My problem here is that I don't understand chemistry lingo. The following is a quote from the Wikipedia article on Francium:
Because of the general appearance of the other elements in its periodic table column, it is assumed that francium would appear as a highly reflective metal, if enough could be collected together to be viewed as a bulk solid or liquid. However preparing such a sample is impossible, since the extreme heat of decay (its longest isotopic half life is only 22 minutes) would immediately vaporize any viewable quantity of the element.
I don't understand what an "isotopic half life" is and why it makes francium apparently impossible to view. Does that mean it explodes after 22 minutes? Or burns? Or... what? What if it existed in an environment that was inhospitable to human life -- say, on average between 100-130 degrees F (38-54 C) with an extremely low percentage of oxygen in the atmosphere? What if it existed as veins of material similar to oceanic currents surrounded by other metallic elements (gallium, mercury, rubidium, caesium)?
My story sets an admittedly less-than-realistic stage involving landscapes built more for the impact of visualizing such a place than any scientific foundation for liquid metal rivers actually existing for any reason, but I like the idea of including names of real materials that such a feature could be composed of and how it might make those rivers behave. Could it be at all plausible for there to be some source generating liquid francium into a stream of other liquid metals, or is the existence of any visible amount of francium simply impossible on the surface of planet Earth even under extreme post-apocalyptic conditions?