What is zircon?
Where is it found?
Radioactivity in gems
Zircon is a zirconium silicate mineral. It is the principle source
of the element zirconium (Zr)
movie of the zircon structure!
Where is it formed and where is it found?
The mineral forms square prismatic crystals and grains in igneous and
crystals are rarer)
usually small and inclusion-rich
Zircon is highly resistant to weathering (maybe the most resistant of any
mineral) This, and its high specific gravity, favor its concentration in
placer deposits and alluvial gem gravels.
Gemstones have historically been recovered from many
Easy to chip
Doubly refractive with very
obvious doubling of facets. A second example showing doubling
of facets. High dispersion.
Contains Uranium (U) and thorium (Th)
decay and leads to damaged crysals ("Metamict") (appear cloudy)
decay of U and Th occur at specific and well known rates. Measurement of
isotopic abundances allows age dating of zircons ! (Thus zircons are very
important minerals in the geological scheme of things.)
Metamict crystals + heat (1450 C) can restore crystallinity
zircon + 800 - 1000 C in reducing environment -> colorless
or blue zircon
Brown zircon + 800 - 1000 C in air-> colorless or golden yellow zircon
Most colorless or blue crystals have been irradiated or heat treated!
Note: UV (sunlight) can modify the color!
Heat treatment makes zircons more brittle, facets tend to abrade more
colorless : brilliant cut (colored stones often in emerald cut) to resemble
diamond: you can distinguished by double refraction (which doubles the
image of the facets), and by signs of wear
|Radioactivity in Minerals
Detection: Geiger counter
Measurement: many scales:
dose the average American receives in one year
nCi = nanoCurie (billionth of a Curie)
Curie is the radioactivity of 1 g of radium
mrem (thousanth of a rem)
rem = roentgen equivalent man (like a sunburn measure)
rad = measure of absorbed dose: quantity of radiation able to deposit 100
ergs of energy)
360 mrem / year or 1 mrem/day
Safety limit: 5,000 mrem to entire body per year
Gems: calculations are complex, but roughly:
Sc 46 or Ta 182 in blue topaz: ~ 25 nCi/g
Thus: for two identically sized stones:
blue topaz (~ 25 nCi/g)
natural zircon (1.0 nCi/g of U238)
over 10 years dose is exactly the same
Does this matter?
the average human contains 200 nCi of K40 and eats and drinks ~ 140 nCi
of K40 / year
the 2 nCi/ g from gems is insignificant.
some images of Zircon
Previous Lecture: Topaz
Crysoberyl, Rutile and Spinel