Crystals, Crystals

I used to be an X-ray crystallographer. So when I saw pictures of crystals in New York Times, I paid attention. It was an article about table salt (sodium chloride), which usually forms the most boring crystals (in my opinion): cubic crystals. What was usually about this substance was that now people can make table salt crystallize in a rhombic dodecahedron form. To your left is a picture of such crystals.

My interest in crystals went way back when I was in junior high. I read my father's college textbook on rocks and crystals in my spare time and learned there are seven crystal forms. I played with our own rock crystal: beautiful, perfectly hexagonal quartz. I was so impressed that nature could produce things so beautiful and symmetrical. Who would have known then I was going to grow crystals out of proteins and nucleic acids years later!

Even though table salt crystals are very boring, I still learned about some of its properties today. I always thought that the reason salt grains sticking to each other (caking) in humid weather was because there was impurity such as calcium salt. In fact, caking is believed to occur as a result of formation of solid intercrystalline bridges. A cubic crystal provides large intercrystalline surface area of contacts, thus the stickier it will be when caking happens. So the way to solve the caking problem is to produce a non-cubic table salt crystal.

To achieve that goal, scientists in India have used glycine as a crystal habit modifier in saturated sodium chloride solution. The result, "round" crystals!

Crystallization seems to be an art rather than science. There are so many tricks that you can try to coerce your favorite molecules to form crystals, but there are way more conditions that will lead you to nowhere. It is amazing that an amino acid can dramatically change the behavior of NaCl in crystals. God knows how many different compounds people have tried to altogether. At least here, they've found a use for NaCl in this new crystal form: easily dispensed salt from a saltshaker.

To learn more about crystals, click here and here.