WoW! Things to Read
Mineral Habits
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.
Isometric
... The first and simplest is the isometric or cubic system. It has three axes. They are all the same length and they all intersect at 90o to each other. Because of the equality of axes, minerals in the cubic system are singly refractive. That does not apply to any other crystal system.
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Minerals that form in the isometric system include all Garnets, Diamond, Fluorite, Gold, Lapis Lazuli, Pyrite, Silver, Sodalite, Shalerite, Spinel.
Tetragonal
... The tetragonal system also has three axes that all meet at 90o. It differs from the isometric system in that the C axis is longer than the A and B axis which are the same length.
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Minerals that form in the tetragonal system include Apophyllite, Idocrase, Rutile, Scapolite, Wulfenite and Zircon.
Orthorhombic
... In this system there are three axes, all of which meet at 90o to each other. However, all the axes are a different length.
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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.
Monoclinic
... The above crystal systems all have axes sides that meet at 90o. In the monoclinic system all the axes are different lengths. Two of them, the A and C axes, meet at 90o, but the third one does not.
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.)
Triclinic
... In this system all the axes are different lengths and none of them meet at 90o.
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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.
Hexagonal
... The above represent every variation of four sided figures with three axes. In the hexagonal system we have an additional axes, which gives the crystals six sides. Three of these are equal in length and meet at 60o to each other. The C or vertical axis is at 90o to the shorter axes.
... Mineralogists sometimes divide this into two systems, the hexagonal and the trigonal, based on their external appearance. However, for purposes of gemology, the above six categories are sufficient.
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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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