Читать книгу Earth Materials - John O'Brien - Страница 87

Mineral crystals are one of nature's most beautiful creations. Many crystals are enclosed by flat surfaces called crystal faces. Crystal faces are formed when mineral crystals grow, and enclose crystalline solids when they stop growing. Perfectly formed crystals are notable for their remarkable symmetry (Figure 4.1). The external symmetry expressed by crystal faces allowed us to infer the geometric patterns of atoms in mineral crystal structures. These patterns inferred from external symmetry have been confirmed by advanced analytical techniques such as X‐ray diffraction (XRD) and atomic force microscopy (AFM).

Figure 4.1 Representative mineral crystals: representative mineral crystals: (a) pyrite.

Source: Courtesy of Doug Moore.

(b) quartz.

Source: Robert Lavinsky. iRocks.com—CCBYSA 3.0, https://commons.wikimedia.org/wiki/File:Quartz‐153455.jpg; last accessed 09/24/2020.

Mineralogists have developed language to describe the symmetry of crystals and the crystal faces that enclose them. Familiarizing students with the concepts and terminology of crystal symmetry and crystal faces is one of the primary goals of this chapter. A second goal of this chapter is to build connections between crystal chemistry (Chapters 2 and 3) and crystallography. This involves an explanation of the relationships between chemical composition, coordination polyhedra and the crystal structures, crystal faces, and crystal forms that develop as crystals grow.