Which of these elements was not found in pitchblende by Marie and Pierre Curie?

In 1894 the newly graduated Marie Sklodowska (1867-1934) met the physicist and chemist Pierre Curie (1859-1906). They married in 1895. In starting the groundwork for her doctoral thesis, Marie Curie began studying uranium, which was at the heart of Henri Becquerel’s 1896 discovery of radioactivity. Her work focused on pitchblende -- now known as uraninite -- a radioactive, uranium-rich mineral and ore with a chemical composition that is largely uranium dioxide.

In her early results Marie Curie reported the high probability of the existence in the mineral of other radioactive elements. Pierre Curie joined her in the research, and in 1898 they discovered radium, thorium and polonium, named after Marie’s native Poland. On 20 April 1902 Marie and Pierre Curie successfully isolated radioactive radium salts from pitchblende. A year later they shared the 1903 Nobel Prize in physics with Becquerel for their investigations of radioactivity.

Plutonium is a radioactive chemical element that is found only in trace amounts in nature -- a few parts per trillion in some concentrated ores of uranium. It was first synthetically produced and isolated using a deuteron bombardment of uranium-238; but this only occurred seven years after Marie Curie’s death.

The picture illustrates pitchblende (uraninite) crystals.