Crystal habits

I am fascinated by crystals, particularly by their regularity. They basically consist of a unit cell that repeats over and over again, identically, across the whole span of the crystal. And if the crystal is big enough to see with the naked eye, that's a very large number of unit cells.

Very simple crystals, for example table salt (NaCl), have a tiny unit cell consisting of 4 atoms of Na and 4 atoms of Cl, arranged in alternating rows, a structure that is called face-centred cubic, because on each face of the cube, there's an atom in the centre of the same type as the atoms on the corners of the unit cell.

An interesting quirk of the simple 1:1 ratio cubic crystal structure is that you can define either Na or Cl as the corners of your unit cell, and it'll still be face-centred cubic.

Diamond's structure is also simple: every carbon atom has four links to four other carbon atoms, arranged in a tetrahedral shape around it. Because unit cells are defined as cubic or rectangular shapes, however, the diamond unit cell is less simple, even if the structure itself is simple.

In both cases, if you copy and paste the unit cell and line up the atoms on the corners, overlapping them if they are displayed as full atoms (the edge of the unit cell "cuts" the atom in half, quarter, or eighth depending on where it's positioned, so the edge atoms are only partly in a given unit cell) the structure repeats and builds in a regular way. You could also build a model of it using physical balls and sticks.

Sometimes, though, the same ingredients can make up some very different crystals. Take, for example, epsom salt (bath salt): magnesium sulphate, MgSO4.

Both of those are magnesium sulphate, but the second has 7 molecules of water for every atom of magnesium built right into the crystal structure.

And even these are still in the realm of simple crystal structures. Only 27 atoms in the unit cell, after all! There are biological molecules which have hundreds of atoms in a single molecule, and I don't know how many molecules make up a crystal unit cell.

Another ridiculously cool thing about crystals is the way they grow. When they're in the right conditions to grow at a slow and steady pace, what happens, more or less, is that one atom or component molecule at a time fits into its place, and by taking its proper place makes the next spot configured to take its particular atom. While it's doing that, it very efficiently excludes anything that is not part of the crystal structure, leading to extremely pure crystals. Things that aren't part of the pure crystal structure generally don't "fit".

And in mentioning "the right conditions", I finally get through my meandering introduction to crystals and to the little bit of news that made me want to post on this subject: some software that can predict the growth of crystals in various conditions and which will be able to help a chemical engineer like me figure out how to make a crystalline product chemical grow the way I want it to, consistently and controllably, from lab to full production scale.

The university's press release mentions predicting crystallization of nuclear waste to make processing easier, and it could also be used to figure out things a little closer to home, such as how to make better crystal structures on the inside of a battery, something which has come up in the last few years as being very important to their proper functioning. I know I'm going to keep my eye out for the feature set when this program goes live—and I may be asking my boss to buy me a copy.

(All crystal images are screenshots from mercury, a crystal structure visualization program created by the fine folks at the Cambridge Crystallographic Data Centre, and the crystal structures themselves were downloaded from the American Mineralogist Crystal Structure Database. Thanks for letting me have fun making pretty pictures with the fruits of your hard work.)

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