The authors of the comic book Superman portrayed Kryptonite as a metallic chemical element from the planet Krypton. However, the suffix -ite generally denotes a compound. Look on your periodic table - there are no elements ending in -ite.
To make matters worse, the element Krypton, presumably a component of Kryptonite if it were a compound, is an inert gas that does not form compounds under ordinary conditions. Superman's anathema is an imaginary compound masquerading as an element.