Tag: crystals

Crystals, Non-Crystals and Quasicrystals

Quasicrystalline ordering of a aluminum-palladium-manganese alloy. Image by J.W.Evans via Wikipedia.
Regular ordering of a halite crystal. The atoms that make up salt crystals are arranged in a cubic shape. The smaller grey atoms are sodium (Na), and the larger green ones are chlorine (Cl).

Daniel Shechtman was just awarded the Nobel Prize in Chemistry (2011). He discovered that matter can exist not only as crystals, which have a regular geometric arrangement of atoms, and amorphous non-crystals that do not, but also as quasicrystals which have a different type of atomic ordering.

The Guardian has an interesting article on Schechtman, whose discovery was roundly disbelieved by other scientists and he was ridiculed for years.

In an interview this year with the Israeli newspaper, Haaretz, Shechtman said: “People just laughed at me.” He recalled how Linus Pauling, a colossus of science and a double Nobel laureate, mounted a frightening “crusade” against him. After telling Shechtman to go back and read a crystallography textbook, the head of his research group asked him to leave for “bringing disgrace” on the team. “I felt rejected,” Shachtman said.

— Sample (2011): Nobel Prize in Chemistry for dogged work on ‘impossible’ quasicrystals in The Guardian

Hat tip to M. Eisenberg for this link.

Salt and Sugar Under the Microscope

Sugar crystals under 40x magnification.

Salt and sugar crystals have wonderfully distinctive crystal forms. They might well be good subjects for introducing minerals, crystals and some of the more complex geometric solids.

Cube shaped salt crystals under 40x magnification.

The salt crystals are clearly cubic, even though some of the grains seem to be made up of overlapping cubes.

The atoms that make up salt's atomic lattice are arranged in a cubic shape, which results in the shape of the salt crystals. The smaller grey atoms are sodium (Na), and the larger green ones are chlorine (Cl).

Salt is an ionic compound, made of sodium and chloride atoms (NaCl). When a number of these molecules get together to form a crystal, they tend to arrange themselves in a cubic pattern. As a result, the salt crystals are also cubic. In fact, if you break a salt crystal, it will tend to break along the planes that are at the surfaces of the planes of the atomic lattice to create a nice, shiny crystal faces. Gem cutters use this fact to great effect when they shape diamonds and other precious stones.

Of course different crystals have different atomic arrangements. The difference is clear when you compare salt to sugar.

A single sugar crystal looks a bit like a fallen column.

Sugar crystals look a bit like hexagonal pillars that have fallen over. According to the Beet-sugar handbook (Asadi, 2007), sugar crystals actually have a monoclinic form, which could end up as asymmetric hexagonal pillars. Salt crystals, on the other hand, have the habit of forming cubes.