Earth's mantle holds moister minerals.

Compared with pork chops or pieces of wood, stones don't hold much moisture. But two geoscientists report that certain minerals from deep inside the Earth contain more water than previously assumed - a finding that alters theories about the internal workings of the planet.

Geologists know that so-called hydrous minerals, such as mica and talc, actually have [H.sub.2.O] or OH locked within their crystalline structures. The most abundant minerals within the mantle, however, are labeled anhydrous because their chemical formulas do not include hydrogen.

In recent decades, experiments have shown that even anhydrous minerals can contain small amounts of water, but researchers were unable to determine the amount of water present. Now, David R. Bell and George R. Rossman of the California Institute of Technology in Pasadena have succeeded in measuring the water content in a broad group of important mantle minerals.

Bell and Rossman passed infrared light through thin mineral samples from the mantle. Bonds between oxygen and hydrogen absorb a specific wavelength of light, allowing the researchers to quantify the hydrogen content of a given sample. In the March 13 Science, they report that certain anhydrous minerals, called pyroxenes, contain small but significant amounts of water--between 200 and 500 parts per million.

Even such a minuscule amount could have important effects because pyroxenes make up between 30 and 40 percent of the mantle, says Bell. Using their findings, the researchers calculated how much water the mantle might hold. "We have something like 80 percent of the volume of the oceans of the world contained down there in these rocks," says Rossman.

The realization that such large amounts of water can reside in anhydrous minerals may help scientists understand the origin of the oceans, he says. Some theorists have suggested that Earth's water originally resided inside the planet and gradually escaped onto the surface through volcanic eruptions. Bell and Rossman's work shows which minerals could have stored the water and in what quantities.

Water in the mantle also plays an important role in determining where volcanoes form and how violently they erupt. "Water is probably the most important single constituent that affects the explosive behavior of volcanic eruptions," says John R. Holloway of Arizona State University in Tempe. The new studies of water in anhydrous minerals will help geoscientists understand the process that creates volcanoes, he says.