Why aren't heavier metals like uranium and mercury found in abundance in the core?

I'm curious as to why there aren't heavier metals in the earth's core than iron and nickel. Why those two? Anyone have any insight?

tiny-tim
Homework Helper
Welcome to PF!

I'm curious as to why there aren't heavier metals in the earth's core than iron and nickel. Why those two? Anyone have any insight?

Hi danielandpenn ! Welcome to PF!

There are … see http://en.wikipedia.org/wiki/Core_(earth_science):
The core of the Earth is still hot because it contains radioactive uranium and thorium atoms. Although the core is mostly iron, it also contains most of the Earth's uranium.

Nickel and iron are more abundant in the universe (or at any rate in the Solar system) than heavier substances, and might therefore dominate the core even if most of these other substances would have accumulated there.

On the other hand, some substances, like iodine, or mercury, might be mostly locked into molecules with less density than iron, or dissolved into subtances with less density than iron, like aluminium or water, and might therefore not be more abundant in the core than in the mantle or crust.

Welcome, Danie

What happens in the core is all pure hypothetical, since we have only some very indirect observations. The wiki ref of tiny-tim is certainly true qualitatively but there is no way of quantifying the concentration of heavy elements in the core as well as their contribution to the terrestrial generated heat. There are more hypotheses going around, none of which can be selected as being the right one.

The iron - nickle abundance is no doubt related to the total abundance of those elements in the solar system, whereas the heavy elements are much more rare.

Another element could be, the order in which compounds became fluid or not, during the -hypothetical- http://www.hopkins.k12.mn.us/pages/high/courses/online/astro/course_documents/earth_moon/earth/geologic_time/iron_catastrophe.htm [Broken]

But in the end it's all a sophisticated guess.

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baywax
Gold Member

Welcome, Danie

What happens in the core is all pure hypothetical, since we have only some very indirect observations. The wiki ref of tiny-tim is certainly true qualitatively but there is no way of quantifying the concentration of heavy elements in the core as well as their contribution to the terrestrial generated heat. There are more hypotheses going around, none of which can be selected as being the right one.

The iron - nickle abundance is no doubt related to the total abundance of those elements in the solar system, whereas the heavy elements are much more rare.

Another element could be, the order in which compounds became fluid or not, during the -hypothetical- http://www.hopkins.k12.mn.us/pages/high/courses/online/astro/course_documents/earth_moon/earth/geologic_time/iron_catastrophe.htm [Broken]

But in the end it's all a sophisticated guess.

Gee, I thought it was one big diamond at the centre... or is that Jupiter

Actually, I'd like to ask a question related to this one.... and that is why is gold associated with volcanic activity? Does it get separated out by extreme heat or is it of greater abundance in the liquid magma surfacing during a volcanic eruption?

Many thanks!

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mgb_phys
Homework Helper

... and that is why is gold associated with volcanic activity?
Gold isn't very chemically active but is soluable at very high pressures. It is carried along cracks by steam/fluids from volcanoes and is deposited out as they cool. So are lots of other metals, there a bunch of elements like platinum and telurium that you find along with gold.

baywax
Gold Member

Gold isn't very chemically active but is soluable at very high pressures. It is carried along cracks by steam/fluids from volcanoes and is deposited out as they cool. So are lots of other metals, there a bunch of elements like platinum and telurium that you find along with gold.

Ah, right! The chinese who built most of the CN and CPR railways across Canada would keep what they called "white gold" (platinum) in their old opium jars. It was considered useless at the time by the CDN and USA placer miners, but the "Coolies" kept it just the same.

If the core of Jupiter is made of diamond, that would mean that carbon is dominating Jupiter's core. One would have to conclude that elements heavier than carbon (such as iron and nickel) are relatively rare on Jupiter. In that case iron and nickel cannot be all that common in the Solar System, and one would have to wonder why they are common on Earth.

That is quite different from the case for hydrogen. Hydrogen is the most common element in the universe, but Earth cannot keep hold of it (except in compounds with heavier elements, such as oxygen), because it is light enough to escape from Earth's gravity well. Jupiter is more massive, and was therefore able to retain more hydrogen.

Carbon, however, would not escape from Earth, at least not over the present age of the Solar System. If a large part of Jupiter is made of carbon, that would be strange.

baywax
Gold Member

If the core of Jupiter is made of diamond, that would mean that carbon is dominating Jupiter's core. One would have to conclude that elements heavier than carbon (such as iron and nickel) are relatively rare on Jupiter. In that case iron and nickel cannot be all that common in the Solar System, and one would have to wonder why they are common on Earth.

That is quite different from the case for hydrogen. Hydrogen is the most common element in the universe, but Earth cannot keep hold of it (except in compounds with heavier elements, such as oxygen), because it is light enough to escape from Earth's gravity well. Jupiter is more massive, and was therefore able to retain more hydrogen.

Carbon, however, would not escape from Earth, at least not over the present age of the Solar System. If a large part of Jupiter is made of carbon, that would be strange.

Very interesting. Do we know what the predominant metal/other is at Mar's core? Does the volcanic activity of Io, next to Jupiter, indicate a core content similar to earth's?

mgb_phys
Homework Helper

Do we know what the predominant metal/other is at Mar's core?
We don't have direct seismic data for mars but it is thought to be similair to Earth. The difference is that Mars is smaller and so it's core cooled and solidifed and so has no (significant) magnetic field.

Does the volcanic activity of Io, next to Jupiter, indicate a core content similar to earth's?
The volcanic activity of Io is due to the energy input from Jupiter, from tidal forces, it is too small to have a liquid core purely from internal heat sources. It does look like it has an iron core from magnetic field measurements made by the Galileo probe.

Gokul43201
Staff Emeritus
Gold Member

Gee, I thought it was one big diamond at the centre... or is that Jupiter

If the core of Jupiter is made of diamond, that would mean that carbon is dominating Jupiter's core.
To my knowledge, there is no real evidence of any significant amounts of carbon in Jupiter's core. The diamond core theory was made up by Arthur C. Clarke for one of his later Odyssey novels.

There is definitely more realistic speculation of metallic hydrogen in jupiter's core, but I don't think that's been verified either.

sketchtrack

To my knowledge, there is no real evidence of any significant amounts of carbon in Jupiter's core. The diamond core theory was made up by Arthur C. Clarke for one of his later Odyssey novels.

There is definitely more realistic speculation of metallic hydrogen in jupiter's core, but I don't think that's been verified either.

Wouldn't it be too hot for diamonds in the core of a planet? Wouldn't they burn up, or would the pressure prevent that?

baywax
Gold Member

I'm sorry, I mentioned the diamond centre of Jupiter as a joke... sort of sarcastic kind of thing.

mgb_phys
Homework Helper

Wouldn't it be too hot for diamonds in the core of a planet?
You need oxygen for things to burn, also Carbon has a rather high boiling point (4000K)
Diamond isn't the most stable form of Carbon, diamonds (slowly) decay into graphite at room temperature - I don't know about at the pressure in the centre of a planet

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baywax
Gold Member

You need oxygen for things to burn and Carbon has a rather high boiling point (4000K)
Diamond isn't the most stable form of Carbon, diamonds (slowly) decay into graphite at room temperature - I don't know about at the pressure in the centre of a planet

So, how long till the engagement diamond turns into a pencil? Probably longer than the license takes to decompose, eh?.

Boiling point of carbon is even a bit higher, 5100K and the melting point (grafite) is 3773K. The temperature of the Earth core is estimated at about http://mediatheek.thinkquest.nl/~ll125/en/core.htm [Broken]. But the tremendous pressure would probably prevent carbon to be fluid.

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sketchtrack

Could one assume that if there is carbon in or near the core, it is probably in diamond form?

mgb_phys
Homework Helper

Could one assume that if there is carbon in or near the core, it is probably in diamond form?

It's probably dissolved in molten iron and so doesn't have any crystal structure.

Suppose that the core of some planet is nearly pure carbon. Then the carbon would not burn or react with other substances. Whether it would be diamond, graphite or something else, not known to us in dayly life, would depend on carbon's diagram of state. This is a parameter space with pressure and temperature as its two parameters. It is divided into several regions (which may overlap) where several forms of carbon may exist (be stable against change into another form). If the entire diagram were known, one might find the point corresponding to the pressure and temperature in the core, and see whether diamond can exist there. (That is, if the pressure and temperature are known. But maybe the diamond region extends to quasi-infinite pressures and temperatures? Quasi-infinite, not really infinite, because eventually degenerate matter would form.)

The question is, whether the state diagram for carbon is completely known. (It might be completely known from first principles, not from experiment, of course.) And the same question might be asked about nickel and iron. The material in the Earth's core might be unusual, too.

Are you saying that the nickel and iron in the core may be different from the nickel and iron as we know it because of the intense pressure and temps?

Yes. I do not know whether they are different, but they might be as different from the usual iron and nickel as diamond is from graphite. (Chemically, they would of course still be the same materials. Only the crystal structure would be different.)

Astronuc
Staff Emeritus

Are you saying that the nickel and iron in the core may be different from the nickel and iron as we know it because of the intense pressure and temps?
Elementally the iron and nickel would not be different, but given a pressure of about 350 GPa, the distances between nuclei, or lattice parameters of any crystal arrangement would be very small. But the core is considered to be liquid.

http://pubs.usgs.gov/gip/interior/
http://www.geol.umd.edu/~mcdonoug/Encyclopedia_Geomagnetism_Paleomagnetism.pdf [Broken]
http://mahi.ucsd.edu/cathy/SEDI2002/ABST/SEDI1-2.html [Broken]

As others have indicated, the heaviest elements are quite rare, and most metals are in the form of oxides, metallic oxides, e.g. silicates and aluminates, carbonates or other complex metal oxides, and sulfides.

List of Periodic Table Elements Sorted by Abundance in Earth's crust
http://www.science.co.il/Ptelements.asp?s=Earth

http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements

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Okay....thank you very much!

There is another point that has not yet been raised except in passing, which is that in nature, because the various elements have various chemical affinities for each other, most elements in nature do not occur in "pure" form, but rather in the form of chemical compounds or mixtures.

The Earth's core is believed to be mostly composed of a mixture of http://en.wikipedia.org/wiki/Goldschmidt_classification#Siderophile_elements" In addition to iron, cobalt, and nickel, the siderophiles include manganese, molybdenum, gold, and the platinum and palladium group elements.

Secondarily, in the core one also expects to find enhanced abundances of the http://en.wikipedia.org/wiki/Goldschmidt_classification#Chalcophile_elements" --- most notably sulfur itself, which is believed by some to be a significant constituent of the liquid "outer core." (Mercury, BTW, happens to be a chalcophile.)

By contrast, uranium and most uranium compounds are http://en.wikipedia.org/wiki/Goldschmidt_classification#Lithophile_elements" and are therefore believed to have become largely concentrated mostly in the Earth's lithosphere (i.,e., its "crust"), rather than in its mantle or core --- despite the relatively high specific gravities of uranium and most of its compounds. [Note that chemical bonds are basically electromagnetic' in nature, and that gravity is actually an extremely weak force compared to the electromagnetic force. (Relatively speaking, the electromagnetic force is about ~1040 times stronger than the gravitational force.) Hence, the chemical bonds between uranium and the other lithophilic elements are easily capable of preventing uranium from sinking to the core under the influence of the earth's relatively weak gravitational field.]

There is a heterodox hypothesis by maverick nuclear chemist http://en.wikipedia.org/wiki/J._Marvin_Herndon" [Broken] that the core is primarily composed of nickel silicide rather than metallic nickel-iron alloy. Nickel silicide has enough of an affinity for uranium that, despite uranium's relatively low cosmic abundance, there might have been enough uranium dissolved in the primitive core for it to later separate out to form a small innermost "kernel" of uranium sulfide with a diameter of ~1--10 km at the center of the inner core. Herndon also believes that this inner "kernel" functions as an intermittent fast fission reactor that drives various geological processes. However, very few "mainstream" geologists take Herndon's "nuclear planet" hypothesis seriously, and there is currently very little evidence of even a circumstantial nature to support it.

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baywax
Gold Member

There is another point that has not yet been raised except in passing, which is that in nature, because the various elements have various chemical affinities for each other, most elements in nature do not occur in "pure" form, but rather in the form of chemical compounds or mixtures.

The Earth's core is believed to be mostly composed of a mixture of http://en.wikipedia.org/wiki/Goldschmidt_classification#Siderophile_elements" In addition to iron, cobalt, and nickel, the siderophiles include manganese, molybdenum, gold, and the platinum and palladium group elements.

Secondarily, in the core one also expects to find enhanced abundances of the http://en.wikipedia.org/wiki/Goldschmidt_classification#Chalcophile_elements" --- most notably sulfur itself, which is believed by some to be a significant constituent of the liquid "outer core." (Mercury, BTW, happens to be a chalcophile.)

By contrast, uranium and most uranium compounds are http://en.wikipedia.org/wiki/Goldschmidt_classification#Lithophile_elements" and are therefore believed to have become largely concentrated mostly in the Earth's lithosphere (i.,e., its "crust"), rather than in its mantle or core --- despite the relatively high specific gravities of uranium and most of its compounds. [Note that chemical bonds are basically electromagnetic' in nature, and that gravity is actually an extremely weak force compared to the electromagnetic force. (Relatively speaking, the electromagnetic force is about ~1040 times stronger than the gravitational force.) Hence, the chemical bonds between uranium and the other lithophilic elements are easily capable of preventing uranium from sinking to the core under the influence of the earth's relatively weak gravitational field.]

There is a heterodox hypothesis by maverick nuclear chemist http://en.wikipedia.org/wiki/J._Marvin_Herndon" [Broken] that the core is primarily composed of nickel silicide rather than metallic nickel-iron alloy. Nickel silicide has enough of an affinity for uranium that, despite uranium's relatively low cosmic abundance, there might have been enough uranium dissolved in the primitive core for it to later separate out to form a small innermost "kernel" of uranium or uranium sulfide with a diameter of ~1--10 km at the center of the inner core. Herndon also believes that this inner "kernel" functions as an intermittent fast fission reactor that drives various geological processes. However, very few "mainstream" geologists take Herndon's "nuclear planet" hypothesis seriously, and there is currently very little evidence of even a circumstantial nature to support it.

Nice work GDP...

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