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 and occasionally, intense blue color can be found as in the sample from the Behive #1 mine in the Boulder Batholith, MT. Instances can even be found when colorless microcline can be found in contact with pale blue albite such as the material from the White Queen Mine, Pala, CA

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; beta ; gamma. 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 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 potassium feldspar (amazonite variety)

• Amazonite CIT 13599 spectrum; Microcline, 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;  beta.
•  Amazonite CIT 15020 spectrum; Microcline, 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; beta;  gamma.
•  Amazonite CIT 15065 spectrum; Microcline in a coarse green perthite from Pearl, Colorado, USA, with 0.8 wt% PbO, plotted for 0.5 mm thick.
•  Amazonite CIT 13756 spectrum; Orthoclase, transparent green crystal with 1.8 wt% PbO from Broken Hill, NSW, Australia, plotted for 0.5 mm thick. Data Files: alpha; beta; gamma.
• Amazonite GRR 2367 spectrum; Orthoclase, transparent green crystal   from Vietnam plotted as 1.0 cm thick. Data Files: alpha 20K; beta 20K; gamma 20K.
• Amazonite GRR 2576 from Mt Ploskaya, Kola Penninsula, Russia. 1.113 mm. Data Files: ~alpha;  ~beta

• 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  beta;  gamma.
• GRR 756 spectrum; Plagioclase (albite), Kenya. a pale blue crystal containing about 0.013 wt% PbO (curve a). When heated to 300 deg C for 25 minutes, it lost much of its color (curve b), but when the heated piece was irradiated with 26.6 Mrads of gamma rays, the blue color returned with even greater intensity (curve c). All spectra shown in the beta polarization plotted for 7.0 mm thickness. 
• GRR 2954, Plagioclse (albite), Beehive #1 Mine, Delmoe Lake, Bourlder Batholith, MT, USA. A blue cleavlandite mass . No spectra at this time 
• GRR 3825, Plagioclase (albite), Eibensteir an der Thaya, Lower Austria, Austria. A blue mass of albite feldspar . No spectra at this time. 
• GRR 3826, Plagioclase (albite), White Queen Mine, Pala, California. A pale blue abite that grows in contact with a near-colorless microcline  No spectra at this time.    

• 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
• Labradorite, Rabbit Hills, Oregon. Red region, plotted as 2.0 mm thick.
• Labradorite, Rabbit Hills, Oregon. Green region, plotted as 1.0 mm thick

Pink albite

• GRR 3236 spectrum, albite, Zagi Mountain, Pakistan, transparent, pink crystals , 3.83 mm thick, viewed on the (001) face. The main absorption at about 538 nm is at a position appropriate for Mn³⁺. The spectrum also has two weak bands near 418 and 448 nm that arise from Fe³⁺ in the tetrahedral Si/Al site. Data Files: E ~ a = ~alpha; E ~ b = ~ gamma.  This color fades in a matter of days when the sample is exposed to bright light.

Link to a collection of references to mostly color and visible spectroscopy of feldspars

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