# Form

## Crystal system and habit

The habit (from the Latin: habitus, external form) of a crystal is determined by the seven crystal systems to which it belongs (see [BRA 66]).

The occurrence frequency of each system in nature is given in Table 1.1.

 Monoclinic 29 Orthorhombic 21 Cubic 15 Triclinic 11 Trigonal 10 Hexagonal 7 Quadratic 4 Total 100

Table 1.1. Bravais' crystalline systems

Metals typically crystallize in an isotropic system (that is, the cubic system).

As far as filtration or drying is concerned, crystals can occur in one of the following forms:

• - tablet;
• - prism;
• - acicular (fibers, needles, rods);
• - pyramid;
• - rhombohedron;
• - tetrahedron;
• - hexahedron (cubes);
• - octahedron;
• - dodecahedron;
• - scale and plate.

Table 1.2 gives the occurrence frequency of these polyhedra according to the system to which the crystal belongs.

The total for each column is equal to 100.

Attrition is often active in crystallizers. Crystal tops and edges are dulled and their form can approach that of river pebbles.

 Monoclinic Orthorhombic Cubic Triclinic Trigonal Hexagonal Quadratic Tablet 42 31 43 30 37 32 Prism 36 46 29 26 42 29 Acicular 15 19 12 15 11 Pyramids 4 5 10 24 Rhombohedron 9 29 Tetrahedron 10 Hexahedron 31 Octahedron 38 Dodecahedron 21 Scale 7 2 15

Table 1.2. Practical shapes of crystalline systems