Feldspar Visible Spectra (generally 350 - 2500 nm)

Pale yellow color in feldspar is due to Fe³⁺ in the tetrahderal Si/Al site. This color is often masked by the pervasive turbidity of common feldspars. Smoky color, the result of radiation damage from the decay of potassium-40, is also common but often masked.

Blue color in the amazonite variety of potassium feldspar (and pale-blue albite as well) is from the interaction of trace amounts of Pb²⁺ in the feldspar with ionizing radiation (again, the radiation usually comes from the decay of potassium-40). Water molecules also must be present in the structure for the coloration to occur. Lead-containing feldspars with a higher degree of Al/Si disorder (typically orthoclase) are green where more the more common ordered feldspars (microcline) become blue from irradiation. Feldspars with intermediate ordering often show both a green and a blue component in the spectrum.

Pale blue color also occurs in plagioclase,

There are varieties of plagioclase feldspar colored green and red from copper ions and colloidal copper found in southwestern Oregon.. 

• Images of representative feldspars

Yellow, iron-containing feldspar

• GRR 146 spectrum; Orthoclase, Itrongahy, Madagascar, pale yellow crystal with Fe³⁺, 10 mm thick. Data Files: alpha 24K; beta 26K; gamma 24K  The spectrum is dominated by absorption from Fe³⁺ in the tetrahedral site.
• GRR 289 spectrum; Plagioclase, Lake County, Oregon, transparent, light amber crystal , plotted as 10.0 mm thick. This spectrum shows both absorption features from the Fe³⁺ in the tetrahedral silicon site in the 380 - 700 nm range (6K) and the features from Fe²⁺ in the calcium site in the 1000 - 2500 nm range. This sample contains about 0.31 wt% Fe. Reference: Hofmeister and Rossman, Determination of Fe²⁺ and Fe³⁺ concentrations in feldspar by optical absorption and EPR spectroscopy. Physics and Chemistry of Minerals 11:213-24 (1984)

Blue to green, lead-containing feldspar (amazonite variety)

• CIT 13599 spectrum; Microcline (variety amazonite), Bahia, Brazil. An attractive blue feldspar with ~300 ppm of Pb, plotted for 1 mm thick. This is representative of the sky blue amazonite feldspars. There is almost no absorption in the gamma direction. More details in Hofmeister & Rossman (1985). Data Files:  alpha 15K ;  beta 15K.
•  CIT 15020 spectrum; Microcline (variety amazonite), turbid blue perthite from Lake George, Colorado, USA, with ~700 ppm Pb plotted for 0.5 mm thick. The steady rise in the baseline towards the ultraviolet is due to scattering from the turbidity in the sample. Data Files:  alpha 16K; beta 16K;  gamma 16K.
•  CIT 15065 spectrum; Microcline (variety amazonite) in a coarse green perthite from Pearl, Colorado, USA, with 0.8 wt% PbO, plotted for 0.5 mm thick.
•  CIT 13756 spectrum; Orthoclase (variety amazonite), transparent green crystal with 1.8 wt% PbO from Broken Hill, NSW, Australia, plotted for 0.5 mm thick. Data Files: alpha 15K; beta 15K; gamma 15K.
• GRR 2367 spectrum; Orthoclase (variety amazonite), transparent green crystal   from Vietnam plotted as 1.0 cm thick. Data Files: alpha 20K; beta 20K; gamma 20K.
• GRR 580 spectrum; Plagioclase (variety oligoclase), South Carolina, pale-blue with ~0.035 wt% PbO, plotted for 10 mm thick. More details in Hofmeister & Rossman (1986). Data Files: alpha 24K;  beta 26K;  gamma 24K.

Grey feldspar (smoky variety)

• GRR 638 spectrum; Sanidine, Eifel, Rhineland, Germany. Smoky color megacryst . Plottted as 5.0 mm thickness. The smoky color is the result of ionizing radiation interacting with Al in the tetrahderal site.

Red and green, copper-containing feldspar (sunstone variety)

• GRR 1615 spectrum; Plagioclase, Ponderosa Mine, Oregon, transparent, deep red color from copper, 2.167 mm thick. Reference: Hofmeister and Rossman, Exsolution of metallic copper from Lake County labradorite. Geology 13, 644-647 (1985). Data Files: unpolarized 24K

Data Files

• no other data files

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last updated 10-Sep-2005