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Ridley_Himself t1_jb7xorj wrote

So, it's not just a matter (no pun intended) of density but one of certain chemical affinities.

There is something called Goldschmidt classification. It's rather outdated but can describe the behaviors of elements in very broad strokes.

Essentially, it classifies elements based on which "phase" they preferentially enter. Lithophiles prefer a silicate rock phase, chalcophiles prefer a sulfide phase, siderophiles prefer a metallic phase, and atmophiles prefer a gaseous phase. In this scheme gold is considered a siderophile and would enter a metallic phase. The iron-rich metallic phase in Earth was denser than the silicate rock, so the former sank to form the core while the latter floated to form the crust and mantle. So thinking in these terms, there would be gold in Earth's core, but it wouldn't necessarily separate from the nickle-iron alloy of the core, especially since it probably only exists in trace concentrations. Of course, you'd have to determine experimentally how gold would behave under the pressure and temperature conditions present in the core.

Goldschmidt classification is nowhere near absolute, which is why we can have gold in the crust.

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forams__galorams t1_jb965ka wrote

> Goldschmidt classification is nowhere near absolute, which is why we can have gold in the crust.

I thought that’s more to do with the following factors:

(1) chemical bonding subtleties mean that partition coefficients are never perfectly into exclusively one phase or the other

(2) core formation is far from a perfectly efficient process with regards to taking certain elements from other layers — even if partition coefficients were perfectly weighted to siderophile, reactions don’t run to completion without being diluted or interrupted, not least because the Earth was not completely molten for very long (if at all).

(3) the crust today (particularly the continental crust) has been modified extensively since whatever was left behind immediately after core formation. Transport and concentration of certain elements from the mantle to the crust and into more localised bits of the crust to form ore deposits has had ~4 billion years of geological processing to occur.

(4) most of the gold that exists in the crust today is thought to have been delivered to the Earth from space after core formation — the late veneer hypothesis eg. Dauphas & Marty, 2002

Is the Goldschmidt classification really so lacking? I know it was developed a long time ago but i thought it was only ever meant to be a broad classification scheme? Seems to fo a good job of that and it does allow for elements to have mixed classifications.

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