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4.6.7 Common crystal forms in each system
Isometric (cubic) system forms
All forms in the isometric system are closed forms. Common crystal forms in the isometric system include the cube, octahedron, dodecahedron, tetrahedron, and pyritohedron (ssss1). These forms may occur alone or in combination with each other. Common isometric minerals, their crystal forms, and form indices are summarized in ssss1.
These form indices are also used to describe cleavage in isometric minerals. These minerals include halite and galena, which possess cubic cleavage {001} with three orientations of cleavage at right angles; fluorite, which possesses octahedral cleavage {111} with four cleavage orientations; and sphalerite, which possesses dodecahedral cleavage {011} with six orientations of cleavage.
ssss1 Common isometric crystal forms, form indices, form descriptions, and minerals.
Crystal form Form indices Form description Minerals that commonly exhibit crystal form Cube {001} Six square faces Halite, galena, pyrite, fluorite, cuprite, perovskite, analcite Octahedron {111} Eight triangular faces Spinel, magnetite, chromite, cuprite, galena, diamond, gold, perovskite Dodecahedron {011} 12 diamond‐shaped faces Garnet, sphalerite, sodalite, cuprite Tetrahedron
ssss1 Common crystal forms in the tetragonal crystal system: (a) tetragonal prism in combination with a pinacoid, (b) tetragonal dipyramid, (c) tetragonal dipyramid in combination with a tetragonal prism.