WoW! Things to Read
by Donald Clark CSM
... Did you know that crystals have habits? No, they don’t take up jogging or eat too much candy. When talking about crystals, habits refer to the way they form. You might already know that diamonds have four sides and quartz crystals have six. These are examples of “mineral habits” or “crystal systems.”
... There are six crystal systems. All minerals form crystals in one of these six systems. You may have seen more than six shapes of crystals, but they are all variations of one of these six habits.
... Each system is defined by a combination of three factors; how many axes they have, their lengths and the angles they meet. An axis is a direction between the sides. The shortest one is A, the longest one C. There is a B axis as well and sometimes a D axis.
Minerals that form in the isometric system include all Garnets, Diamond, Fluorite, Gold, Lapis Lazuli, Pyrite, Silver, Sodalite, Shalerite, Spinel.
Minerals that form in the tetragonal system include Apophyllite, Idocrase, Rutile, Scapolite, Wulfenite and Zircon.
Minerals that form in the orthorhombic system include Andalusite, Celestite, Chrysoberyl, (including alexandrite,) Cordierite, (Iolite,) Danburite, Epidote, (except zoizite, includes tanzanite and thulite,) Enstatite, (Hemimorphite,) Fibrolite/Sillimanite, Hypersthene, Olivine, (Peridot,) Sulphur, Topaz and Zoisite.
Minerals that form in the monoclinic system include Azurite, Brazilianite, Crocoite, Datolite, Diopside, Jadeite, Lazulite, Malachite, Orthoclase Feldspars, (including albite moonstone,) Staurolite, Sphene, Spodumene, (including Hiddenite and Kunzite.)
Minerals that form in the triclinic system include Amblygonite, Axinite, Kyanite, Labradorite, Microline Feldspar, (including Amzonite and Aventurine,) Plagioclase Feldspars, (indcluding Labradorite,) Rhodenite and Turquoise.
Minerals that form in the hexagonal system include Apatite, Beryl, (aquamarine, emerald, heilodore and morganite,) Taaffeite and Zincite.
... Again, the trigonal system is a subsystem of the hexagonal. Most gem references will list these as hexagonal.
... Minerals that form in the trigonal system include all varieties of Quartz, (agates, aventurine, chalcedony, jasper, tiger eye, citrine, amethyst, etc.,) Benitoite, Corundum, (rubies and sapphires,) Hematite, Rhodochrosite and Tourmaline.
... Amorphous materials are not minerals. They include Amber, Chrysocolla, Glass, Ivory, Jet , Moldavite, Obsidian and Opal.
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