GEOLOGY 101:

Crystal Systems ❖

A crystal system is a method of classifying crystalline substances into 7 crystallographic systems (sometimes 6 if not including trigonal, which can be grouped in with hexagonal.) A crystal can be defined based on elements of symmetry, often called symmetry operations. These include centre of symmetry, axis of symmetry, plane of symmetry, axis of rotatory inversion, screw-axis of symmetry, & glide-plane of symmetry.

The 7 crystals systems include: isometric, trigonal, hexagonal, tetragonal, orthorhombic, monoclinic, & triclinic. Some may also mention amorphous (defined as lacking a crystal structure) when talking about crystal systems.

  • Isometric (Cubic) System

    The first system is the isometric or cubic system. It has three axes, all of which are the same length. The three axes in the isometric system all intersect at 90º to each other. Minerals of this system tend to produce crystals of equidimensional or equant habit. Isometric crystal forms include:
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  • Tetragonal System

    Like the cubic system, the tetragonal system has three axes that all meet at 90 degrees. However, it differs from the isometric system in that the C axis is longer (or shorter) than the A & B axis, which are the same length. Because of this, it has a rectangular inner structure. Tetragonal crystal forms include:
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  • Hexagonal System

    These crystals have a hexagonal, 6-sided form. They have four axes, three are equal in length & lie at an angle of 120° from each other. The fourth is either longer or shorter but must be at a right angle toward the other corners. Crystals in the hexagonal crystal system are among the easiest to recognize. Their cross section, viewed from the termination, is usually hexagonal. Hexagonal crystal forms include:
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  • Trigonal System

    The trigonal crystal system is often considered a subdivision of the hexagonal crystal system. They share many of the same features, however trigonal minerals have threefold symmetry, whereas true hexagonal minerals have sixfold symmetry. Trigonal crystal forms include:
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  • Orthorhombic System

    In this system there are three axes, all of which meet at 90º to each other. However, all the axes are different lengths. Crystals of this system tend to be of prismatic, tabular, or acicular habit. It is the second largest crystal system, after Monoclinic. Orthorhombic crystal forms include:
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  • Monoclinic System

    In the monoclinic system all the axes are different lengths. Two of them, the A & the C axes, meet at 90 degrees but the third one does not. The monoclinic system is the largest crystal system, holding nearly 1,400 different mineral species or approximately 27% of all known minerals. Monoclinic crystal forms include:
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  • Triclinic System

    The final & least symmetrical crystal system is known as triclinic. In the triclinic system, all the axes are different lengths & none of them meet at 90º. Because of this, they often lack the sharp, angled points commonly seen in crystals such as Quartz. Triclinic crystal forms include:
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  • Amorphous

    While the term “crystalline” refers to minerals comprised of a repeating arrangement of atoms, Amorphous or non-crystalline refers to materials that do not have a definite internal structure. Although some materials may look like crystals, their atomic structure is what matters. These materials are sometimes called mineraloids.

    Pictured above are two simple illustrations showing the structural difference between an amorphous substance & a crystalline one. As we can see, there is no regular repeating three-dimensional array of atoms (no crystal lattice) on the left one as compared to the right. This random & unstructured atomic arrangement means that these mineraloids never produce well-formed crystals. They also do not exhibit the property of cleavage because they lack internal planes of weakness. Amorphous materials include:
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