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
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
Vietnam plotted as 1.0 cm thick. Data Files: alpha
20K; beta 20K; gamma 20K.
- Amazonite from the Kola Penninsula, Russia. 1.113 mm.
- 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
- 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
- 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
20K; E ~ b = ~ gamma 20K.
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