Webster's English Dictionary

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uraninite

ura.ni.nite \yu.-'ra--n*-.ni-t\ n [G uranin uraninite (fr. NL uranium) + E 
   -ite] : a mineral UO2 that is a black octahedral or cubic oxide of uranium 
   , that contains thorium, the cerium and yttrium metals, and lead that often 
   when heated yields a gas consisting chiefly of helium, and that is the 
   chief ore of uranium