WELL-FORMED CRYSTALS are objects of great beauty and extreme rarity. Conditions have to be just right for them to grow and any later changes in conditions must act to protect rather than destroy them. Even if they do grow and survive, many are destroyed by people during mining and other activities. Survivors are therefore of great interest. The crystals shown are about 60 per cent their real size.
A WELL FORMED CRYSTAL has certain regular (symmetrical) features. One feature is that sets of faces have parallel edges. Another feature of many crystals is that for every face, there is a parallel face on the opposite side. Crystals may have three types of symmetry. If a crystal can be divided into two, so that each half is a mirror image of the other, the line that divides them is called a "plane of symmetry". If a crystal is rotated about an imaginary straight line and the same pattern of faces appears a number of times in one turn, then the line is an "axis of symmetry". Depending on how many times the pattern appears, symmetry about an axis is described as twofold, threefold, fourfold, or sixfold. If a crystal is entirely bounded by pairs of parallel faces then it has a "centre of symmetry".
Crystals of the same mineral may not look alike. The same faces on two crystals may be different sizes and therefore form different-shaped crystals. Crystals may also grow with a variation of "form". Shown here are three forms found in the cubic crystal system, illustrated by pyrite.
THE INTERNAL ATOMIC STRUCTURE of crystals determines their regular shape and other properties. Each atom has its own special position and is tied to others by bonding forces. The atoms of a particular mineral always group in the same way to form crystals of that mineral.
In early crystallography, the study of crystals, one of the most significant deductions was made by the Abbe Hauy in 1784. In 1808, Dalton defined his theory that matter was built up from tiny particles called atoms. In 1895, Rontgen discovered X-rays and, in 1912, von Laue realized that these might help determine the arrangement of atoms within a solid. This was the start of our understanding of the inside of crystals.
Some crystals split along well-defined planes called cleavage planes which are characteristic for all specimens of that species. They form along the weakest plane in the structure and are direct evidence of the orderly arrangement of atoms.