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Mobile oxygens make silicate magma flow.

Silicon, that marvelous material at the core of glasses, ceramics, sand, computer chips, even magma spewing from volcanoes, has revealed yet another side of its versatile nature to scientists studying its properties.

Researchers knew that silicon-oxygen compounds, called silicates, consist of tetrahedral formations, in which four oxygen atoms surround each silicon. But sometimes, silicon forms a brief relationship with a fifth oxygen atom, geologist Jonathan F. Stebbins from Stanford University reports in the June 20 NATURE. Using nuclear magnetic resonance, he has demonstrated the existence of the anomalous silicon in a liquid silicate. He and others think the overcrowded silicon plays a key role in making this liquid flow.

"It's basically a defect and it increases the rate at which things move around," says Frank J. Spera, a geologist at the University of California, Santa Barbara, who studies the molecular dynamics of silicates. "A small concentration can be enough of a disruption [to the liquid's structure] to produce important changes: It breaks down the viscosity of a fluid."

The viscosity of silicon-based magma influences volcanic eruptions. While less viscous magma flows, thicker magmas tend to trap gases inside where they "build up like a pressure cooker" until the volcano explodes, says Ian Farnan, a Stanford chemist working with Stebbins. An understanding of the dynamics of the magmas may enable geologists to better assess how violent an eruption will be, he adds.

Theorists had predicted the existence of five- and higher-fold coordinate silicons, which represent intermediate, denser phases of silicate. These would allow an oxygen to shift from one silicon partner to another and consequently enable the liquid to flow. "It's moving closer at the same time that another [oxygen] is moving slightly further away," says Farnan. The researchers caught the oxygen in this dance by rapid cooling, or quenching, liquid silicate, thereby causing the atoms to freeze in their places.
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Title Annotation:silicate discovered that flows because it can take on an extra oxygen atom
Author:Pennisi, Elizabeth
Publication:Science News
Date:Jun 29, 1991
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