CRYSTALLOGRAPHY PART 7 (Crystal forms 1)

CRYSTAL FORMS

A crystal form refers to a set of crystal faces which have the same symmetry elements (or crystal faces that are related by symmetry).

Simple crystals may have only one crystal form but more complex and complicated crystals may have a combination of several different forms.

a)    Some common forms in the Non – Isometric System

                            i.            Pinacoids (or Parallelohedrons)

It consists of 2 parallel faces which intersects one crystallographic axis and are parallel to the others

-         { I 0 0 } is the form symbol for a Front Pinacoid

-         { 0 I 0 } is the form symbol for a side Pinacoid

-         { 0 0 I } is the form symbol for a basal Pinacoid

Figure 22:  Pinacoids (front, side and basal)

 

                          ii.            Prisms:

It is a crystal form composed of 3, 4, 6, 8 or 12 faces, all of which are parallel to the c crystallographic axis and intersect one or two other horizontal axes.  Examples of prisms include Trigonal Prisms, Tetragonal prisms, Rhombic prisms and hexagonal prisms.  Some prisms are called Prisms of the 1^st order while others are called Prisms of the 2^nd order.

-         Prisms of the 1^st order are those whose faces intersect two horizontal axes and are parallel to the rest.

-         Prisms of the 2^nd order are those whose faces intersect one horizontal axis and are parallel to the rest.

 

 

Figure 23:  Tetragonal and Hexagonal Prisms (showing the difference between 1st and 2nd order prisms).