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Geosciences LibreTexts

5.2: Crystal Systems

  • Page ID
    3194
  •  Table \(\PageIndex{1}\): Table of Crystal Systems

    Crystal System Diagram Examples

    Cubic 

    The cube is composed of 6 square faces
    at 90° angles to each other. Each
    face intersects one of the crystallographic
    axes and is parallel to the other two.

    Cubic.jpg

    Diamond 

    Garnet 
    Spinel

    Tetragonal 

    The tetragonal system also has three axes
    that all meet at 90°. It differs from the
    isometric system in that the C axis is longer or
    shorter than the A axes,
    which are the same length.

    Tetragonal.jpg Zircon

    Hexagonal

    In the hexagonal system, we have an additional axis,
    giving the crystals six sides. Three of these
    are equal in length and meet at 120° to each other. 
    The C or vertical axis is at 90° to the horizontal axes.
    Mineralogists sometimes divide this into two systems, 
    the hexagonal and the trigonal, based on their external
    appearance (see following).

    Hexagonal.jpg

    Aquamarine

    Beryl 
    Emerald
    Heliodor
    Morganite
    Apatite

    Trigonal

    The trigonal system is a subsystem of the 
    hexagonal system according to some institutes,
    therefore some gem references will list these as
    hexagonal.

    Trigonal.jpg

    Amethyst

    Benitoite
    Citrine
    Corundum
    Quartz 
    Ruby 
    Sapphire 
    Tourmaline 

    Orthorhombic

    In this system, there are three axes all of which
    meet at 90° to each other. However, all the axes
    are of different length.

    Orthorhombic.jpg

    Alexandrite

    Andalusite
    Chrysoberyl
    Iolite
    Peridot
    Tanzanite 
    Topaz
    Varisite

    Monoclinic

    The above crystal systems all have axes sides
    that meet at 90°. In the monoclinic system, all
    the axes are different lengths. The A axis is inclined
    to the C axis. The B axis is at 90° to them. 

    You can imagine this as a matchbox that slants to one side.

    Monoclinic.jpg

    Azurite

    Diopside 
    Feldspar
    Kunzite 
    Nephrite 

    Triclinic

    In this system all the axes are different 
    lengths and none of them meet at 90°.

    You can imagine this as a matchbox which slants to two sides.

    Triclinic.jpg Turquoise