This describes the way the mineral’s atoms come together to form crystals. The process of crystallization can happen very slowly, over millions of years, or quickly, in as short a span as minutes. Typically it takes a long time for minerals to form. The chemistry of how crystals develop governs how “unit cells” link together. The resulting crystal faces have one of seven possible crystal systems, which refer to the arrangement of the crystal faces. They are:
1. Isometric, or cubic
These are cube-shaped crystals that include copper, diamond, fluorite, galena, gold, halite, iron, leucite, magnetite, pyrite, and spinel.
Appearing like square prisms, these include anatase, apophyllite, autunite, calomel, cassiterite, chalcopyrite, rutile, vesuvianite, wulfenite, and zircon.
Shaped like hexagons, these crystal forms include the minerals apatite, beryl, ettringite, hanksite, molybdenite, and vanadinite.
Similar in crystal form to hexagonal crystals, this class is nonetheless different in its modified shapes. Includes arsenic, bismuth, calcite, cinnabar, corundum, elbaite, hematite, magnesite, pyrargyrite, rhodochrosite, and sturmanite.
This class includes prisms and flattened tabular forms. The minerals belonging here include aragonite, barite, brookite, chrysoberyl, danburite, marcasite, staurolite, sulfur, topaz, wavellite, and zoisite.
Monoclinic crystals look like flattened cubes and are very common. Among the minerals here are acanthite, azurite, chalcocite, clinoclase, crocoite, creedite, diopside, epidote, euclase, muscovite, olivenite, orpiment, realgar, spodumene, stilbite, talc, and vivianite.
This class consists of complex shapes with the least symmetry of any group. It contains the minerals albite, axinite, chalcanthite, cylindrite, inesite, kermesite, kyanite, microcline, rhodonite, turquoise, vauxite, and wollastonite.
All specimens from the David J. Eicher Mineral Collection; images © David J. Eicher