Natural Occurrence
Astatine is the rarest naturally occurring element that is not a transuranic element, with the total amount in Earth's crust estimated to be less than 28 grams (1 oz) at any given time. Any astatine that was present at the Earth's formation has long since decayed, and the minute amounts of astatine existing currently have formed through the decay of heavier elements. While it was previously thought to be the rarest element occurring on the Earth, astatine has lost this status to berkelium, atoms of which can be produced by neutron capture reactions and beta decay in very highly concentrated uranium-bearing deposits.
Six astatine isotopes occur naturally (astatine-214 to astatine-219). Because of their short half-lives, they are found only in trace amounts. There is no data indicating that astatine occurs in stars.
Four out of these isotopes (astatine-215, astatine-217, astatine-218, and astatine-219) are found due to their production in major natural decay chains. Francium-223, the father isotope of astatine-219, alpha decays with a probability of only 0.006%, making this astatine isotope extremely rare even compared to the other astatine isotopes; this is in spite of its half-life being the longest of the natural astatine isotopes (56 seconds). Astatine-219 decays to polonium-215, which itself beta decays to astatine-215 with an even smaller probability of 0.00023%. The entirety of North and South America combined, considered to a depth of 16 kilometers (10 miles), contain only about one trillion astatine-215 atoms at any given time. Astatine-218 is found in nature as a result of polonium-218 beta decay; as with francium-223 and polonium-215, decay to an astatine isotope is not the primary decay mode. However, the astatine-217 isotope has a straight chain leading directly to astatine; its father isotope (francium-221) decays exclusively to this nuclide. Given that its fathers, grandfathers, and so on each decay exclusively to only one nuclide, this gives only one possible way for the starting nuclide in the neptunium series (neptunium-237) to decay – via eventual production of astatine-217.
The isotopes with mass numbers 214 through 216 are found as the result of triple alpha decay of the naturally present protactinium isotopes protactinium-226, protactinium-227, and protactinium-228. However, these isotopes are extremely rare, so much so that they are often not cited as natural astatine isotopes.
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