Crystal (mineral)

B

Tetragonal

This system comprises crystals with three axes, all perpendicular to one another; two are of equal length.

C

Orthorhombic

This system comprises crystals with three mutually perpendicular axes, all of different lengths.

D

Monoclinic

This system comprises crystals with three axes of unequal lengths, two of which are oblique (that is, not perpendicular) to one another, but both of which are perpendicular to the third.

E

Triclinic

This system comprises crystals with three axes, all unequal in length and oblique to one another.

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F

Hexagonal

This system comprises crystals with four axes. Three of these axes are in a single plane, symmetrically spaced, and of equal length. The fourth axis is perpendicular to the other three. Some crystallographers split the hexagonal system in two, calling the seventh system thus formed trigonal or rhombohedral.

A few elements and compounds can crystallize in two different systems, giving rise to substances which, although identical in chemical composition, are different in virtually all their physical properties. For example, carbon crystallizes in the isometric system to form diamond, and in the hexagonal system to form graphite. Although diamond is in the same system as salt and garnet, it is in a different class. it crystallizes in tetrahedrons (solids with 4 faces) or octahedrons (solids with 8 faces); the latter is possible in the garnet-salt class, the former is not.

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