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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4.5 Crystal systems

      6  4.6 Indexing planes in crystals

      7  4.7 Twinned crystals

      8  4.8 Crystal defects

      9  4.9 Polymorphs and pseudomorphs

      Crystallography emphasizes the long‐range order or crystal structure of crystalline substances. It focuses on the symmetry of crystalline materials and on the ways in which their long‐range order is related to the three‐dimensional repetition of fundamental units of pattern during crystal growth. In minerals, the fundamental units of pattern are molecular clusters of coordination polyhedra or stacking sequences (Chapter 2). The ways in which these basic units can be repeated to produce crystal structures with long‐range order are called symmetry operations. Crystallography is also focused on the description and significance of planar features in crystals including planes of atoms, cleavage planes, crystal faces, and the forms of crystals. In addition, it is concerned with crystal defects which are local imperfections in the long‐range order of crystals.

      4.1.1 Crystals and crystal faces

Photos depict representative mineral crystals: representative mineral crystals: (a) pyrite. (b) quartz.

      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.

       Motifs and nodes

      When minerals begin to form, atoms or ions bond together, so that partial or complete coordination polyhedra develop (Chapter 2). Because the ions on the corners and edges of coordination polyhedra have unsatisfied electrostatic charges, they tend to bond to additional ions available in the environment as the mineral grows. Eventually, a small cluster of coordination polyhedra is formed that contains all the coordination polyhedra characteristic of the mineral and its chemical composition. In any mineral, we can recognize a small cluster of coordination polyhedra that contains the mineral's fundamental composition and unit of pattern or motif. As the mineral continues to grow, additional clusters of the same pattern of coordination polyhedra are added to form a mineral crystal with a three‐dimensional geometric pattern – a long‐range, three‐dimensional crystal structure. Clusters of coordination polyhedra are added, often one atom or ion at a time, as (1) the crystal nucleates, (2) it becomes a microscopic crystal, and, if growth continues, (3) it becomes a macroscopic crystal. Growth continues in this manner until the environmental conditions that promote growth change and growth ceases.

      4.2.1 Simple symmetry operations

      Symmetry operations may be simple or compound. Simple symmetry operations produce repetition of a unit of pattern or motif using a single type of operation. Compound symmetry operations involve motif repetition using a combination of two symmetry operations. Simple symmetry operations, discussed in this section, include (1) translation, by specific distances in specified directions, (2) rotation, about a specified set of axes, (3) reflection, across a mirror plane, and (4) inversion, through a point called a center. These operations are discussed below and provide useful insights into the geometry of crystal structures and the three‐dimensional properties of such crystals.

       Translation

      Two‐dimensional translations are defined by two unit translation vectors (taand tb ort1and t2, respectively).