Visible and Near-Infrared Reflectance Spectroscopy for Investigating Soil Mineralogy: A Review

Table 1

Spectral features of some predominant soil minerals.

Absorption band	Origin		Diagnostic feature	Description	Assigned soil minerals
∼465 nm	Electronic transition	^6A1g → ^4T1g,4Eg	Width	Narrow	Goethite
				Wide	Hematite
∼650 nm		^6A1g → ^4T2g	Position	Left → right	Goethite → hematite
∼900 nm		^6A1g → ^4T1g	Position	Left → right	Hematite → goethite
∼1400 nm	Molecular vibration	OH	Double absorption band (1395 nm and 1415 nm)		Kaolinite
			Asymmetry	>1	Kaolinite
				<1	Montmorillonite
∼1900 nm		H2O	Position	<1910 nm	Montmorillonite/vermiculite
				>1910 nm	Kaolinite/illite
			Depth	Intense → shallow	Montmorillonite → illite → kaolinite
			Width	Narrow	Montmorillonite
				Wide	Vermiculite/illite
∼2200 nm		Al-OH	Double absorption band (2163 nm and 2208 nm)		Kaolinite
			Depth	Intense	Kaolinite
				Shallow	Vermiculite/montmorillonite/illite
			Asymmetry	>1	Kaolinite
				<1	Montmorillonite
>2300 nm		Fe-OH and Mg-OH	Additional absorptions (2340 nm and 2445 nm)		Illite
			Triple absorption features between 2330 and 2360 nm		Chlorite