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Uranium more Common than Zinc

Posted: Sun Oct 08, 2017 4:42 am UTC
by jewish_scientist
I was reading a story and the author mentioned that the super-special element made up only half of a percent of the crust. That actually seemed rather large to me, so I googled "composition of the earth's crust" and found the paper The Composition of the Earth's Crust. Isn't nice when things work out like that.

The section on the crust does not get more specific than 1% (page 39 of pdf), so I went to the next best thing, the section on igneous rocks (page 25 of pdf). That is when things got weird. Apparently, uranium makes up 0.8% and zinc makes up 0.4%. :? It gets even worse with lead being 0.2%. I went to the next page for an explanation, and found another table that gives different values for these same elements in MUCH smaller amounts. So I repeat: :? .

P.S. What notation is being used on columns 2 and 4 of the table on page 25. At first I thought that it was regular decimal notation, but that would mean that thier estimations for 5 elements, including copper, changed by >2 degrees of magnitude.

Re: Uranium more Common than Zinc

Posted: Sun Oct 08, 2017 12:50 pm UTC
by Liri
my first guess, that it's higher than expected due to the decay of larger elements to much more stable u-238, is correct, but that contributes an almost negligible amount

uranium is the heaviest primordial element, so it doesn't strike me as odd that things would "bunch up" there

the table on p25 is showing parts of a gram per gram of rock (there's a guide to reading the table in the column on the left), not percentages

they state in the text that the first table is presenting earlier estimates that they are including for comparison

Re: Uranium more Common than Zinc

Posted: Sun Oct 08, 2017 7:29 pm UTC
by Eebster the Great
According to Jefferson Lab, Uranium has an abundance of 2.7 ppm in the crust and 3.2 ppm in the ocean. Zinc has an abundance of 70 ppm in the crust and 49 ppm in the ocean, while lead has 14 ppm and 0.03 ppm, respectively. That's a pretty big difference.

Re: Uranium more Common than Zinc

Posted: Mon Oct 09, 2017 2:23 pm UTC
by Xenomortis
My guess is that lead and zinc deposits are more common in sedimentary layers (limestone, etc), so you're getting a warped view when you only look at the composition of igneous rock.

Re: Uranium more Common than Zinc

Posted: Tue Oct 10, 2017 10:36 am UTC
by Soupspoon
I had a cursory glance at the tables, and couldn't quite determine whether the quantities were molar (number of atoms), by weight (each uranium atom is roughly 11/3rds the weight of a zinc one) or by density (pure uranium seems to be roughly 8/3rds that of pure zinc, but compounded/alloyed, with the other thibgs like silicon, that'd change).

There's a bit that discusses the percentage masses of lithosphere, hydrosphere and atmosphere near one of the tables, but my brief skim (around the two tables referenced in the OP) when the question first came up didn't reveal the measure. Could there be an internal inconsistency?

Re: Uranium more Common than Zinc

Posted: Tue Oct 10, 2017 9:09 pm UTC
by elasto
Also, this study is from 1924.

Isn't there something more up to date you'd prefer to be referencing...?

Re: Uranium more Common than Zinc

Posted: Tue Oct 10, 2017 9:20 pm UTC
by Zamfir
Wikipedia?

https://en.m.wikipedia.org/wiki/Abundan ... %27s_crust

Has a table comparing several sources

Edit: TIL that noble metals are actually rare due to their nobility. Since they do not react to form stone-like materials, they do not float on iron. And therefore most Gold$$$$ has sunk to the core of the earth. Makes perfect sense, but I never connected the dots there. Would this work for bitcoins?

Re: Uranium more Common than Zinc

Posted: Wed Oct 11, 2017 2:56 am UTC
by Meteoric
What, letting hard drives sink to the center of the earth? Sounds difficult.

It would give a new meaning to "bitcoin mining" though.

Re: Uranium more Common than Zinc

Posted: Fri Oct 20, 2017 5:41 am UTC
by ericgrau
If the heavier elements sink to the core, would that mean uranium sinks to the core too? Its density is almost the same as gold. Is most of the uranium in the core perhaps? What's the ppm of uranium in the entire earth and/or core?

Also found this, which bodes well for core uranium: https://www.researchgate.net/publicatio ... h%27s_core
And this, which does not bode well for core uranium: https://physics.stackexchange.com/quest ... -than-gold

Re: Uranium more Common than Zinc

Posted: Fri Oct 20, 2017 6:46 am UTC
by Eebster the Great
Doesn't this depend on more than just density? Shouldn't it depend on solubility, for instance?

Re: Uranium more Common than Zinc

Posted: Sun Nov 05, 2017 12:39 am UTC
by Himself
Yes, solubility is a key thing. Uranium will form compounds and go into rock more readily than gold. Since it doesn't fit well in many crystal lattices it is also one of the last components to crystallize in a cooling magma. As a result it tends to get concentrated in continental crust.