Identifying Minerals

• Different types of minerals absorb and scatter incident energy differently for different wavelengths of light!!!

• These differences in absorption and scattering for different wavelengths can be used to identify the minerals.

• We examine the maxima and minima of spectral reflectance curves – minima are caused by molecular absorption, and we call these absorption features or absorption bands.

What causes absorption features?

• Electronic transitions – absorption in bands out to about 1.0 um.

• Vibrations –‘shake, rattle and roll’ – at the molecular level – absorption in bands beginning at about 0.7 um and continuing beyond the TIR.

What causes absorption features?

• Electronic processes– Crystal field effect: an electron is moved from a lower level to a

higher level by the absorption of a photon with the exact energy difference between the two states (remember, Q=hλ). Occurs in Ni, Cr, Co, Fe, etc. and absorption bands are typically small.

– Charge transfer absorptions: caused when an electron is transferred to another ion or ligand due to the absorption of a photon. They cause large absorptions in the UV extending into the visible. This is the cause of red color of iron oxide.

– Conduction bands: photon of a specific energy causes a shift of an electron into the electronic lattice of certain materials (dielectrics, not metals). Occurs in the visible to NIR regions. This is the cause of the yellow color of sulfur.

– Color centers: irradiation of an imperfect crystal (one with defects) causes an electron to shift into the defect.

Electronic Processes - examples

Crystal field effect absorption caused by Fe2+. Fe 29 has 53.65% FeO, Fe 91 has 7.93% FeO.

Charge transfer absorption caused by Fe2+. Fe2O3 (hematite) and FeOOH (geothite).

Conduction bands caused by S and HgS.

What causes absorption features?

• Vibrational processes– Bonds in a molecule vibrate, the

frequency is dependent on the type of bond and the atom masses. Vibration can involve either displacement and/or rotation.

– Individual bonds absorb at wavelengths greater than about 0.7um

– Portions of a molecule and entire molecules absorb at longer wavelengths.

– Certain materials have important vibrational absorption bands: water, hydroxyl, carbonates, phosphates, borates, arsenates, vanadates.

Identifying Minerals

• We can use all of these absorption features to determine the chemical composition of a spectral reflectance curve.

Spectral Libraries

• Library containing spectra of various materials.

• An unknown spectrum can be compared to the spectra in the library in order to identify it.

• Important to note: data from spectrometers is collected in radiance, but must be converted to reflectance factor to compare to other samples.

• With an adequate spectral library, geological remote sensing can be done without field data providing ‘ground truth.’ Example: Hematite, water and carbon dioxide on Mars.

Cuprite, NV

• True color (LANDSAT TM bands).

• We are interested in mapping the minerals in the non-vegetated regions.

Cuprite, NV

• Derived from the electronic absorption features (0.4 to 1.2 microns)

• Fe2+ and Fe3+ bearing minerals

• Grain size can be determined using saturated bands

• The absorption features are broad, so specific mineralogy is more difficult to determine

Cuprite, NV

• Derived from the vibrational absorption features (2 to 2.5 microns)

• OH, CO3 and SO4 bearing minerals

• Can use absorption depths to determine the amount of mineral in a pixel

Mapping Mine Waste

California Gulch Superfund Site, Leadville, CO: Pyrite (and Fe-bearing secondary minerals) are indicative of acidic mine waste

ASTER (Thermal) Image of the Andes

ASTER

Cuprite, NV: classification of ASTER data (SWIR bands 4,6,8 on left). Blue=kaolinite, red=alunite, light green=calcite, dark green=alunite+kaolinite, cyan=montmorillonite, purple=unaltered, yellow=silica or dickite.

Emissivity

• Another way we can identify minerals is through their emissivity spectrum (as opposed to their reflectance spectrum), located primarily in the TIR range.

Thermal Emission Spectroscopy

Geological remote sensing does not necessarily require hyperspectral data

• Example: Finding river valleys under Sahara Sand

– Microwave SAR penetrates meters of sand, if sand is dry.

– Buried river valleys revealed by Shuttle Imaging Radar (SIR).

Death Valley: SIR-C Image

Alluvial Fan with variable particle sizes

Death Valley Alluvial Fan:AVIRIS hyperspectral image

A dendritic drainage in east-central Columbia : SIR-A radar

Oman mountains• On the east side of the Arabian

Peninsula, in Oman, are the Oman mountains, large parts of which are composed of ophiolites.

• These are ultramafic igneous rocks (peridoties; some gabbros), first extruded as lavas with shallow intrusives below, that moved as ocean floor away from a spreading ridge.

• On contacting a continental mass at a subduction zone, the ophiolites may subduct but otherwise can also be thrust on (obducted) to the continental edge.

• In this Landsat image the ophiolites are the dark bluish-black masses