From Crystal Shapes to the Origin of Life

Participant Profile

Yukio Saito

Yukio Saito

When you hear the term "crystal shape," what comes to mind? Is it a clean shape enclosed by flat surfaces, like quartz or a diamond? Or is it a complex shape, like a snowflake with six arms and intricate side branches? Although we simply call them crystals, why do these differences in shape occur?

This diversity in crystal shapes is observed during growth and is an example of the general problem of what state is selected under non-equilibrium conditions. In our laboratory, we are trying to theoretically understand these problems of state selection under non-equilibrium conditions. We also use computer simulations to examine the validity of our theories.

For example, in simulations of crystal growth in a solution, changing the growth conditions can produce a variety of morphologies, as shown in Figure 1. These correspond to the various shapes of actual crystals.

Figure 1: Various shapes of crystals formed by the aggregation of atoms (white) diffusing in a solution. Clockwise from top right: polyhedral crystal, skeletal crystal, dendritic crystal, and fractal crystal. The colors represent the passage of time.

Understanding the reasons for morphology determination in this way could be useful for controlling shapes. For example, our current information society is supported by advanced microfabrication technology on semiconductor surfaces, but it is said that we are approaching the limits of what can be achieved with external fine processing. Instead, basic research is underway on the self-organization of crystal surface morphologies, exploring whether atoms themselves can spontaneously create surface microstructures (Figure 2).

Figure 2: (b) Instability of steps on a crystal surface. (a) Meandering of a single step and (b) bunching of many steps.

Furthermore, some crystals have two types of structures, right-handed and left-handed, even though their constituent atoms or molecules are highly symmetrical. The shapes of these two types of crystals are mirror images of each other; quartz is one such example. This kind of left-right symmetry, or chirality, is also found in the organic matter that makes up life (Figure 3). However, for some reason, in terrestrial life, only left-handed amino acids and right-handed sugars exist. Understanding how to selectively create right-handed and left-handed crystals may be linked to the origin of life on Earth.