Dale Mace, of Erwin Elementary School, North Carolina, USA, asks :-

Which grows faster - salt or sugar crystals?

John Campbell, a retired physicist at the University of Canterbury and who has grown crystals as diverse as quartz and ruby, responded.

About the same if the usual way of growing crystals in primary schools is used.

What distinguishes a crystal from other forms of matter is that in a crystal the atoms or ions form into very regular arrays such that one type of ion or atom will have the same pattern of ions or atoms around it. For example, common salt is NaCl and consists of a cubic array of chlorine ions (Cl-) with a sodium ion (Na+) at the centre of every cube of chlorine ions.

These atomic patterns are determined by which has the lowest energy for the particular ions or atoms involved. If the crystal is allowed to grow freely and slowly then the shape of the actual crystal indicates the atomic arangement. For example, common salt crystals, if you look at them with a magnifying glass, are worn little cubes; fluorite (CaF2), a different type of cubic array, forms opposing pyramids with a square base; and crystals involving only carbon can form diamond, which is a cubic structure, graphite, which because of its flate plate structure is used as a solid lubricant, or fullerenes, which are nano-particles of carbon in the form of individual hollow spheres (C60 or buckyballs) or hollow icosohedral (C540) or even hollow tubes.

To form a crystal, time must be allowed for the atoms or ions to find the crystal surface and to jiggle their way into the regular array. So most crystal growing isn't fast. Salt crystals can be grown at home or school by dissolving as much salt as possible in water, pouring the filtered solution into a glass, and suspending into it a length of cotton or string to provide nucleation centres. As the liquid slowly evaporates over a few days the excess salt is precipitated out onto the string.

This process can be sped up by a way I used to demonstrate in undergraduate laboratories. Melt NaCl in a platinum crucible, keep its temperature constant in a cylindrical electric heater, lower a previously grown seed crystal to touch the surface, then slowly pull the seed upwards whilst slowly rotating it to mix the molten salt. Sodium chloride crystals about 10 cm long and about 3 cm wide could be grown in a few hours.

The fastest crystal growth I have ever seen was when sodium thyosulphate was melted in a very clean spherical flask and allowed to cool below its melting temperature. Energetically the solution wants to form a solid but being very pure there is nothing to start the nucleation, ie there is no dust or rough bits inside the flask. When a small crystal of sodium thyosulphate was dropped into the flask there was a loud click as the whole solution turned into one (rough) crystal.