Waco Gem and Mineral Club, Waco, Tx

1.5 Crystal Shapes


1.5: Crystal Shapes

Crystals come in wonderful and amazing shapes that are based upon their underlying chemical structure. Some common forms are cubic, tretragonal, monoclinic, triclinic, hexagonal, trigonal, and orthorhombic. The shape of a crystal is an important trait that can help you identify a mineral, so you should familiarize kids with these basic crystal forms. Here are brief descriptions of each:

Cubic: very symmetric and orderly, shaped like a square cube, with 6 faces, or sides (note, however, that some are shaped like octahedrons—or diamond-shaped—with 8 faces, and still others are shaped like dodecahedrons, with 10 faces)

Tetragonal: shaped like cubic crystals that have been stretched out along one axis.

Monoclinic: these are shaped like tetragonal crystals that have been skewed or tilted in one angle.

Triclinic: triclinic crystals are similar to monoclinic ones but aren‘t usually symmetrical from one side to the other; they can look like monoclinic crystals that someone stepped on and squished!

Hexagonal: these crystals look like six-sided prisms; viewed from the top, they look like hexagons.

Trigonal: similar to hexagonal, but possessing a 3-fold axis of rotation instead of the 6-fold axis of hexagonal crystals.

Orthorhombic: these crystals look like two elongated pyramids stuck together, but they‘re skewed at a bit of an angle.

Because it can be difficult to visualize these systems using words alone, you should get a book illustrating different crystal forms and bring in pictures and samples of minerals that illustrate each (for instance, a cubic pyrite or fluorite crystal; a hexagonal quartz crystal).

A couple of activities provide kids with hands-on fun in learning about these shapes. In one, build crystal shapes using tinker toys or dowels and Styrofoam balls or gum drops and toothpicks. (Kids especially like the last option because they get to eat the results!)

Another way to illustrate crystal shapes in hands-on fashion is through making models by folding colorful construction paper, cardstock, or thin cardboard and pasting or taping them together. Darryl Powell (aka Diamond Dan) has prepared a couple of great resources you may wish to purchase to use with your club‘s kids. These include Corundum Carl’s Great Crystal Adventure (introduces crystallography and includes 13 crystal models that can be cut out and folded into 3-dimensional crystal shapes, along with a recipe for growing crystals) and Crystal Clips V (a CD-ROM holding over 900 mineral and crystal drawings in both color and black-and-white in TIFF and JPEG formats). These resources may be purchased from Diamond Dan Publications, c/o Darryl Powell, phone 585-278-3047.