Elements heavier that iron are all formed in supernovas, correct? So why are they found in concentration? For example, why is gold found in gold nuggets?
As I understand it, the heavier elements would be formed as a result of fusion. But I’d expect this to occur on a somewhat random basis. In other words, I wouldn’t expect an entire section of a star to turn into pure gold. Rather, I’d expect an atom here and and atom there to undergo whatever fusion chain creates gold while the atom next to it takes a slightly different path and becomes, say, uranium or tungsten or whatever. In addition, I’d expect the process of being blown into inter-stellar space at an enormous velocity and temperature to mix things up pretty well.
So what gives? How and why do heavier elements get concentrated? Why aren’t, say, gold atoms more or less uniformly distributed?
IANA geologist, but most metals are concentrated by some sort of mechanism, often aqueous. E.g superhot water underground goes through rock, selectively dissolves metal, which as the water cools closer to the surface the metal comes out of solution. There is also a circular argument in that (i) if the metal is not concentrated we don’t notice it (e.g there are small amounts of all metals in almost any soil or rock sample) and (ii) in the rare times it is concentrated by a processs such as above it we go “hmm, wonder why the only time I see a metal it is concentrated” .
IANA geologist either, but it seems to me there must be other processes that produce concentrated minerals. Perhaps there are regions of the supernova explosion that are at the right temperature for producing (say) gold, so a huge chunk of gold (still with some other stuff mixed in) gets made? It would only be an extremely small region of the whole supernova, relatively speaking, needed to produce a “vein” of gold in the Earth’s crust.
They are not formed in concentration, they accumulate in concentrations, for the exact same reason that any other element or compound is concentrated – it or other material is leached away from a deposit and redeposited elsewhere. In a recent book on planetary origins, I read that iron, nickel, and the material that alloys easily with them tended to concentrate as planetary cores (being the preponderant “heavy” substance), and other materials which did not tend to react or alloy but which were refractory (e.g., silicon-magnesium compounds) were laid down next, leaving the crust with non-refractory materials which did not fit either those criteria. Gold and uranium are two examples of heavy materials which meet those criteria. Iridium, cosmically more common than either but tending to alloy with iron, is a prime example of a heavy metal depleted in the crust compared to cosmic abundance – accounting for the K-T iridium spike from the Chicxulub impact.