Introduction to Crystal Systems and Symmetry

Crystallography is the science of crystal structure and symmetry. All crystalline minerals belong to one of seven crystal systems, defined by the lengths and angular relationships of their crystallographic axes.

The cubic (isometric) system has the highest symmetry, with three equal axes at 90 degrees. It includes many familiar minerals: diamond, garnet, pyrite, fluorite, halite, and magnetite. Common crystal forms are the cube, octahedron, and dodecahedron.

The hexagonal system has four axes: three equal horizontal axes at 120 degrees and one vertical axis. Beryl and apatite are hexagonal minerals. The trigonal system is sometimes classified as a division of hexagonal, distinguished by three-fold symmetry. Quartz, calcite, tourmaline, and corundum are trigonal.

The tetragonal system has three axes at 90 degrees, with two equal (a = b) and one different (c). Zircon, rutile, and cassiterite are tetragonal.

The orthorhombic system has three unequal axes all at 90 degrees. Olivine, topaz, and barite are orthorhombic.

The monoclinic system has three unequal axes, two at 90 degrees and one inclined. This is the most common crystal system for minerals. Gypsum, orthoclase, and epidote are monoclinic.

The triclinic system has the lowest symmetry: three unequal axes at unequal angles. Plagioclase feldspars, kyanite, and turquoise are triclinic.