13.6: Silicate Structures in General

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In Chapter 6 we discussed silicate mineral structures. Here we take another look at silicate structures in light of Pauling’s rules and other crystal-chemical principles. Because oxygen and silicon are the two most abundant elements in the Earth’s crust, and because the (SiO₄) tetrahedron is such a stable complex, silicate minerals are extremely stable and abundant in crustal rocks and sediments. They dominate igneous and metamorphic rocks, and also many sedimentary rocks. An individual SiO₄ tetrahedron has a charge of -4. Because minerals must be charge balanced, silicon tetrahedra in crystals must share oxygen ions, or must bond to other cations.

As we have seen previously, sharing of oxygen between tetrahedra is a form of polymerization. Quartz and tridymite (SiO₂), for example, are highly polymerized. In most of the SiO₂ polymorphs, two (SiO₄)^4– tetrahedra share every oxygen atom (see Figure 6.25 in Chapter 6). The strength of each Si-O bond is 1; each Si⁴⁺ bonds to four oxygen, and each O^2– to two silicon, so charge balance is maintained and the overall formula is SiO₂.

Polymerization is absent in some silicates, such as olivine, (Mg,Fe)₂SiO₄ (see Figure 6.94 in Chapter 6). Instead, cations link individual silicon tetrahedra. In many silicates, a combination of oxygen sharing between tetrahedra, and the presence of additional cations, leads to charge balance. The more oxygen sharing, the fewer additional cations needed.

In still other silicates, Al³⁺ replaces some tetrahedral Si⁴^+. Consequently, more additional cations must be present to maintain charge balance. In albite, for example, Al³⁺ replaces one-fourth of the tetrahedral Si⁴⁺. Na⁺ ions between tetrahedra maintain charge balance. Albite’s formula is NaAlSi₃O₈, which we may write Na(AlSi₃)O₈ to emphasize that both Al³⁺ and Si⁴⁺ occupy the same structural sites. In anorthite, another feldspar, Al³⁺ replaces half the Si⁴⁺, resulting in the formula Ca(Al₂Si₂)O₈. Besides feldspars, tetrahedral aluminum is common in micas, amphiboles, and, to a lesser extent, in pyroxenes.

The feldspars and other minerals, in which tetrahedra form a 3D network, contain large atomic sites that can hold large cations including Na⁺, K⁺, and Ca²⁺. Most crystal structures are more closely packed and large cations will not fit. This explains why rocks rich in alkali and alkali-earth elements always contain feldspar or, less commonly, feldspathoid minerals.