Why is There Uranium in Rocks? – A Geochemical Exploration
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Why is There Uranium in Rocks? – A Geochemical Exploration

By Dr. Zoomie

So I’m in Australia now and, as with my previous post, I thought I’d take an opportunity to describe one more source of radiation I’ve encountered – the rocks. In particular, Australia has some world-class uranium deposits as well as other mineral deposits that have high levels of uranium – it’s reasonable to wonder why. And it mostly comes down to geochemistry. I know this sounds a bit esoteric, but let me see if I can keep it down-to-Earth. There are two kinds of geochemistry – how elements behave when they’re in molten rock and how they behave in the environment – let’s start with the first.

Picture a magma chamber deep underground – as it cools, minerals begin to form and to settle to the bottom of the magma chamber and the minerals that form first tend to be rich in iron and magnesium and to be formed of relatively small atoms. Uranium, thorium, and potassium – the most common radioactive atoms – are large and they don’t fit easily into the tight crystal structure. So these first minerals that form tend to be bereft of radioactivity and, since the radioactive elements remain in the residual melt, this (and the minerals it forms) tend to be more radioactive than the original molten rock. This process continues with the melt becoming increasingly radioactive with each successive round of fractional crystallization. At the end, the majority of the uranium, thorium, and potassium is in the tiny fraction of liquid rock that remains and the minerals that form contain the largest atoms – this is where the remaining radioactivity ends up. These are the minerals that go into forming granite and other light-colored igneous rocks – these rocks contain elevated levels of radioactivity. This is why granite can be radioactive and why light-colored igneous rocks tend to be more radioactive than those that are black or dark-colored. And when, in the course of geologic events, these granites are subjected to tremendous heat and pressure, they sometimes recrystallize – called metamorphism – into metamorphic rocks that are also enriched in radioactivity.

In Australia, the radioactivity finds is way into granites as well as into a number of ores; with the ores the mechanism is sort of interesting. In particular, ores that are likely to contain uranium of thorium tend to include other atoms that are close to the same size as uranium and/or that have similar chemical properties. These include ores of rare earth elements, titanium, niobium, and others. Here, the uranium, thorium, or potassium can replace these other atoms of similar size and chemical properties – uranium substituting for europium, cerium, or gadolinium for example. Thus, many ores can have radioactivity in them.

So that’s how radioactivity finds its way into igneous rocks – but we also find it in coal, petroleum, natural gas, black shales, and other materials that are not igneous rocks. Not only that,, but we also find isotopes of radium, polonium, lead, and a dozen other elements in all of these rocks – it’s natural to wonder where those come from as well. So let’s talk about the environmental geochemistry of these elements next, and then we’ll get into the other isotopes and where they come from. And with the environmental geochemistry, uranium is the most interesting.

Here’s the thing – uranium is soluble in water that contains oxygen and it falls out of solution in anoxic water (that’s water that lacks oxygen). So now picture rain falling from the skies, picking up oxygen from the atmosphere as it falls, and splashing down onto the granite rocks of the mountains below. As we’ve seen, this granite contains traces of uranium and the water, running over the granite, leaches some of this uranium from the rock. As the water collects into rivulets and streams and rivers it remains oxygen-saturated (also carrying enough oxygen to support fish and other aquatic organisms), flowing downstream until, perhaps, it enters a swamp or other wetland, where it pools and stands fairly still. Here, the lush vegetation sheds leaves into the water; they sink to the bottom and join the trunks of fallen trees and other plants, decaying in the depths. This decay removes oxygen from the water and, at some point, the water contains too little oxygen for the uranium to remain in solution; it precipitates out and joins the decaying plant matter in the muck. Over the eons, these decaying plants go on to form coal, petroleum, or natural gas and the uranium can be locked within the coal or is carried within the brine that accompanies most hydrocarbon deposits. Eons later, when we dig up and burn the coal the uranium remains in the ash; when we extract the hydrocarbons it comes to the surface with the brines, often coating the insides of the pipes.

Of course, there’s more than uranium here – uranium decays to thorium which is, itself, radioactive; thorium decays as well, and it takes over a dozen of these steps before uranium-238 (for example) finally arrives at stable lead-206 (U-235 winds up at Pb-207 through somewhat fewer steps). These radioactive nuclides – called a decay series – are present in the coal, the brines – and in the scale lining the pipes and tanks as well, and they can cause some problems for the oil extraction industries. And, to complete the picture, these are also present in the granite – any granite that’s more than a few million years old will also have these decay progeny.

So that’s why the uranium (and thorium, radium, radon, etc.) are in the rocks – and in the pipes, coal, flyash, and so many other natural objects. And that’s why I so frequently get elevated radition levels when, for example, flying above a cenetery (from the granite headstones), driving past a granite building, surveying natural gas and petroleum storage adn processing facilities, and so forth.

There’s one other interesting outcome of all of this too – the natural nuclear reactor found in Gabon in the 1970s. This reactor formed at a time when the atmosphere contained enough oxygen to make uranium soluble (so it could collect in a single location) and when the Earth was still young enough that the fissile U-235 was present in sufficient quantity to sustain a critical chain reaction. During a relatively narrow window of time, nature could produce a working nuclear reactor – in this case, it operated off and on for a few hundred thousand years. I’ve written about that elsewhere, so I won’t go into that again here.

Image Reference:


Photograph captured by James St. John