Most soils are a mixture of organic material, clay, sand, and silt. Due to their large surface area and negative charge clays play a critical role in nutrient availability. Common plant nutrients are positively charged, such as CA++, Mg++, K+ and Na+. The ability to hold and store positively charged ions is called cation exchange capacity (CEC). Clay also acts as a binder for soil particles, helping soil hold moisture and reduce nutrient loss due to leaching. The most common clay minerals in soil include kaolinite, illite, and montmorillonite. We also compared Kaolinite to Halloysite in Figure 1 due to their chemical and spectral similarities. Typical soil sampling necessitates expensive or tedious lab analysis. Most commonly this is done with an XRD that involves grinding the sample down to a powder significantly manipulating it. Another challenge is that typical soils are dark decreasing the reflectance signal when conducting field spectroscopy. To address this problem, we deployed Spectral Evolution’s NaturaSpec™ series to maximize signal to noise ratio and spectral resolution as well as demonstrate added utility in field soil spectroscopy. Spectral Evolution’s field spectroradiometers combined with the contact probe, benchtop probe, and EZ ID™ software allows unmixing of signal and identification of clay minerals without manipulating the soil sample. These methods are nondestructive in-situ measurements. In Figure 3 we show examples of spectral unmixing soil reflectance to identify clay minerals in their natural conditions.

Various clays can attract water molecules and swell—which can affect soil drainage capabilities. O-H stretch combinations, and a metal O-H bend allow clay minerals to show absorption. In the NIR regions between 2200 and 2500nm, there are combination vibrations associated with a metal bend and an O-H stretch. For clays like illite, kaolinite and smectite/montmorillonite, absorptions near 2200nm are due to Al-OH. H2O, OH and CO3 combination vibrations and overtones typically show clay mineral absorptions.

Kaolinite shows characteristic absorptions, doublets around 1400 and 2200nm. At 1400nm, the absorption 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 to show at the 1400nm band. Water bound in the lattices of smectite will show absorbance at 1400 and 1900nm due to vibrations. The combination bands that are caused by vibrations of bound water appear at somewhat shorter wavelengths — 1400 and 1900nm. Shoulders of absorbed water will appear around 1468 and 1970nm and will dominate by remoistening. 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.

The NaturaSpec Plus field portable spectroradiometer from Spectral Evolution has the features required for accurate and fast soil and clay identification and analysis. With its industry leading resolution and sensitivity, it collects clear spectra from clays and soil organic matter that are essential for accurate analysis. Equipped with optional EZ-ID software, it can unmix optically complex spectrum and match against built in or custom spectral libraries for nearly any soil application.