CRYSTAL SYSTEMS AND SYMMETRY CLASSES
It can be proved mathematically that among the innumerable possible crystal shapes there are only 32 distinct types of symmetry, known as symmetry classes. These fall into six major groups called crystal systems, the cubic, tetragonal, hexagonal (with the trigonal subsystem), orthorhombic, monoclinic and triclinic systems. The most common minerals of the earth’s crust, the feldspars, crystallise in the monoclinic and triclinic systems.