All the crystal forms of cubic system can be referred to three axes of equal lengths which intersect at right angles. These axes are interchangeable and of equal lengths therefore they are denoted by a1, a2 and a3.The a1 axis runs from front to back, a2 runs from left to right and a3 runs from top to bottom. The values of angle alpha, beta and gamma is 90 degree.
The symetry classes which belong to this system are :
Galena Type or Hexaoctahedral Class
Pyrite Type or Diploidal Class
Tetrahedrite Type or Hexatetrahedral Class
This class takes it's name from the common mineral galena (PbS), which belongs to it. It is also called as the Normal Type of cubic system since it shows the highest degree of symmetry of this system.
The symmetry of this class can be deduced from the typical form cube.
Planes 9 - 3 Axial and 6 Diagonal.
Axes 13 - 3of four fold, 4 of three fold and 6 of two fold.
A centre of symmetry.
1) Cube - Cube is a six faced solid. Each face is a square and cuts one crystallographic axis and is parallel to other two. The Miller symbol of the form is (100).
2) Rhombdodecahedron - It's a solid having twelve rhomb shaped faces. Each face cuts two crystallographic axes at equal distance and being parallel to the third. It's Miller symbol is (101).
3) Octahedron - It's a solid having eight equilateral triangular faces, each face cuts all the three axes at equal distance. It's Miller symbol is (111).
4) Tetrahexahedron - It's a solid having 24 faces , each face is an isosceles triangle. Each face is parallel to one axis and cuts other two at different lengths. It's Miller symbol is (210).
5) Trisoctahedron - It's a solid having 24 faces, each face an isosceles triangle. Each face cuts two axes at an equal length, and the third at a greater length. It's Miller symbol is (221).
6) Trapezohedron - It is also called icositetrahedron. It has 24 faces, each face a trapezoid. Each face cuts two axes at an equal length and the third at a smaller distance. It's Miller symbol is (211).
7) Hexaoctahedron - It's asolid having 48 faces each face a scalene triangle. Each face cuts the three axis at unequal lengths. It's Miller symbol is (321).
Galena - Combination of Cube and Octahedron
Fluorspar - Cubes
Leucite, Analcite - Trapezohedra
Spinel, Magnetite - Simple Octahedra, somtimes combined with rhombdodecahedron or trapezohedra.
Garnet - Rhombdodecahedron.
Diamond - Octahedral crystals.