Crystal Systems

Now let's turn our attention to the seven crystal systems and look at their descriptive properties. Unlike most presentations and references we'll start with the least symmetrical and build up to the more symmetrical.

Triclinic System

Triclinic crystals have three distinctly different axis lengths, and the angles between each are aslo unique. Since everything about the crystal is unique, ther are no symmetry axis, and hence we have defined the triclinic system.

The three crystal axis will be labeled a, b, anc c in this example and those that follow. The angles between axis will be listed as alpha , beta, and gamma. The triclinic system can be defined by a b c, and alpha beta gamma. (Where the symbol "" can be pronouced, "not equal to".)

triclinic minerals include: kyanite, axinite,and microcline

Monoclinic System

Monoclinc crystals have a b c, but two of the crystal axis are 90º, and one is "something else". So that is defined as alpha = gamma = 90º, and beta 90º.

monoclinic minerals include: lazulite, sphene (titanite), epidote, and heulandite.

Orthorhombic System

Orthorhombic crystals have three different axis lengths, a b c, but the angles between axis are all 90º. Hence alpha = beta = gamma = 90º. This crystal system is more easily identified than the previous systems as it has substantially more symmetry and the crstals are far more regular.

orthorhombic minerals include: hemimorphite, olivine, barite, and ilvaite .

Tetragonal System

Tetragonal crystals have two equal axis, and one different, a = b c. All three angles are equal to 90º, thus alpha = beta = gamma = 90º. These crystals tend to look less platey and more like columns. Cross sections should be roughly square.

tetragonal minerals include: apophyllite, rutile, idocrase (vesuvianite), and cassiterite.

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