Question

Describe briefly the seven systems of crystals.

Answer 1

Cubic System Also known as the isometric system. All three axes are of equal length and intersect at right angles. Based on a square inner structure. Crystal shapes include: Cube (diamond, fluorite, pyrite) Octahedron (diamond, fluorite, magnetite) Rhombic dodecahedron (garnet, lapis lazuli rarely crystallises) Icosi-tetrahedron (pyrite, sphalerite) Hexacisochedron (pyrite). Common Cubic Crystals: Diamond Fluorite Garnet Gold Pyrite Silver Spinel Tetragonal System Two axes are of equal length and are in the same plane, the main axis is either longer or shorter, and all three intersect at right angles. Based on a rectangular inner structure. Crystal shapes include: Four-sided prisms and pyramids Trapezohedrons Eight-sided and double pyramids Icosi-tetrahedron (pyrite, sphalerite) Hexacisochedron (pyrite). Common Tetragonal Crystals: Anatase Apophyllite Chalcopyrite Rutile Scapolite Scheelite Wulfenite Zircon Hexagonal System Three out of the four axes are in one plane, of the same length, and intersect each other at angles of 60 degrees. The fourth axis is of a different length and intersects the others at right angles. Based on a hexagonal (6-sided) inner structure. Crystal shapes include: Four-sided prisms and pyramids Twelve-sided pyramids Double pyramids Common Hexagonal Crystals: Apatite Aquamarine Beryl Cancrinite Emerald Goshenite Morganite Sugilite Zincite Trigonal System (Rhombohedral System) - Axes and angles in this system are similar to the Hexagonal System, and the two systems are often combined as Hexagonal. In the cross-section of a Hexagonal crystal, there will be six sides. In the cross-section of a Trigonal crystal there will be three sides. Based on a triangular inner structure. Crystal shapes include: Three-sided prisms or pyramids Rhombohedra Scalenohedra Common Trigonal Crystals: Agate Amethyst Aventurine Calcite Carnelian Citrine Hematite Jasper Phenakite Quartz Rhodochrosite Rose Quartz (rarely crystallises) Ruby Sapphire Smoky Quartz Tigers Eye Tourmaline Orthorhombic System (Rhombic System)Three axes, all of different lengths, are at right angles to each other. Based on a rhombic (diamond-shaped) inner structure. Crystal shapes include: Pinacoids Rhombic prisms Pyramids Double pyramids Common Orthorhombic Crystals: Alexandrite Andalusite (Chiastolite) Celestite Chrysoberyl Chrysoberyl Danburite Dumortierite Enstatite Hemimorphite Iolite Tanzanite Topaz Zoisite Monoclinic System There are three axes, each of different lengths. Two are at right angles to each other and the third is inclined. Based on a parallelogram inner structure. Crystal shapes include: Basal pinacoids and prisms with inclined end faces Common Monoclinic Crystals: Azurite Chrysocolla Diopside Epidote Gypsum Hiddenite Howlite Kunzite Lazulite Moonstone Muscovite (Mica) Petalite Serpentine Spodumene Staurolite Vivianite Triclinic System All three axes are of different lengths and inclined towards each other. Based on a 'triclinic' inner structure, meaning 'three inclined angles'. Crystal forms are usually paired faces. Common Triclinic Crystals: Amazonite Aventurine Feldspar Kyanite Labradorite Rhodonite Turquoise Amorphous No crystal structure. Most of these are either cooled too quickly to crystallise - such as obsidian or moldavite, or are organic - such as amber. Common Amorphous Minerals: Amber Moldavite Obsidian Opal