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Uranium's density is greater than most elements, so you would expect it to settle to the bottom of a volume of fluid. In the case of the Earth, which was molten in the beginning, you might then expect uranium to have settled to the core.

What stopped all of the Uranium from sinking down, so that we still have the deposits in the crust? Or more widely, why do we have heavy elements in the crust at all?

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Uranium is primarily lithophilic (it readily combines chemically with oxygen), but also is chalcophilic (it readily combines chemically with sulfur). It definitely is not siderophilic (it does not readily dissolve in molten iron). The first two means that uranium compounds are easily formed, which are of course much less dense than is pure uranium. That uranium is not siderophilic means that the mechanisms that led to the differentiation of the Earth had a lesser effect on uranium than it did on metals that easily alloy with iron.

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  • $\begingroup$ I believe there are significant thermally driven flows in the earths liquid core which will cause mixing of all materials throughout the core- these forces are far greater than buoyancy and will allow uranium and other heavy materials to appear near the surface. Of course the fission of uranium and other heavy (unstable) materials is responsible for sustaining this temperature differential in the first place... $\endgroup$ – Floris Jul 30 '14 at 0:41
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Uranium does not exist as native metal and is usually combined with oxygen forming silicates, phosphates, carbonates, sulphates, arsenates, and vanadates, which are stable minerals found in the Earth's crust. Uranium is held in the crust because of its chemistry.

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