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ssss1 Crystallographic axes (positive ends labeled) and intersection angles for the major crystal systems: isometric, tetragonal, orthorhombic, monoclinic, triclinic, and hexagonal systems.

Crystal forms

ssss1 A pyritohedron, a closed form in which all faces have the same general relationship to the crystallographic axes.

Closed crystal forms

Open crystal formspedionspinacoidsprismspyramidsdomessphenoids

ssss1 Different types of dipyramid forms in the trigonal, tetragonal, and hexagonal systems.

Source: Klein and Hurlbut (1985). © John Wiley & Sons.

The most common crystal forms in each system are discussed later in this chapter, after we have presented the language used to describe them. More detailed discussions are available in Nesse (2016) and Klein and Dutrow (2007).

axial ratios2

Axial ratios are essential to understanding how crystallographic planes and crystal forms are described or indexed by reference to the crystallographic axes as discussed in the section that follows.