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I read about this a few days ago. Most people don't realise it but we're surrounded by fissile nuclear materials in every single rock we see. Most of the actinide elements are lithophilic, which means they preferentially bind chemically to the rock forming minerals in the crust and mantle....the silicates and such. Sedimentary rocks are particularly noted for containing radioactive elements and minerals. Most of our Uranium is found in them (unconformity hosted deposits, roll front hosted deposits in sandstones and mudstones, quartz pebble conglomerates etc). Actually, in the Gabon, there are naturally occurring nuclear reactors which have been active for quite a few billion years. At present they're not as active as they used to be, but the mineralogy of the cavities and such they're in is quite unique, because of the higher than normal radioactivity.
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