Symmetry and crystal classes

The shape and symmetry of crystals attracted the attention of early crystallographers and, until the internal structure of crystals could be determined, was an important method of classification of minerals. The external shape, or habit, of a crystal is described as isometric (Uke a cube), prismatic (like a prism, often with six sides), tabular (like a rectangular tablet or thick plate), lathy (lath-like) or acicular (needle-Uke). An examination of the disposition of crystal faces, which reflected the symmetry of the crystal, led to an appreciation that all crystals could not only be allocated to one of the seven crystal systems but also to one of 32 crystal classes. 

The crystal class mirrors the internal symmetry of the crystal. The internal symmetry of any isolated object, including a crystal, can be described by a combination of axes of rotation and mirror planes. 

The symmetry of the internal structure of a crystal is obtained by combining the point group symmetry with the symmetry of the lattice. It is found that 230 different patterns arise. These are called space groups. Every crystal structure can be assigned to 

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