Undoubtedly, at some point in your life (perhaps as part of chemistry classes in elementary or high school) some of you tried to make a nice single crystal using the method of crystallization from a saturated solution. You probably tried to grow a nice, big crystal of bluestone (copper sulfate pentahydrate), table salt (sodium chloride) or maybe even citric acid, and some probably tried their hand at crystallizing ordinary table sugar (sucrose).
Those who used blue stone and citric acid for the experiment certainly succeeded in growing beautiful, regular and relatively large crystals, with clearly defined edges and planes, while those who tried with table salt probably did not get such beautiful crystals, but tiny cubes stuck to each other.
Those beautiful, regular crystals that look like some kind of a regular geometric body we call single crystals. Thus, the single crystal of sodium chloride looks like a cube, the single crystal of chromium and aluminium alums like an octahedron, and the single crystal of nickel sulfate heptahydrate is very similar to a cuboid. They really grow like regular geometric bodies – such external shape is a consequence of their regular internal structure, that is, the way in which the molecules in a certain crystal are arranged.
Twinned crystals are nothing else but a cluster of such single crystals that found themselves to be too close to each other during growing, so they grew into one another, or one started to grow on top of the other. Although such crystals are unsuitable for some applications (e.g. single crystals of silicon are needed to make electronic elements), they are no less beautiful, which some of us find to be quite sufficient…
Author of the text and photos: Alen Bjelopetrović, Dr.Sc.