CLASSIFICATION OF CRYSTALS 8 (The Triclinic system)

7)    The Triclinic System

Figure 36: The crystallographic axes of the triclinic system

 

This system has the following characteristics:

-         3 unequal crystallographic axes all at oblique angles.

 That is;

-         a ≠  b ≠ c.

-         Its axial angles are such that α ≠ γ ≠ β ≠ 90^o

-         The diagnostic property of this system is the lack of axes and planes of symmetry.

The Triclinic system has 2 classes as follows:

a)     The Asymmetric (Triclinic pedial) class

b)    The Triclinic holosymmetric (Triclinic pinacoidal) class, which is the holosymmetric class with the following properties:

-         A center of symmetry

-         No planes of symmetry

-         No axes of symmetry

-         Examples include: plagioclase feldspars and kyanite.

CLASSIFICATION OF CRYSTALS 6 (The Orthorhombic system)

5)    The Orthorhombic System

Figure 34: The crystallographic axes of the orthogonal system

-         This system has 3 perpendicular crystallographic axes with unequal lengths.  That is

-         a ≠  b ≠ c.

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

-         The unit form for this system is a straight prism with a rectangular base.

-         The diagnostic property of the orthorhombic system is 3ⁱⁱ along the c crystallographic axis.

The orthorhombic system has 3 classes as follows:

a)     The orthorhombic holosymmetric (orthorhombic bipyramidal) class

b)    The orthorhombic hemimorphic (orthorhombic pyramidal) class

c)     The orthorhombic sphenoidal class.

The holohedral class of the system is the orthorhombic holosymmetric (orthorhombic bipyramidal) class with the following elements of symmetry:

-         A center of symmetry

-         3 planes of symmetry

-         3 axes of symmetry; 3ⁱⁱ.

-         Examples include: barite, amphiboles, andalucite, sillimanite, olivine group.

Forms in the Orthorhombic system

1.     Pinacoids

2.     Prisms

3.     Bipyramids