Reflectance Spectroscopy has been used by chemists for years. It was "re-discovered" by the remote sensing discipline. Sensors such as Thematic Mapper on the Landsat V Satellite, the NASA-JPL aircraft based scanner AVIRIS (Airborne, Visible, InfraRed, Imaging Spectrometer) and various commercial aircraft systems all use information from the visible through short infrared bands for mapping, vegetation investigation, mineral exploration and environmental monitoring. Geologists have recognized that this method has far reaching applications beyond remote sensing, especially in mineral exploration, core logging, alteration zone delineation and lithology mapping.

Reflectance spectroscopy can be defined as the technique that uses the energy in the Visible (0.4-0.7µm), Near Infrared (0.7-1.3µm) and Short Wave InfraRed (1.3-2.5µm) wavelength regions of the electromagnetic spectrum to analyze materials.

	This figure is from Bob Agars.

Certain atoms and molecules absorb and reflect chemical bond energy at diagnostic wavelengths as a function of their atomic structures.

The manifestation of this takes the form of a reflectance spectrum, with absorption features, wavelength positions and distinctive profiles that can be used to identify mineral and organic phases.

This figure shows part of a kaolinite SWIR spectrum with components labeled.

[A] Doublet absorption feature for the Al-OH bonds with two minima. "FWHM" ­ measurement for the Full width at half maxima of the absorption feature. "Position" indicates the value,.measured in nanometers (or microns m ). "HULL" is the baseline, which is described by a parabolic curve through the SWIR region.

Each mineral detected within the SWIR region has a fairly unique set of spectral characteristics combined into this reflectance spectrum. The features have characteristic frequencies or wavelength positions and bandwidths. Both the spectral features and the hull or background component are influenced by multiple variables, the presence of which (such as iron oxides) are not always visibly detectable within the SWIR wavelength region of 1.3 to 2.5 µm.

The reflectance properties, particle size, degree of sample orientation, presence of surface water and interlayer water, associated organic and inorganic phases, particle orientation, degree of structural ordering, data collection and instrument parameters all influence the spectral curve and absorption features. The profile of the feature is also of great importance as this will change with changes in the listed variables.

SWIR is particularly sensitive to the OH, H2O, CO3, CH, NH4 bonds. The minerals detected include clay minerals (kaolinite, illite, dickite, halloysite, smectite, palygorskite) and other phylosilicates (serpentines, talc, pyrophyllite, chlorites), carbonates (calcite, dolomite, malachite, siderite, magnesite), hydroxides ( gibbsite, brucite, diaspore) selected sulfates (jarosites, alunites, alunogen), amphiboles, micas, epidotes, zeolites and topaz.

Therefore the application to exploration becomes apparent as these are usually pivotal minerals for defining a deposit type. This sensitivity is a function of the molecules present in the mineral phases, especially water, hydroxyls and carbonate.

Table I summarizes the common mineral groups detected by SWIR and the positions of their major spectral features.

A more detailed explanation of Reflectance Spectroscopy can be found:

• in the Spectral International publication "Applied Reflectance Spectroscopy";
• on the USGS website at http://speclab.cr.usgs.gov/
• on the CCRS website at http://www.ccrs.nrcan.gc.ca/ccrs/eduref/tutorial/tutore.html