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.

There are also rare varieties of plagioclase feldspar colored green and red from copper ions and colloidal copper. 

• 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, ploted 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

  Back to the list of minerals

Back to the Mineral Spectroscopy home page  

last updated 10-Sep-2005