Uraninite named, was either unknown, impossible or not economically feasible). The mineral has been known at least since the 15th century from silver mines in the Ore Mountains, on the German/Czech border. The type locality is the historic mining and spa town known as Joachimsthal, the modern day Jáchymov, on the Czech side of the mountains, where F. E. Brückmann described the mineral in 1772. Pitchblende from the Johanngeorgenstadt deposit in Germany was used by M. Klaproth in 1789 to discover the element uranium. All uraninite minerals contain a small amount of radium as a radioactive decay product of uranium. Marie
Period 7 element corresponding to the filling of the 5f electron shell; actinium, a d-block element, is also generally considered an actinide. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. Of the actinides, thorium and uranium occur naturally in substantial, primordial, quantities. The radioactive decay of uranium produces transient amounts of actinium, protactinium and plutonium, and atoms of neptunium are occasionally produced from transmutation reactions in uranium ores. The other actinides are purely synthetic elements, although the first six actinides after plutonium would have been produced during the Oklo phenomenon (and long since decayed away),