Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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the indices of a single face, called the form face, are chosen and placed in brackets to indicate that they refer to the form indices. The rule for choosing the form face is generally to select the top face if there is one, or the top right face if there is one, or the top right front face if there is one. In the case of the octahedron, the top right front face is the face that intersects the positive ends of the a1‐axis (front), the a2‐axis (right), and the a3‐axis (top) and has the Miller indices (111). The form indices for all octahedral crystals are the Miller indices of the form face placed between curly brackets {111}. Similarly the form indices for the cube (see Figure 4.22), in which the faces intersect one axis and are parallel to the other two are {001}, the Miller indices of the top face, whereas the form indices for the dodecahedron, in which each face intersects two axes at unity and is parallel to the third is {011}, the indices for the top, right face.

Schematic illustration of five common forms in the isometric system: (a) cube, (b) octahedron, (c) dodecahedron, (d) tetrahedron, (e) pyritohedron.

      Many other forms exist. Every crystal form has a form index, which is the Miller index of the form face placed in brackets. Each form consists of one or (generally) more faces and each face possesses a Miller index different from that of every other face in the form. Every crystal system has a characteristic suite of forms that reflect the unique characteristics of the crystal lattice of the system, especially the relative lengths of the three crystallographic axes that directly or indirectly reflect the lengths of the unit cell edges. The forms that are characteristic of each class (space point group) in each crystal system are beyond the scope of this text (see Klein and Hurlbut 1985). However, a brief review of some common forms in each crystal system is appropriate.

      4.6.7 Common crystal forms in each system

       Isometric (cubic) system forms

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 left-brace 1 ModifyingAbove 1 With bar 1 right-brace Four triangular faces Tetrahedrite, sphalerite
Pyritohedron {h0l} 12 pentagonal faces Pyrite
Schematic illustration of 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.
Crystal forms Form indices Form description Minerals that commonly exhibit crystal form
Tetragonal dipyramid {111} {hh1} {011} {0kl} and variations Eight triangular faces; top four separated from bottom four by mirror plane Zircon, rutile, cassiterite, scheelite, wulfenite, vesuvianite, scapolite
Tetragonal prism {010} {110} and variations Four rectangular faces parallel to c‐axis Zircon, scheelite, vesuvianite, rutile, malachite, azurite, cassiterite, scapolite
Tetragonal disphenoid {0kl} Four triangular faces; alternating pairs symmetrical about c‐axis Chalcopyrite
Basal pinacoid {001} Pair of faces perpendicular to c‐axis Vesuvianite, wulfenite

       Tetragonal system forms

       Hexagonal system (hexagonal division) forms