This article is meant to be an introduction to determinative crystallography, to aid you in identification of mineral species and to broaden your understanding of the complexity of crystal structure.

Mineral crystals come in a seemingly infinite range of shapes and forms. Behind this tremendous diversity of form there is a very strict order, because crystals always grow according to simple mathematical laws.

For me, the most exciting aspect of mineral study is their beautiful and elegant crystal form. This crystal form is only a visible expression of the minerals invisible atomic structure. By studying the crystal geometry, you may fit a crystal into one of only six basic crystal systems, and they are as follows: isometric, tetragonal, hexagonal, orthorhombic, monoclinic and triclinic.

ISOMETRIC (Cubic System)

Crystals are generally blocky in appearance or ball-like, with many similar, symmetrical faces. Characteristic forms are cubes, octahedra and dodecahedra. A few common isometric minerals are: galena, fluorite, gold, diamond and the garnet family.

Crystals are often long and slender or needle-like. Characteristic forms are 4-sided prisms, pyramids and dipyramids. A few common tetragonal minerals are chalcopyrite, zircon, vesuvianite, scapolite, and rutile.

Crystals are generally prismatic or columnar, with a rounded triangular or hexagonal cross-section. Characteristic forms are 3 or 6-sided prisms, pyramids and rhombohedra. A few common hexagonal minerals are quartz, calcite, tourmaline, apatite and beryl.

Crystals are generally short and stubby with diamond-shape or rectangular cross-sections. Characteristic forms are: 4-sided prisms, pyramids and pinacoids. A few common orthorhombic minerals are sulfur, topaz, stibnite and olivine.

Crystals are stubby with tilted matching faces at opposite ends forming a somewhat distorted rectangular solid. Characteristic forms are prisms and pinacoids. A few common monoclinic minerals are epidote, monazite, crocoite, and brochantite.

Crystals are usually flattened, with sharp edges and thin cross-section. There are no right angles on the faces or edges. All forms are pinacoids or pdeions. A few common triclinic minerals are microcline, kyanite, rhodonite and pectolite.

References

Berry & Mason, Determinative Mineralogy, (1968). Sinkaknkas, Mineralogy, (1964).