[pjungwir]

Something I've been curious about lately is: why do we find elements on Earth clumped together, for example veins of iron or gold? I can understand coal or oil since that comes from something organic that put it there. But what about elemental substances? When I throw stuff into my blender or spice mixer it gets pretty homogeneous. Surely an exploding star ought to mix things up better than that. So is there something that brings the iron & gold back together again? I don't know if this is a question for an astrophysicist, chemist, or geologist, but I suppose HN has all three. :-)

[jandrewrogers]

Veins of heavy metals are created by the chemistry of hydrothermal systems. High-pressure super-heated water is a great solvent and will leach many things out of the surrounding rocks in a chemistry and temperature/pressure dependent way.

These hydrothermal systems can be structurally enormous and form gradients e.g. due to differences in rocks and temperature at different depths. This acts a bit like a distillation column. Every metal has different characteristic solubility and will precipitate out of solution in boundary areas where amount of metal in solution exceeds the solubility under those conditions. Since these are fluids, the solution is circulating, bringing fresh saturated solution to the boundary area.

In some regions like Nevada, there are places where very large and diverse gradients are exposed e.g. you will find copper, silver, gold, tin, mercury, lead, uranium, etc veins of various quality distributed across the same very local geological formation as different metals were deposited in different places within the same hydrothermal system.

A limitation on formation is that it typically requires a relatively large hydrothermal system with stable properties and appropriate chemistry over millions of years. The kinds of places that have large hydrothermal systems also tend to be geologically unstable.

[cmyr]

I remember reading in one of John McPhee's geology books (collected as [1], and one of the best things I've ever read) about a particular way that diamonds can form; it had a name (like a diamond jet or a diamond burst or something) but the basic idea is mesmerizing: super-heated water containing dissolved carbon would find some fissure in the surrounding rock and explosively expand into it, and in the process it would cool rapidly and diamonds would crystallize out of the solution. I'm stuck with the visual of a seam of diamonds popping into existence in an instant, which is not what we normally think of as a geological timescale.

[1] https://en.wikipedia.org/wiki/Annals_of_the_Former_World

[PKop]

Similar process here with gold

https://news.ycombinator.com/item?id=38935662

[fnordpiglet]

I’m surprised it’s not due to settling in magma. Does this mean on, say, the moon we would see a uniform scattering of elemental atoms ?

[PaulHoule]

... there's a reason why gold is a precious metal! See also https://en.wikipedia.org/wiki/Hydrometallurgy

[pixl97]

Iron banded formations...

https://en.wikipedia.org/wiki/Banded_iron_formation

Earthquakes make gold veins in an instant.

https://en.wikipedia.org/wiki/Banded_iron_formation

Also plants performing agromining.

It's not a big blender, there are all kinds of processes forming gradients.

[cratermoon]

How do earthquakes make gold veins? Something like this? https://www.nature.com/articles/nature.2013.12615

[pixl97]

Oops, that is exactly what I meant to post.

[cratermoon]

That's a fascinating process. Even the slow way of forming veins is something that I didn't know until today. Science blows my mind.

[borissk]

You pasted the same link twice.

[sandworm101]

Many natural processes self-sort material over time. In the below vid simple sand in a rotating tube self-sorts according to crystal size, forming veins of like material. Over geological timeframes, tiny little effects add up to seemingly unnatural organization.

https://youtu.be/Zvh5FU6XwaI

[throwaway5752]

It is because of geological processes like https://en.wikipedia.org/wiki/Massive_sulfide_deposits. The vast majority of heavy metals on Earth are in the core, because of gravity. Convection moves elements from inner layers of the Earth to the outer ones, and sometimes they dissolve in the ocean or get stuck in a buoyant piece like a continent before falling back down the gravity well.

It is also because chemistry, periodic table stuff related to valence electron interaction. You actually don't find veins of iron, for example you find bands of iron oxide ore like hematite. The reason those are present is one-off because iron dissolved in water precipitated out when plants evolved to product oxygen gas via photosynthesis. Gold is just relatively nonreactive so it is found in relatively pure forms, like nuggets, that get eroded out of igneous formations.

[xenadu02]

Also remember things are not like the movies. Most gold is not found in large single chunks. It is found as tiny inclusions scattered within a larger piece of rock, sometimes put there by flash deposition deep in the crust (earthquake or whatever lowers pressure, water instantly flashes to steam, dissolved minerals and floating particles fall out).

The same goes for other materials: They are often interspersed with lots of other minerals or the ore form is actually several different compounds in random mixtures.

Another example is Aluminum which comes from Bauxite. We say Bauxite as if it was one single thing but its actually a varying mix of Al(OH)3, AlO(OH), FeO(OH), Fe2O3, Al2Si2O5(OH)4, TiO2, FeTiO3, FeOTiO2, and traces of others. The actual composition varies within and between veins.

[photochemsyn]

A remarkably large number of minerals (I've heard estimates as high as 2/3 of all mineral forms) are produced via microbiological activity, e.g. selective oxidation, precipitation, etc. A famous example is the natural uranium reactor Oklo from 2 billion years ago (Congo), formed most likely by microbial precipitation of uranium from seepage water. Today people are investigating the use of microbial uranium precipitation to clean up mine tailings, accident sites, waste leakage etc. from the nuclear sector:

https://www.sciencedirect.com/science/article/abs/pii/S03043...

[hermitcrab]

Definitely not an expert on this. But IIRC most of the heavy metals, like gold, present at the earth's creation will have migrated to the core, due to their density. However some heavy metals will have arrived by meteroite and I guess these will be relatively localized. Also various chemical and geological process will tend to separate out different elements over long timescales.

[umvi]

I imagine it's sort of like how if you pour a bunch of liquids together (oil, water, etc), even if you blend them up (ignoring emulsification) they will naturally separate into layers based on density, buoyancy, or some other property that causes them to separate again when subject to natural forces.

[jessriedel]

There are lots of natural processes that will tend to sort atoms by density or chemical type, e.g., heavier things settle downward. That's why the Earth's core is a very different composition than its crust.

[hutzlibu]

There are also centrifugal forces at play, meaning heavier elements get also pushed upward.

[pfdietz]

Heavier phases settle. Heavier elements will rise to the surface if they go into lower density phases. This is exactly what happened with uranium, which is concentrated by a factor of 1000 in the Earth's continental crust.

[genman]

This post about how gold veins are produced may help to understand this https://news.ycombinator.com/item?id=38732825

[thsksbd]

Clearly the metals and their oxides phase segregate, so the question is: "Why do you assume that molten rocks mix?"

What mental model do you have that leads you to the wrong answer?

[bee_rider]

Their model seems to be that the elements mix like in a blender. It isn’t clear to me if that’s wrong or not. In particular, two seemingly reasonable processes could the that the elements mix, and then have some additional process to separate, or that they simply don’t mix in the first place. In the former case, their model isn’t wrong, it is just missing an understanding of that last process.

In general I think it is good to investigate both options: maybe I’m just wrong, or maybe I’m mostly right but am just missing a couple steps.

We’ve got lots of people commenting with processes that will sort the elements back out, so I think op was mostly right, just missing a lot of little processes.

[colordrops]

There are plenty of phenomena in life where things clump together. Just look at oil and water, or different sand particle sizes when shook together in a container. It's going to be something similar at a cosmic scale.

[hugh-avherald]

https://en.wikipedia.org/wiki/Clustering_illusion

[javajosh]

I have always assumed that such metals came in big blobs and then got smeared out over geological time. Could be wrong though.

[frostix]

Most likely do to some process resulting in separation by density. You’d probably also need a geologist to contribute who understands previous states of earth. I’d hazard a guess that most of it occurred over time which much of earth was in a sort of fluid hot state and things clumped together. Structures you see today might be due to the surrounding materials that also tended to clump and how things cooled over time (slow, fast, etc.) There’s also the fact earth still has active convection going on heating things up, spinning them around, letting gravity pull, rising up, cooling, and then more complex motions like plate tectonic movement, fracture, etc. I suspect it would be pretty difficult to say exactly why any specific mineral deposits tend to follow the structures we find them in due to how complex the process is but someone may know.