Kryptonite is no longer just the stuff of fiction feared by caped superheroes. A new mineral matching its unique chemistry - as described in the film Superman Returns - has been identified in a mine in Serbia. According to movie and comic-book storylines, kryptonite is supposed to sap Superman's powers whenever he is exposed to its large green crystals. The real mineral is white and harmless, says Dr Chris Stanley, a mineralogist at London's Natural History Museum. "I'm afraid it's not green and it doesn't glow either - although it will react to ultraviolet light by fluorescing a pinkish-orange," he told BBC News.

Researchers from mining group Rio Tinto discovered the unusual mineral and enlisted the help of Dr Stanley when they could not match it with anything known previously to science. Once the London expert had unravelled the mineral's chemical make-up, he was shocked to discover this formula was already referenced in literature - albeit fictional literature. "Towards the end of my research I searched the web using the mineral's chemical formula - sodium lithium boron silicate hydroxide - and was amazed to discover that same scientific name, written on a case of rock containing kryptonite stolen by Lex Luthor from a museum in the film Superman Returns.

"The new mineral does not contain fluorine (which it does in the film) and is white rather than green but, in all other respects, the chemistry matches that for the rock containing kryptonite." The mineral is relatively hard but is very small grained. Each individual crystal is less than five microns (millionths of a metre) across.

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"'Knowing a material's crystal structure means scientists can calculate other physical properties of the material, such as its elasticity or thermochemical properties," explained Dr Le Page. "Being able to analyse all the properties of a mineral, both chemical and physical, brings us closer to confirming that it is indeed unique."

Finding out that the chemical composition of a material was an exact match to an invented formula for the fictitious kryptonite "was the coincidence of a lifetime," he added.

The mineral cannot be called kryptonite under international nomenclature rules because it has nothing to do with krypton - a real element in the Periodic Table that takes the form of a gas. Instead, it will be formally named Jadarite when it is described in the European Journal of Mineralogy later this year. Jadar is the name of the place where the Serbian mine is located.