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Crystal Symmetry
The external shape of the crystal is a manifestation and honest expression
of how its internal arrangement, and that the atoms that make up the
crystal have a special arrangement regular, and that arrangement is
repeated units in the space to form a single crystal. By studying the
position of the faces, the distribution of crystal edges and solid angles at
the center of the crystal, it is possible to reach some symmetrical
properties. This organized arrangement in the crystal is known as the
phenomenon of crystal symmetry.
The degree of symmetry varies from one crystal to another, but it is fixed
and does not change for the crystal of a single mineral, and it is one of its
distinctive properties. The symmetry generally has three elements known
as the elements of symmetry, which are

1- Plane of sym
2- Axis of sym
3- Center of sym

These elements may be found together in one crystal, or some of them
are found only in crystals of less symmetric minerals, and sometimes
there is no symmetry at all, as is the case in crystals of some chemical
compounds, such as the crystal of aqueous calcium thiosulfate.
1- Plane of Symmetry:
The plane of symmetry divides the crystal into two equal and similar
parts, provided that one of the two halves is the mirror image of the other
half. It is noted that any solid face, edge or angle on one side of the plane
corresponds to a face, edge or angle on the other side, and a single crystal