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The Seven Systems

...from "The Petrograph, May, 1999", (The Mineral & Gem Society of Castro Valley, California).


Hexagonal quartz crystal

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.

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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.

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