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On the rocks

2:34pm, April 17, 2007

New research explains why a cancer-causing form of chromium has been turning up in ground and surface waters far from industrial sources.

Chrome plating and dye manufacturing are among the industries that generate chromium (VI), a form that the element assumes in certain compounds. But recently, researchers have discovered the toxic agent in regions—including California and parts of Mexico and Italy—beyond the reach of industrial contamination.

In these cases, "it was obvious that [chromium (VI)] had to be coming from a natural material," says Scott Fendorf, an environmental chemist at Stanford University.

Fendorf and his coworkers focused on the mineral chromite, found in certain rocks and soils common to the Pacific coasts and other seismically active areas. Over time, chromite slowly releases chromium (III), a relatively benign form of the element.

The researchers reacted chromite with birnessite, a manganese-containing mineral that often form

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