Crystal Forms

There are three elements of crystal forms. The face, edge and solid angle.

• Faces – Faces are the external expression of the internal atomic structure. Faces can be ‘like’ if they have same properties and ‘unlike’ if they have different properties.
• Edges – The line created by the intersection of two adjacent faces is termed as edge.
• Solid angles – Solid angle is a point created by the intersection of three or more than three planes.

Forms – It is a group of faces which have a like positions with respect to the crystallographic orientations. Forms may be classified in the following three ways –

1st way	2nd way	3rd way
Simple form – when a crystal is made up of all alike faces the form is called simple form. EX- Cube, Octahedron etc.	Open form – These are the form which doesn’t enclose the space all by themselves. EX- pinacoid, prisms etc.	General form – It is one in which the indices are unrestricted in magnitudes.
Combination form – when a crystal is made up of two or more simple form, it is called combination form EX- combination of prisms and pinacoid.	Closed form – These are the form which can enclose the space all by themselves. EX- pyramid etc.	Special form – It is one in which there is only one possible value of indices. EX- octahedron (111)
		Restricted form – when the forms are neither special nor general but some part of their indices are fixed and some part is variable, the resulted form is called restricted form. EX- Prisms (hkl) [l is always zero], trisoctahedron of cubic system [h must always equal to k]

besides above forms can also be classified in four important types –

1. Holohedral forms – These forms exhibits the highest degree of symmetry possible in a system.
2. Hemihedral forms – These forms show half the number of faces required for the full symmetry of the system. EX- tetrahedron is the hemihedralform of octahedron.
3. Hemi-morphic forms – This form have unlike faces at the different ends of the axis of symmetry. This axis is called polar axes.
   • Thus half of the faces of holohedral forms are grouped at the one end of the axis and none at the other end.
   • This forms lack centre of symmetry.
4. Tetartohedral forms – They show only a quarter of the number of faces of the corresponding holohedral form.
   • These forms have neither plane nor centre of symmetry.
5. Enantiomorphic forms – These forms occur in two positions which are mirror image to each other.
   • They can not be converted into each other by ay means of rotation.
   • They do not have either plane or centre of symmetry.

Common crystal forms

• Pedion – it has only one face
• Pinacoid – it is an open form, consisting of two faces which cuts one crystallographic axis and parallel ot the other two axis.
• Prisms – it is an open form, consisting of four faces which are parallel to the vertical axis and cuts one or more horizontal axis at equal or unequal lengths
• Pyramids – it is an closed form, consisting of eight faces which always cut the vertical axis and one or more horizontal axis at equal or unequal lengths
• Domes – it is an open form but intermediate between the prisms and pyramids, as it cuts vertical axis and one horizontal axis. These are also called as horizontal prisms.
• Dimetral prisms – it is formed by the combination of three pinacoid which together becomes closed form. They only occurs in the orthorhombic, monoclinic and triclinic systems.