The Seven Systems
Petrograph 5/99
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Hexagonal quartz crystal
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How crystals shape up: Just a bit of information from "Color Treasury of Crystals," from our library. Symmetry, habit, faces, form, axes--there are pages of diagrams and text clearly defining the vocabulary of crystals (you have to read this book) and pages of wonderful colored pictures.
- The Cubic System has the fundamental face intersecting the three crystallographic axes at equal distance from the origin and contains only closed forms; about 12% of known minerals crystallize in this system including copper, gold, silver, galena, argentite, fluorite, rock salt, magnetite, analcite, garnet, uraninite (pitchblende), zinc blende, tetrahedrite, pyrite and cuprite.
- The Hexagonal System includes both closed and open forms; among the latter are prisms and pyramids; about 8% of known minerals crystallize in this system, notably beryl, pyrrhotite, covellite, molbdenite, apatite, vanadinite and nepheline.
- The Trigonal System includes most characteristic forms of rhombohedrons, hexagonal prisms and pyramids; nearly 9% of minerals crystallize in this system including corundum, hematite, calcite, siderite, rhodochrosite, tourmaline proustite, dolomite, dioptase, benitoite, magnesite, quartz and cinnabar.
- The Tetragonal System also contains prisms of different orders, bipyramids, scalenohedrons and trapezohedrons; some 10% of minerals crystallize in this system including cassiterite, rutile, anatase, zircon, vesuvianite, scheelite, wulfenite and chalcopyrite.
- The Orthorhombic System contains minerals with rhombic-shaped cross-sections perpendicular to the length of the crystal; some 22% of known minerals crystallize in this system including sulfur, stibnite, marcasite, aragonite, strontianite, witherite, andalusite, topaz, olivine, chalcocite, bournonite and cordierite.
- The Monoclinic System has only prisms and pinacoids; nearly 32% of minerals belong in this system including wolframite, azurite, malachite, colemanite, gypsum, crocoite, vivianite, sphene, datolite, epidote, orthoclase, many amphiboles and pyroxenes.
- The Triclinic System has fundamental forms that are pinacoidal and crystals result from the combination of pinacoids; only 7% of minerals crystallize in this system such as wollastonite, rhodonite, axinite, kyanite, microcline and the plagioclases.