CLASSIFICATION OF CRYSTALS 4 (The Hexagonal system)

1)    The Hexagonal System

Figure 32: The crystallographic axes of the hexagonal system

-         The crystals of this system are referred to by 4 crystallographic axes;

-         3 horizontal axes of equal lengths at 120^o to each other labeled a₁ a₂ a₃ and a fourth vertical axis labeled c which is perpendicular to the horizontal axes.

NB: the a₃ crystallographic axis is positive behind the origin and negative in front of the origin.

-         Its axial angles are such that α = β = 90^o and γ = 120^o.

-         The unit structure is a prism with a hexagonal base.

-         The diagnostic property of this system is a unique 1^vi axis on the c crystallographic axis.

The Hexagonal system has 7 classes as follows:

a)     The Hexagonal holosymmetric (Dihexagonal bipyramidal) class or the Beryl type.

b)    The Hexagonal hemimorphic (Hexagonal pyramidal) class

c)     The Trigonal Bipyramidal class

d)    The Hexagonal Bipyramidal class

e)     The Dihexagonal hemimorphic (Dihexagonal pyramidal) class

f)      The Ditrigonal Bipyramidal class

g)     The Hexagonal Trapezohedral class.

The holohedral class of the system is the Hexagonal holosymmetric (Dihexagonal bipyramidal) class or the Beryl type with the following elements of symmetry:

-         A center of symmetry

-         7 planes of symmetry; 4 axial and 3 diagonal

-         7 axes of symmetry; 1^vi and 6ⁱⁱ

-         An example is Beryl (BeAl₂Si₆O₁₈)

Forms in the Hexagonal system

1.     Basal pinacoids, {0 0 0 I }

2.     Hexagonal Prisms of 1^st and 2^nd order, {I 0 I 0 } and {I I 2 0 }

3.     A dihexagonal Prism made up of 12 faces.

4.     Hexagonal bipyramids of 1^st and 2^nd order.