Aimee Stoddart, of Stirling School, asks :-

Why do minerals have the shapes they do?

Alan Cooper, a mineralogist at the University of Otago, responded.

The shape of a mineral is a reflection of how the atoms (which are the smallest building blocks of all substances) are arranged in a regular array which is specific to each type of mineral.

The way in which differently sized atoms are arranged in minerals can be likened to how you would fit tennis and soccer balls together. Clearly the tennis balls would fit neatly in the holes between the larger soccer balls. So also in minerals, one atom occupies the gap between others. Neighbouring atoms in minerals are often arranged in regular patterns, with three atoms occupying the corners of a triangle, or four at the corners of a square, or six at the corners of a hexagon. As more atoms are transported to the site of growth these crystals grow larger, perhaps elongated, and show triangular, cubic, or hexagonal form. Mineralogists use the regular shapes of minerals as a means of identifying them.

If the mineral grows, or crystallises, on the edge of an open space it is able to build a regular crystal. On the other hand, if the mineral is growing with lots of others, its external shape may well be irregular, with all the crystals small and squashed together.