13.7.1: Framework Silicates

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13.25.png — Figure 13.25: The atomic arrangements in two framework silicates

Framework silicates consist of a three-dimensional polymerized network of Si or (Si,Al) tetrahedra. Figure 13.25 shows two examples: quartz and sodalite. Quartz is a hexagonal mineral and sodalite is cubic; these symmetries are apparent in Figure 13.25. Most framework silicates, however, have less symmetry.

Quartz is the most common framework silicate. In quartz and several other SiO₂ polymorphs, oxygen links each silicon tetrahedron to four others. Different SiO₂ polymorphs have different arrangements of tetrahedra; some contain 4-, 6-, or 8-membered loops, and some contain channels. See Chapter 6 (Section 6.4.1) for further discussion of the different polymorphs.

The two photos below (Figures 13.26 and 13.27) show a monstrous 260-kg quartz crystal from Brazil, and amethyst from Mexico. Other photos of quartz were in Chapters 1, 3, and 6: Figure 1.8, Figure 3.44, Figure 3.61, Figure 6.26, and Figure 6.27.

13.26.png — Figure 13.26: A 260 kg quartz crystal from Minas Gerais, Brazil

13.27.png — Figure 13.27: Amethyst (quartz) from Veracruz, Mexico. The largest crystal is 5.8 cm long.

In sodalite, all the feldspars, and a number of other framework silicates, tetrahedral Al³⁺ replaces some Si⁴⁺. This leads to a charge deficiency. However, three-dimensional (SiO₄) polymerization creates holes between tetrahedra. The openings, sometimes irregular, can accommodate alkalis, alkali earths, and other large cations to make up the missing charge. In sodalite, Na⁺ occupies the large site. The sodalite structure also contains Cl^– anions between tetrahedra.

In orthoclase and other feldspars, K⁺, Na⁺, or Ca²⁺ occupy large distorted sites with coordinations of 6 to 9, depending on the feldspar. In other framework silicates, including some feldspathoids (for example, analcime) and zeolites, the openings between silica tetrahedra are large enough to hold molecular water.

Figure 13.28 shows the atomic arrangement in natrolite, one of the more common zeolites. Blue spheres show H₂O molecules. Because of their many channels and cage-like openings, zeolites are used as molecular sieves and as absorbents in many applications. The photo in Figure 13.29 shows an example of natrolite from central France. Figure 7.56 (Chapter 7) shows a photo of chabazite, another zeolite species.

13.28.png — Figure 13.28: The atomic arrangement in natrolite, one of the most common zeolites

13.29.png — Figure 13.29: Natrolite crystals on basalt from the Puy de Dôme, central France. The specimen is 5.3 cm tall.