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The main focus of the work was on the mineral
calcite, which has more than 300 identified crystal forms that can
combine to produce at least a thousand different crystal variations.
Crystals can form a thousand different shapes by combining the basic
forms of the positive rhombohedron (a prism with six faces, each a
rhombus), negative rhombohedron, steeply, moderately and slightly
inclined rhombohedrons, various scalahedrons, prism and pinacoid. De
Yoreo and Dove first determined that when combined with magnesium, the
corners formed by the intersection of atomic steps flatten and roughen,
leading to flattening of the crystal's corners and elongation and
roughening of the crystal shape.
When combined with acidic amino acids, both the
step and crystal shapes changed to reflect the handedness (whether the
molecule was right-handed or left-handed) of the amino acids.
Molecular simulations showed that the step edges provided the most
favorable binding environment for the acids.
When citrate, a naturally occurring inhibitor and
therapeutic agent, was used in the experiment, the change in crystal
shape again mimicked the change in step shape, and molecular models
also identified the steps as the preferred interaction sites.
In the last experiment, calcite crystals were
combined with a protein extracted from abalone nacre, a pearly
substance that lines the interior of many shells, and is most perfect
in the mother-of-pearl. The changes were step-specific and directly
determined the shape of the macroscopic crystals.
"Although crystal growth modifications are diverse,
the source of shape changes in these studies is clear," De Yoreo said.
"Crystal shape is controlled by step-specific interactions between
growth modifiers and individual step edges on pre-existing crystal
faces. Through this research, our team has shown that the classic
theories of growth merge smoothly with the models proposed to explain
shape modification." |
