Word to the Wise: Coordination

One of the fundamental aspects of crystal structures, on the local scale, is the coordination of individual atoms or ions. Coordination is the number of neighboring atoms that are bonded to the atom of interest. Because the arrangement of these coordinating or bonded atoms is regular throughout a structure, the geometry of coordination is also an important feature. The centers of the coordinating atoms can be used to define the geometry of a coordination polyhedron. The focus of the lead article in this issue of Rocks & Minerals is silicates, or silicate minerals (Heaney 2023), and the fundamental structural unit of all silicates is the coordination polyhedron composed of four oxygen ions bonded to a central silicon, the geometry of which can be described by a tetrahedron. Thus, the coordination number (of the Si) is four, and the coordination geometry is tetrahedral. Let’s use the structure of the most abundant silicate mineral in Earth’s crust, quartz, to illustrate the concepts behind coordination. Figure 1 is a rendering of the quartz structure in which the Si and O ions are drawn to scale for their measured ionic radii. The O ions are dark blue; the Si ions (not seen in the drawing) are light blue. In this image the much larger O ions block your view of Si ions below them. This type of rendering is often called space-filling and is the most accurate depiction of the structure if you could image it by some type of microscopy. Every Si ion in the structure is bonded to (coordinated by) four O ions. Because we are using drawings, rather than microscopic images of a real crystal, we can create any type of rendering CO O R D I N AT I O N