CRYSTALLOGRAPHY PART 7 (Crystal forms 2)

                            i.            Dome (or Dihedron):

It is an open crystal form consisting of 2 non – parallel faces.  Each face cuts the c crystallographic axis and one other horizontal crystallographic axis and is parallel to the other. { I 0 I } or { 0I I }.

Figure 24:  A dome

 

                          ii.            Pyramid:

It is an open form composed of 3, 4, 6, 8 or 12 non – parallel faces that meet at a common point. Each face intersects all three crystallographic axes.  The Tetragonal, Trigonal, Orthorhombic, and Hexagonal systems all produce pyramids. 

Figure 25:  A Pyramid

 

                       iii.            Bipyramid:  It is an open form with 6, 8, 12, 16 or 24 triangular faces that intersect all 3 crystallographic axes. Just like the pyramid, there are also  Tetragonal, Trigonal, Orthorhombic, and Hexagonal bipyramids.

 

 

 Figure 26:  Bipyramids

                       iv.            Scalenohedrons:  It is a closed form with 8 or 12 faces grouped in symmetrical pairs.  There are Tetragonal Scalenohedrons (with 8 faces) and Hexagonal scalenohedrons with (12 faces).  Scalenohedrons are built up of [1]scalene triangles.

 

Figure 27:  A scalenohedron

 

                          v.            Rhombohedrons:

It is a closed form composed of 6 rhomb shaped faces in which each face intersects 3 crystallographic axes and is parallel to the fourth.

 

 

 Figure 28:  A Rhombohedron

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[1]A scalene triangle is one whose sides have unequal lengths.