The LA Times features a rather interesting chemical mystery today. Rocks being collected by a woman on a beach in San Diego County reportedly caught fire in her pocket, giving her some rather nasty burns. But what on Earth were these rocks and how can anything that can spontaneously ignite have survived so long out on a beach?

Spontaneous combustion can occur naturally, but is pretty rare. A few instances are listed, such as coal seams or even large compost heaps or shipments of pistachio nuts. But these examples only tend to catch fire when they’re present in large quantities. There are plenty of pyrophoric materials out there, such as white phosphorus (one of several allotropes of the element) that gained notoriety during its use by US forces in Fallujah and tert-butyllithium, responsible for the death of Sheri Sangji. But by their very nature they don’t hang around long because they’re so unstable and combust in air. So what was it?

The Times gives some details about the rocks – one grey and marbled the other green (‘Teenage Mutant Ninja Turtle green’, apparently). Both stones were ‘streaked and flecked’ with bright orange deposits too. Tests by labs suggest that there were indeed phosphorus compounds on the rocks. The Times approached some chemists but they were stumped and couldn’t come up with an answer as to what could have caused the fire. White phosphorus can appear tinged yellow, thanks to the presence of red phosphorus but would surely burn up unless it was somehow protected – could the stones have been picked up out of a rock pool? But that wouldn’t explain how the chemical got there in the first place. Has anyone out there got any suggestions?

Patrick Walter


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