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Question about the composition of dirt

  1. Jul 1, 2015 #1
    I was wondering if I was holding a handful of ordinary dirt would there likely be some amount of every naturally occurring element in it?
     
  2. jcsd
  3. Jul 2, 2015 #2

    Borek

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    Staff: Mentor

    Quite likely, although many of them can be present in amounts that are way below detection limits of our best analytical methods.
     
  4. Jul 2, 2015 #3
    I thought that might be the case, but I wasn't sure. Thanks for the conformation.
     
  5. Jul 3, 2015 #4
    That didn't sound like confirmation...or conformation. You can imagine some obvious exceptions, like the transient, trans-Uranium elements. Among the more mundane, I could believe there isn't a single atom of Helium. It won't be forming any compounds and as a gas at STP it would not be very likely to be in a random handful of soil- assuming you're not standing over a natural gas source. Astatine is pretty doubtful too. Besides being incredibly rare, with the most stable form having a 1/2 life of 8 hours, even if you got lucky it wouldn't be there for long.
     
  6. Jul 3, 2015 #5

    Borek

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    Staff: Mentor

    Whenever you have alpha decay, there is helium produced. It slowly diffuses out of any solid it is trapped in, but its presence seems to be inevitable. Especially taking into account fact uranium is present almost everywhere, at ppm or ppb levels, and it decays mostly by α.

    Interesting point about astatine though. Could be an interesting exercise - estimate concentration of all elements produced by various decay chains, assuming 100 ppb level or uranium in the dirt. Express the result in atoms per handful :wink:
     
  7. Jul 3, 2015 #6
    Pull-quote from Wikipedia:

    "Astatine is the rarest naturally occurring element that is not a transuranic element. The total amount in the Earth's entire crust (quoted mass 2.36 × 1025 grams) is estimated to be less than one gram at any given time."

    If I did this right, using an average atomic weight of dirt of 30, I get 1 atom out of each 1.7E26 atoms in the Earth's crust is astatine. If a handful of dirt is 454 grams, that's 9.1E24 atoms of dirt with 0.05 atoms of astatine per handful...a 5% chance of it being there.
     
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