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4.1.1 Crystals and crystal faces
Crystal faces
ssss1 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 (ssss1 and ssss1) 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 (ssss1). 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.