(1) Zircon

COMPOSITION	ZrSiO4
HARDNESS	7-7.5
CRYSTAL SYSTEM	Tetragonal
COLOR	yellow-brown/red/colorless ("Hyacinth")
FRACTURE	Conchoidal
RI	High
Dispersion	Strong
SPECIFIC GRAVITY	(High) 4.6-4.7 (Low due to radiation damage)4.0

See a movie of the zircon structure!

(2) Where is it formed and where is it found?

• the mineral forms square prismatic crystals and grains in ingeous and metamorphic rocks (large 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 localitites

(3) Important characteristics:

• 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)
• undergoes radioactive decay and leads to damaged xals ("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.)

1. Metamict crystals + heat (1450 C) can restore crystallinity
2. Brown zircon + 800 - 1000 C in reducing environment -> colorless or blue zircon
3. Brown zircon + 800 - 1000 C in air-> colorless or golden yellow zircon
4. MOST COLORLESS OR BLUE CRYSTALS HAVE BEEN HEAT TREATED !!
   • 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
5. Note: UV (sunlight) can modify the color!
6. Heat treatment makes zircons more brittle, facets tend to abrade more

   RADIOACTIVITY IN MINERALS:

   Detection: Geiger counter
   Measurement: many scales:

   • 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)

   Average dose the average American receives in one year

   • 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.

   

   Questions on the lecture

   

   View some images of Zircon

   

   "Topaz"

   "Crysoberyl, Rutile and Spinel"

   

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