Crystal Systems Continued

Cubic System

Cubic Crystals have three equal axis lengths and three angles all at 90º. a=b=c and alpha = beta = gamma = 90º. These crystals may look like cubes, pyramids, or be almost round (like many garnets). The symmetry is very high and the system supports many many possible shapes. The cubic system is also called isometric.

cubic crystals include, fluorite, pyrite, analcime, tetrahedrite, garnet, and galena.

Trigonal System

Trigonal crystals have three equal axis, a1=a2=a3, the angles must be less than 120º but must not be 90º. alpha1=alpha2=alpha3=60º beta 90º. The trigonal axis has a three fold axis of symmetry, but is not hexgonal (described next.) Trigonal crystals maybe described by a hexagonal unit cell, but they lack hexad rotation. They are sometimes called rhombohedral.

trigonal minerals include: calcite, sidereite, dolomite, and rhodochrosite.

Hexagonal System

Hexagonal crystals have a1 = a2 a3 (two equal axis), beta = 90º, and alpha1 = aplpha2 = alpha3 = 60º. They have a six fold axis of symmetry. Cross sections are often perfect hexagons and fairly easy to spot in the crystal world.

heagonal minerals include: quartz, vanadinite, apatite, beryl, and mimetite.

For a more in-depth discussion (Written by Mike Howard - Illustrated by Darcy Howard) on crystal symmetry and crystal groups you can visit: Rockhounds.com


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