13.6.4: Elemental Substitutions in Silicates

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While quartz is usually 99.9% SiO2, most minerals have variable chemistries due to elemental substitutions. Consistent with Pauling’s rules, the nature and extent of substitutions depend primarily on ionic size and charge and on the nature of atomic bonding in a mineral’s structure. Because silicate minerals are dominantly ionic, the nature of bonding is relatively constant; size and ionic charge control most substitutions. Figure 13.7 showed the relative sizes and charges of the most common elements in silicate minerals. Elements of similar size and charge may occupy similar sites in crystal structures without causing distortion or charge imbalance. As an example, Ca²⁺, Mn²⁺, Fe²⁺, and Mg²⁺ substitute for each other in many silicates (and other minerals), including garnets and pyroxenes.

The extent to which elements may substitute for each other is often limited. Natural garnets may have any composition described by the formula (Ca,Mn,Fe,Mg)₃Al₂Si₃O₁₂. In contrast, the substitution of Ca²⁺ for Mn²⁺, Fe²⁺, or Mg²⁺ in pyroxenes is limited at all but the highest temperatures (due to the large size of Ca²⁺ compared with the other ions). Consequently, a miscibility gap is found between orthopyroxene and clinopyroxene. A similar gap exists in the Ca-Mn-Fe-Mg carbonate system.

Similarity in size and charge allows K⁺ and Na⁺ to substitute for each other in feldspars, amphiboles, and other minerals. Fe³⁺ and Al³⁺ replace each other in minerals such as garnet and spinel. These are both examples of simple substitutions. In a simple substitution the substituting ion has the same charge as the one it replaces. Sometimes simple substitutions are described using equations such as Fe²⁺ = Mg²⁺ or Fe³⁺ = Al³⁺.

Other elemental substitutions are more complex. For example, Ca²⁺ may replace Na⁺ in feldspar. To maintain charge balance, Al³⁺ replaces Si⁴⁺ at the same time, and we describe the coupled substitution as Ca²⁺Al³⁺ = Na⁺Si⁴⁺ or just CaAl = NaSi. This substitution relates the feldspar end member anorthite (CaAl₂Si₂O₈) to albite (NaAlSi₃O₈) and occurs in other minerals too. Ions related by coupled substitutions never have charge differences greater than 1.

Anions, too, may substitute for each other in minerals. In micas and amphiboles, for example, F^– or Cl^– may replace (OH)^–. More complex substitutions in micas involve the replacement of (OH)^– by O^2-, which requires some compensatory substitution to maintain charge balance. In scapolite and a few other minerals, (CO₃)^2- or (SO₄)^2- may replace Cl^–. To add further complexity, in some minerals, substitutions involve vacancies. For example, in hornblende, □ Si = KAl is a common substitution. The □ symbol shows a vacancy. The table below lists some of the more common and most important elemental substitutions.

Some Typical Elemental Substitutions
possible substitutions	example minerals
Na⁺ = K⁺	alkali feldspar, hornblende, micas
Ca²⁺ = Mg²⁺ = Fe²⁺ = Mn²⁺	pyroxenes, amphiboles, micas, garnet, carbonates
F^– = Cl^– = OH^–	amphiboles, micas, apatite
Fe³⁺ = Al³⁺	garnet, spinels
Ca²⁺Al³⁺ = Na⁺Si⁴⁺	plagioclase, hornblende
Al³⁺Al³⁺ = (Mg,Fe)²⁺Si⁴⁺	pyroxenes, amphiboles, micas
□Si⁴⁺ = K⁺Al³⁺	hornblende
O^2- = (OH)^– plus other substitution to keep charge balance	biotite, titanite

While the elemental substitutions listed above occur in many minerals, including many nonsilicates, they do not necessarily occur in all. Generally, limited substitution is because ions vary in size, and some atomic sites can accommodate larger or smaller ions than other. For example, there is only very minor substitution of Fe³⁺ for Al³⁺ in corundum, but unlimited substitution of Fe³⁺ for Al³⁺ takes place in garnet. Complete solid solution exists between andradite (Ca₃Fe₂Si₃O₁₂) and grossular (Ca₃Al₂Si₃O₁₂). Similarly, periclase is always close to end member MgO, rarely forming significant solid solution with FeO or with MnO. Nonetheless, the substitutions listed are common, occur in more than one mineral class, and explain most of the mineral end members discussed in earlier chapters in this book. A glance back to Chapters 6 and 7, for example, will show that elemental substitutions, including the nature and extent of solvi, are about the same in pyroxenes, amphiboles, and carbonates. Major substitutions for all are Ca²⁺ = Mg²⁺ = Fe²⁺ = Mn²⁺.