John O'Brien - Earth Materials

Figure 4.24 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.

All forms in the isometric system are closed forms. Common crystal forms in the isometric system include the cube, octahedron, dodecahedron, tetrahedron, and pyritohedron ( Figure 4.24). These forms may occur alone or in combination with each other. Common isometric minerals, their crystal forms, and form indices are summarized in Table 4.6.

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.

Table 4.6 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		Four triangular faces	Tetrahedrite, sphalerite
Pyritohedron	{h0l}	12 pentagonal faces	Pyrite

Figure 4.25 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.

Table 4.7 Common tetragonal crystal forms, form indices, form descriptions, and minerals.

Tetragonal crystals can possess closed and/or open forms. Common closed crystal forms in the tetragonal crystal system include different eight‐sided dipyramids. Common open forms include four‐sided and eight‐sided prisms and pyramids, as well as pinacoids and pedions. Typical crystal forms and associated minerals in the tetragonal crystal system are shown in Figure 4.25and Table 4.7.

Common crystal forms in the hexagonal system include 6–12‐sided prisms, dipyramids, and pyramids. Pinacoids and pedions are also common. Some selected examples of common crystal forms and minerals in the hexagonal system are illustrated in Figure 4.26and Table 4.8.

Common closed crystal forms in the trigonal system include the six‐sided rhombohedron, the 12‐sided scalenohedron, six‐sided hexagonal dipyramids and three‐sided trigonal pyramids. Three‐ and six‐sided prisms, and pinacoids and pedions are also common. Many forms common in the hexagonal division also occur in trigonal crystals, but not vice versa. Common crystal forms and representative minerals in the trigonal crystal system are summarized in Figure 4.27and Table 4.9.

Figure 4.26 Common crystal forms in the hexagonal crystal system (hexagonal division): (a) hexagonal prism closed by a basal pinacoid, (b) hexagonal dipyramid.

Table 4.8 Common hexagonal crystal forms and associated hexagonal and trigonal minerals.

Crystal forms	Common form indices	Form description	Minerals that commonly exhibit crystal form
Hexagonal dipyramid	and variations	12 triangular faces inclined to c‐axis; top six separated from bottom six by mirror plane	Apatite, zincite
Hexagonal prism	and variations	Six rectangular faces parallel to c‐axis	Apatite, beryl, quartz, nepheline, corundum, tourmaline
Basal pinacoid	{0001}	Pair of faces perpendicular to c‐axis	Apatite, beryl, corundum

Figure 4.27 Common crystal forms in the trigonal system: (a) trigonal dipyramid, (b) rhombohedron, (c) scalenohedron.

Common crystal forms in the orthorhombic system include four‐sided rhombic prisms and pyramids, and eight‐sided dipyramids. Pinacoids are the dominant form and pedions are also common. Common crystal forms and associated minerals in the orthorhombic crystal system are indicated in Figure 4.28and Table 4.10.