6.4.9: Ring Silicates and Paired Tetrahedral Silicates

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Ring Silicates and Paired Tetrahedral Silicates
The most common ring silicates
beryl tourmaline cordierite	Be₃Al₂Si₆O₁₈ (Na,Ca)(Fe,Mg,Al,Li)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄ (Mg,Fe)₂Al₃(Si₅AlO₁₈)
The most common paired tetrahedral silicates
epidote zoisite lawsonite	Ca₂(Al₂,Fe)(SiO₄)(Si₂O₇)O(OH) Ca₂Al₃(SiO₄)(Si₂O₇)O(OH) CaAl₂Si₂O₇

Ring silicates, generally containing rings of six silicon tetrahedra (see Figure 6.24), are relatively rare. Beryl, tourmaline and cordierite are the only common examples. A few obscure and uncommon minerals have rings of three tetrahedra.

Beryl, tourmaline, and cordierite are sometimes beautiful gemstones. But, the more common varieties are often rather drab. Figures 6.89 and 6.90 show examples of common beryl and tourmaline. Because these mineral have atoms arranged in hexagonal rings, their crystals often exhibit hexagonal symmetry, as can be seen in Figure 6.89. Figure 6.108, later in this chapter, shows an example of gemmy tourmaline. It is similar to one we saw in Figure 4.13 (Chapter 4). Figures 4.1 and 4.30 (Chapter 4) show examples of aquamarine and emerald, both gemmy varieties of beryl.

Paired tetrahedral silicates contain linked pairs of silicon tetrahedra that create an Si₂O₇ group (see Figure 6.24 earlier in this chapter). Epidote, zoisite, and lawsonite are the most common examples. But, epidote and zoisite contain paired tetrahedra and tetrahedra in other configurations as well. The photos, in Figures 6.91, 6.92, and 6.93 show beautiful examples of these minerals. More common specimens are not so pretty.

6.91.png — Figure 6.91: Epidote from Prince of Wales Island, Alaska. The specimen is 5.2 cm across.

6.92.png — Figure 6.92: Zoisite from Tanzania. The crystal is 7.2 cm tall.

6.93.png — Figure 6.93: Lawsonite from just north of the Golden Gate Bridge, San Francisco. The largest crystals are about 5 mm across.

Beryl and tourmaline contain beryllium and boron, respectively – these elements are generally quite rare. But, they concentrate in some pegmatites, and that is where most beryl and tourmaline are found. Cordierite and zoisite are both common metamorphic minerals. Lawsonite is a rare high pressure metamorphic mineral associated with metamorphism in subduction zones. The chemistry of tourmaline is highly variable, but the other minerals are generally close to the end member compositions listed in the blue box above.