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Calcite on the edge of growth, dissolution.

Wheter incorporated into seashells or deposited as limestone, marble or chalk, the mineral calcite participates in a variety of biological and geologic processes. To help elucidate how calcite fulfills it svaried roles, researchers have developed a new technique for observing hitherto hidden details of the way calcite crystallizes and dissolves.

Physics graduate student Paul E. Hillner of the University of California, Santa Barbara, geoscientist Andrew J. Gratz of the Lawrence Livermore (Calif.) National Laboratory and their co-workers use an atomic force microscope to observe the step-by-step addition or removal of calcium and carbonate ions at a calcite crystal surface. Although they can't detect individual atoms, they can clearly see the apparent movement of edges as ions settle into layers to produce characteristic patterns of steps.

These findings represent "extremely important first observations [by atomic force microscopy] of in situ crystal growth ... of a mineral," comments Richard J. Reeder of the State University of New York at Stony Brook, who has also studied calcite growth.

Hillner and his colleagues describe their work in the April GEOLOGY.

The researchers track changes in surface features during crystal growth by passing a concentrated solution of calcium carbonate dissolved in water across the surface of a calcite sample. By making the solution highly alkaline, they slow the deposition process sufficiently to allow time for repeatedly scanning the surface to detect any changes.

"Our growth rate is so small that it would take us years to grow a reasonably sized crystal," Hillner notes. The researchers can control the rate of crystallization or dissolution by slightly varying the solution concentration.

The resulting images reveal surprisingly complex processes taking place as ions shuttle between solution and crystal surface. Layers grow by the formation of broad steps spiraling outward from surface outcroppings. These steps, each only 0.3 nanometer high, give the crystal surface a distinctive, terraced appearnce. In contrast, dissolution produces sharply etched pits.

"Calcite grows by adding material right at a step," Gratz says. The researchers find scant evidence of ions landing on a flat surface, then diffusing to their final resting places at edges.

And there's much more to see. "It's its own little world," Hillner says. "There's so much going on, it's hard to figure out what to attack next." Hillner and Gratz are now putting together an apparatus that would allow them to study calcite growth and dissolution at elevated temperatures.

"Carbonate minerals exhibit a rich surface chemistry that creates hundreds of crystalline forms in nature," they conclude. "It may now be possible to relate each form not only to the natural environment in which it grows, but also to its detailed growth mechanism."
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Author:Peterson, Ivars
Publication:Science News
Date:Apr 18, 1992
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