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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 It's rather outdated but can describe the behaviors of elements in very broad strokes.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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mfb- t1_jb8ccrb wrote

> I understand that gravity gets less and less as you go deeper

The gravitational acceleration decreases as you descend through the core but it's still always inwards, so the center is the deepest point in the potential.

Yes, you have zero-g in the very center, but also the highest pressure.

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Bbrhuft t1_jbfl2dd wrote

There was a 1990s theory that the centre of the Earth's core was natural uranium nuclear reactor that provided a some of Earth's internal heat, possibly helped drive the Earth's magnetic field. This extraordinary theory was proposed by J. Marvin Herndon who published a fantastical paper in 1993 claiming that he inner core was was composed of natural uranium and it was a natural fission reactor, a georeactor.

The idea was supported by plausible, at the time, geochemistry, that appeared to suggest that uranium could differentiate from iron-nickel and form an inner uranium core at the center of the Earth. However, most at the time disagreed, said the uranium content wasn't high enough to form a uranium core and/or the chemistry at high pressures prevent the separation of uranium from other metals.

Also, futher, advances in geoneutrino detection, that's neutrinos emited by radioactive decay and hypothetical inner core fission, ruled out a fission reactor well below the energy output proposed by J. Marvin Herndon.

Didn't stop him promoting the idea however, of a weaker fission reactor, or one that was more active in the past when there was more fissionable 235U.

Also, there's other scientists, who proposes variations on the Core Fission Hypothesis. A recent claim proposed a uranium oxide fission reactor at the inner/outer core boundary. This was published in a reputable journal. They proposed a 30 TeraWatt georeactor. That's insane.

I must look into this more, at it seems some scientists seriously think this is possible.

Refs.:

Herndon, J.M., 1993. Feasibility of a nuclear fission reactor at the center of the Earth as the energy source for the geomagnetic field. Journal of geomagnetism and geoelectricity, 45(5), pp.423-437.

Rusov, V.D., Pavlovich, V.N., Vaschenko, V.N., Tarasov, V.A., Zelentsova, T.N., Bolshakov, V.N., Litvinov, D.A., Kosenko, S.I. and Byegunova, O.A., 2007. Geoantineutrino spectrum and slow nuclear burning on the boundary of the liquid and solid phases of the Earth's core. Journal of Geophysical Research: Solid Earth, 112(B9).

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Funkybeatzzz t1_jb7ra2x wrote

The core is thought to be solid or immensely viscous. If anything settles it would be on the outer layer of the inner core. Gravity is about 4.3m/s^(2) here and the radius is about 20% of the whole earth. From seismic wave analysis the density of the inner core is 3% less than iron. This means most likely that it’s mostly iron and lighter elements. Gold isn’t lighter than iron but I guess there could be very small traces.

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