The color in zircon (ZrSiO4) comes from both
uranium ions and radiation damage. U4+ enters the zircon structure
because, like Zr4+, it has a large ionic radius. U4+
substituting in zircon causes the blue color of heat-treated zircon. Over
geologic time, the uranium undergoes radioactive decay and the resulting
radiation damage centers cause a range of red-brown and amber colors. Heat
treatment removes the radiation damage centers and restores the blue
color with which zircon presumably originally crystallized. Green colors
are associated with a mixture of the blue color from uranium and red-brown
color from radiation damage centers. Highly radiation-damaged zircons are
called metamict zircons. Colorless zircons have little uranium.
Zircon Visible Spectra (400 - 1200 nm)
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Zircon 1844 ; Brownish-red zircon
from Chanthaburi, Thailand, colored by radiation damage. Data files: a
(24K) ; c (24K)
Zircon 1486 ; Yellowish-green,
partially metamict zircon from Sri Lanka colored by radiation damage. Sharp
lines from U4+ are seen in the spectrum of the 4.4 mm thick
sample. Data files: a (24K)
Zircon 1898; Blue zircon from Burma, near
the Thailand border. This 2.65 mm thick crystal may have been heat-treated
to remove a brownish-red initial color. Data files: a
(20K); c (20K)
Zircon; medium green zircon from Burma(?).
This 6 mm thick crystal has moderately intense alpha activity. Data files:
Thorite; Green thorite compared to yellow-green zircon.
Both of these show an uranium spectrum.
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