Читать книгу Earth Materials - John O'Brien - Страница 111

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.

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