The Centre of the Earth

[johnmarks9]

Surely the central core will, after four billion years, mostly comprise the densest material available (>20g/cc). By all accounts this seems to be osmiridium (if more iridium) or iridosmine (if more osmium), the difference in elemental densities differing by less than 0.03g/cc.

Given Cox´s estimates of terrestrial abundances and a terrestrial volume of 10²⁷cc, the core of the earth should be a sphere of osmiridium, a little over 100kms in radius.

Would you agree?

[Jess Tauber]

Assuming the two elements don't mix with the siderophile elements that make up most of the mass, and you'd have to nomenclaturally differentiate it from the inner core generally. But would there be actually enough of these elements to make up such a massive central core? Wouldn't gold be more abundant?

Jess Tauber

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[Rene]

Hi John

Nice to hear from you.

I recall reading about this in the context of how elements like osmium and iridium behave.

Os and Ir, like Ni, Au, and Pt, are strongly siderophilic—meaning they prefer to reside in metallic phases like the Earth’s core rather than in silicate minerals. They readily alloy with Fe, which is itself a siderophile element.

As a result, Os and Ir are diluted within the Fe-Ni matrix of the core, not separated into a discrete osmiridium sphere. Their alloying behaviour and extreme scarcity mean they exist only at trace levels, even in the core. I’m not aware of any physical mechanism that would allow them, once alloyed, to migrate to the very center and coalesce into a pure, dense sphere.

If such a concentrated high density mass did exist, I expect we would likely have detected it by now through seismic or gravitational anomalies at the heart of the planet. Rather than there being just an outer core and an inner core, we'd have to postulate an additional ultracore for which there is no observational evidence or geophysical requirement.

Instead, what we observe is consistent with a gradually solidifying inner core composed primarily of iron and nickel, with trace amounts of other siderophile elements like Os and Ir alloyed throughout.

It’s a good question. What prompted it?

René

[johnmarks9]

Thanx René.

I´ve long been interested in geochemistry, volcanoes, meteorites, seismology, etc., for the insights they shed into the 100 naturally occurring elements.

Of course, it early became apparent to me that we knew less about the centre of the earth than anything on the surface - even the surface of stars!

Exploration of the elements revealed the heaviest, viz. Os & Ir. My reflections on the composition of the inner core went from there.
I note your metallurgical arguments. Are you saying that eutectics behave like solutions, e.g. of CuSO₄ in water, and that "solutions" of metals in each other, i.e. alloys, never separate under gravity? U-235 was separated from U-238 by centrifuging the mixture of isotopes in UF₆. I had thought terrestrial gravity would act over the aeons on the mixture of siderophiles in the core.

I agree that seismology would likely have detected any discontinuity, though I can´t think of any gravitational nor magnetic anomaly that would show it since they´re both radially symmetrical.

In 1936, Lehmann discovered the Inner Core, about 850∓100 miles in radius, ten times the radius of my osmiridum kernel. Presumably phase distinguishes Lehmann´s boundary and her solid inner core. How could this solid core be further investigated?

[Jess Tauber]

Just as an aside, note that Os, atomic number 76 (and thus a Lucas number, which in the periodic system tend to be located in positions within orbital rows corresponding to half- and completely filled orbitals (in the exact case up to atomic number 18, half- versus completely filled depending on whether the Lucas atomic number is odd or even, part of a general trend that also includes Fibonacci atomic numbers, which strongly trend towards INITIAL positions in half- or completely filled orbitals, all the way up to atomic number 89) BEHAVES (at least in its monatomic, gaseous phase) AS IF it were a noble gas, with significantly reduced reactivity. Its six valence electrons apparently being 'interpreted {?}' as if they were the six p- electrons found in most of the noble gases (though that's just MY interpretation). I wonder whether this might have any effect on how it 'alloys' with other core metals. Don' know how Ir might correspond to any of these trends, if at all.

Jess Tauber

Jess Tauber

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[johnmarks9]

Ha! Ha! Jess, be careful what you say there!

You´ll have the Bent left-steppers arguing Os be classified with the inert gases in the sense that they class Be with He :)

[Rene]

On 19 Jun 2025, at 08:46, johnmarks9 <johnm...@hotmail.com> wrote:

Thanx René.

I´ve long been interested in geochemistry, volcanoes, meteorites, seismology, etc., for the insights they shed into the 100 naturally occurring elements.

Of course, it early became apparent to me that we knew less about the centre of the earth than anything on the surface - even the surface of stars!

Exploration of the elements revealed the heaviest, viz. Os & Ir. My reflections on the composition of the inner core went from there.
I note your metallurgical arguments. Are you saying that eutectics behave like solutions, e.g. of CuSO₄ in water, and that "solutions" of metals in each other, i.e. alloys, never separate under gravity? U-235 was separated from U-238 by centrifuging the mixture of isotopes in UF₆. I had thought terrestrial gravity would act over the aeons on the mixture of siderophiles in the core.

I agree that seismology would likely have detected any discontinuity, though I can´t think of any gravitational nor magnetic anomaly that would show it since they´re both radially symmetrical.

In 1936, Lehmann discovered the Inner Core, about 850∓100 miles in radius, ten times the radius of my osmiridum kernel. Presumably phase distinguishes Lehmann´s boundary and her solid inner core. How could this solid core be further investigated?

Tx John for your reply, and your follow-up question.

My knowledge of metallurgy is no better than average.

As I understand it, your mention of isotope separation in UF6 brings up an important distinction. That process exploits tiny mass differences between isotopes in a gaseous state using high-speed centrifugation. In contrast, the Earth’s core involves elements alloyed in a metallic liquid or solid—where atomic masses may differ by orders of magnitude, but the atoms are bonded in a dense, cohesive phase.

Trace elements like Os and Ir, which are dissolved in the Fe–Ni matrix at very low concentrations (ppm or ppb), are there because they're siderophiles, which prefer to form metallic bonds with Fe. While I don’t fully understand the nature of the bonding involved, or the basis for the preference, these atoms aren’t like droplets in a liquid; they instead substitute into the crystalline or liquid metallic lattice. In that sense, such a metallic alloy—even as a eutectic—behaves more like homogeneous solution than a suspension. Put differently, gravitational forces are apparently insufficient to overcome the strength of metallic bonding between Os or Ir and Fe under core conditions.

From what I have read, estimates of the core's composition can only be indirectly obtained by matching results from high-pressure experiments and theoretical calculations with seismic observations.

If the inner core was made primarily of Os and Ir:

• this would apparently change its "seismic velocity structure" but no such deviations, AFAIK, have been observed, which supports the conclusion that the inner core is predominantly Fe–Ni alloy, not Os–Ir; and
• it'd be reasonable to expect that the metallic remnants of disrupted planetary bodies (e.g., from collisions during early Solar System formation) might produce osmiridium-rich asteroids or meteorites. However, we don't observe such objects which absence supports the idea that cores of at least Earth-like planets aren't primarily composed of Os and Ir.

René

[Rene]

On 19 Jun 2025, at 23:03, johnmarks9 <johnm...@hotmail.com> wrote:

Ha! Ha! Jess, be careful what you say there!

You´ll have the Bent left-steppers arguing Os be classified with the inert gases in the sense that they class Be with He :)

John and Jess

Curiously, there is some chemistry to the idea of associating Os in group 8 with the inert gases in group 18.

Historically, one group numbering system placed Fe, Co, Ni, and the platinum group metals in a three-column-wide Group VIIIB, while the noble gases occupied Group VIIIA—suggesting a kind of symmetry across different block types.

In modern terms, the so-called "noble metals"—including Os—are sometimes viewed as transition-metal analogues of the noble gases, based on their relative chemical stability and reluctance to oxidise under standard conditions. In the same manner, the sf metals can be associated with the halogen nonmetals; the d metals with the biocentric nonmetals; and the p metals with the metalloid nonmetals, resulting in an eightfold taxonomy. This approach extends the tradition of contrasting the highly reactive alkali metals with the highly reactive halogen nonmetals.

Individually, the parallels are interesting.

Xenon in the +8 oxidation state forms xenon tetroxide (XeO4), a pale yellow explosive oxide. Osmium forms osmium tetroxide (OsO4), also yellow and highly oxidizing. There are striking similarities in the oxyfluoride compounds too: XeO2F4 and OsO2F4, as well as XeO3F2 and OsO3F2.

The first compound between a noble metal and a noble gas—tetraxenonogold(II) undecafluorodiantimonate(V), AuXe42+(Sb2F11−)—was synthesized in 2000. Ironically, the synthesis of the first noble gas compound back in 1962—xenon hexafluoroplatinate (XePtF6)—involved another noble metal, platinum. It was later shown that this compound is actually more complex, containing both [XeF]+[PtF5]− and [XeF]+[Pt2F11]−.

Jess, re "six valence electrons": in terms of its electron configuration, Os has eight valence electrons, not six—specifically, [Xe]4f14 5p6 6s2. Correspondingly, Xe too has eight valence electrons, 6p and 2s.

René

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[Jess Tauber]

Thanks for the heads up, Rene- I think I meant to write 'six d-electons, which including the two s-electrons WOULD give eight. I'm starting to have some memory issues (again). This is why getting older sucks eggs (though I would think the alternative is far worse for most)....

Four decades ago I took an introductory electron microscopy course at my undergraduate school, and in the laboratory section we all had to learn how to use the ultramicrotome and also how to stain the thin sections with osmium tetroxide. We had to be VERY careful with the stuff, not just because it was very expensive.

Jess Tauber

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[johnmarks9]

It´s generally held, since Alvarez, Penfield et al. (1980) that the Chixculub crater, Mexico, was the result of the K-T extinction event 66 million years ago.

The meteorite concerned is thought to have comprised mostly osmiridium (particularly iridium), from geological markers.

The Hayabusa asteroid-mining initiatives are predicated on the discovery of asteroids composed mostly of platinum and other 5d-metals.

So aggregating asteroids could have seeded the formation of the Earth, but it´s all guesswork seemingly beyond empirical test at present.

Thanx for the discussion.

John

[Rene]

On 22 Jun 2025, at 23:51, johnmarks9 <johnm...@hotmail.com> wrote:

It´s generally held, since Alvarez, Penfield et al. (1980) that the Chixculub crater, Mexico, was the result of the K-T extinction event 66 million years ago.

The meteorite concerned is thought to have comprised mostly osmiridium (particularly iridium), from geological markers.

The Hayabusa asteroid-mining initiatives are predicated on the discovery of asteroids composed mostly of platinum and other 5d-metals.

So aggregating asteroids could have seeded the formation of the Earth, but it´s all guesswork seemingly beyond empirical test at present.

Thanx for the discussion.

John

While the Chixculub impactor left a significant geochemical iridium signature, it's not thought to have been composed mostly of iridium or osmiridium. Rather, the elevated iridium levels in the boundary layer are consistent with a carbonaceous chondrite — a primitive stony meteorite — which contains iridium at ppm levels, much higher than in Earth’s crust but still trace overall. Iridium serves here more as a fingerprint than as a major constituent.

Re asteroid mining, the Hayabusa missions were designed primarily as sample-return missions — not as mining initiatives. Hayabusa targeted the S-type asteroid Itokawa, and Hayabusa2 sampled the C-type asteroid Ryugu. Neither asteroid is especially metal-rich. While M-type asteroids may indeed be enriched in PGEs, AFAIK no asteroid has been confirmed to consist mainly of platinum or related metals.

René

[Rene]

I forgot to add that PGEs are rare at the Earth’s surface because they were mostly drawn into the core during planetary differentiation, due to their strong tendency to alloy with iron.

In contrast, most asteroids haven’t undergone this process, so PGEs remain in the crustal material—making them about 100 times more abundant in some asteroids than in Earth’s crust. This enrichment is consistent with their overall cosmic abundance.

Even so, "100 times more" still only means concentrations around 100 parts per billion, so we’re still talking about trace levels, just comparatively elevated.

René

PS Where I earlier wrote, "iridium at ppm levels" I meant to say, "iridium at sub-ppm levels".

[Jess Tauber]

Guys- I remember (vaguely) some TV-broadcast science fiction movie aired some years ago where space aliens came to earth to steal our gold, the latter element being rare everywhere in the universe. And in the sequel to 'Independence Day' we're told that the alien mothership mothership (so huge its diameter spans the entire Atlantic Ocean) comes here to suck out the earth's metallic core. One would think that they'd have a good choice of cores to suck out in a multiplanet system like ours, but perhaps the writers weren't aware of such things (most screenwriters not being terribly scientifically literate).

Jess Tauber

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[Rene]

A related item: "Metal asteroid Psyche has a ridiculously high 'value.' But what does that even mean?"

https://www.space.com/psyche-metal-asteroid-composition

[Jess Tauber]

I was reading about Psyche just two days ago. Turns out it may NOT have a solid metallic core after all, just a metal-rich rocky core. That'll make any future mining operations a little trickier, as more separations and refining will be necessary. I've thought about how to use solar energy in this regard, by constructing a gigantic mass-spec analogue, and letting the masses of the ablated particles themselves determine their targets, where you can build up ingots/bars as final products for shipment. A setup like this would also be able to provide for isotopically pure elements, not just chemically pure ones.

Jess Tauber

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[Rene]

On 28 Jun 2025, at 22:24, Jess Tauber <tetrahed...@gmail.com> wrote:

I was reading about Psyche just two days ago. Turns out it may NOT have a solid metallic core after all, just a metal-rich rocky core. That'll make any future mining operations a little trickier, as more separations and refining will be necessary. I've thought about how to use solar energy in this regard, by constructing a gigantic mass-spec analogue, and letting the masses of the ablated particles themselves determine their targets, where you can build up ingots/bars as final products for shipment. A setup like this would also be able to provide for isotopically pure elements, not just chemically pure ones.

Jess Tauber

Thanks Jess.

There sure is a lot of information out there about Psyche, which has a diameter of 157 miles. Being so big, that’s what results in its astronomical value.

I’ve read that if it could somehow be transported back to our planet, the iron alone would be worth US $10,000 quadrillion. The gross world product (GWP) in 2015 was only about $73.7 trillion, so our economy would promptly collapse.

Psyche may have started out as a planet that got bumped around so badly during the formation of the solar system that the outer layers of rock were blasted away, revealing a core similar to Earth’s.

What’s left may be chunk of metal made up almost entirely of iron, nickel and (presumably) a number of other rare metals like gold, platinum, copper, cobalt, iridium and rhenium.

OTOH, according to Nasa:

"Up until recently, the scientific consensus was that the asteroid Psyche consisted mostly of metal. The more recent data indicates that the asteroid is possibly a mix of metal and silicate, the same material found in glass and sand. The best analysis indicates that Psyche is likely made of a mixture of rock and metal, with metal composing 30% to 60% of its volume. The asteroid’s composition has been determined by radar observations and by the measurements of the asteroid’s thermal inertia (how quickly an object gains or re-radiates heat)."

https://science.nasa.gov/solar-system/asteroids/16-psyche/

30 to 60% of still results in a notional value of US $3000—6000 quadrillion.

René

[Jess Tauber]

Well, even the silicate portion might be of value, for making concrete aggregate, given the depredations of beaches and sea bottoms here on earth seeking sand sources, it might go far to alleviating that issue. As for Psyche causing the bottom of our economy to fall away, that might only occur if we continue to rely on things like precious metals to back up currencies (and I thought we'd gotten off the gold and silver standard ages ago, despite Fort Knox). In the future, given current shifts towards services and information bases, metals may no longer have the sway they once had. And also remember that despite the wealth possibly contained in Psyche, that's practically NOTHING compared to what will likely be present in the core of planet Mercury, whose own rocky crust and mantle were similarly blown off a long, long time ago. A bit more work there, but much bigger rewards.

Jess Tauber