Radioactive alchemy: diamonds from coal.

Superman once made a diamond by squeezing a lump of coal. Now, research suggests he need not have lifted a finger: Coal may already harbor tiny diamond crystals, created naturally by radioactive decay over millions of years.

Tyrone L. Daulton of Argonne (Ill.) National Laboratory and Minoru Ozima of the University of Tokyo discovered nanometer-size diamonds within a coal-like rock from Russia, they report in the March 1 Science. They propose that the diamonds formed when uran ium atoms in the rock underwent fission, breaking up into high-energy ions.

"When the uranium atom decays, it produces two fission fragments. These fission fragments are like bullets fired into the carbon structure. When they tear through the carbon structure, they break bonds between the carbon atoms," says Daulton.

After the fission fragments have passed, the carbon atoms move back together and reform bonds. Sometimes these new bonds pull the carbon atoms into the compact crystal structure of a diamond, suggests Daulton.

This process represents an unusual method of forming diamonds. Most diamonds, especially gem-size stones, develop in the mantle, deep inside the planet.

High pressures there squeeze carbon atoms into their most compact form.

In the 1970s, Russian scientists proposed that the uranium naturally present in coal and other carbon-rich rocks could form microscopic diamonds through fission. To test this theory, they examined an unusual rock called carburanium, which contains high pe rcentages of carbon and uranium.

Using X-ray diffraction analysis, the Russian workers failed to find diamonds in the carburanium. Daulton and Ozima, however, analyzed the carburanium with a high-resolution transmission electron microscope, a much more sensitive method. They identified u ltrasmall diamonds ranging in size from 2 nanometers to 40 nanometers and containing a few thousand to a few million carbon atoms.

Lest anyone get the idea of mining coal for its diamonds, it would take about a quadrillion (1015) of these nanodiamonds to equal the volume of a 1-carat solitaire.

Stephen E. Haggerty of the University of Massachusetts at Amherst says that Daulton and Ozima "seem to have firmly established that these are diamonds."

But Haggerty wonders whether other processes aside from radiation could have created the nanodiamonds.

To resolve the issue, Daulton hopes to test the radiation theory with an experiment using Argonne's linear accelerator. Sending an ion beam into a carbon-rich material may tell whether radioactive decay of uranium can create nanodiamonds in nature.