I don't understand how the aging works. Pure calcite does not contain uranium. How do we know the calcite didn't form in an environment already containing trac

I don't understand how the aging works. Pure calcite does not contain uranium. How do we know the calcite didn't form in an environment already containing traces of old decaying uranium and thus trapped both thorium and uranium as the crystal formed? Edit: I think I've figured it out (from the wiki[1]) > Thorium is not soluble in natural water under conditions found at or near the surface of the earth, so materials grown in or from this water do not usually contain thorium.[citation needed] In contrast, uranium is soluble to some extent in all natural water, so any material that precipitates or is grown from such water also contains trace uranium, typically at levels of between a few parts per billion and few parts per million by weight. So what I'm guessing is happening is this: Running water is dissolving uranium and going down a cave wall. The water contains no thorium because thorium is not soluble and the water is running (i.e. by the time the uranium decays it will not be on the wall anymore). Calcite forms using the minerals in the running water, and traps the dissolved uranium. Only then can it decay inside the crystal into thorium. [1] https://en.wikipedia.org/wiki/Uranium%E2%80%93thorium_dating https://en.wikipedia.org/wiki/Uranium%E2%80%93thorium_dating