13.7.5: Ring Silicates

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Tourmaline is the only common mineral in which all tetrahedra link to form independent 6-member rings (Figure 13.40). Other minerals, including beryl, Be₃Al₂Si₆O₁₈ and cordierite, (Mg,Fe)₂Al₄Si₅O₁₈, contain rings but they also contain tetrahedra joined in other ways. In tourmaline, 6-membered rings bond to octahedral Fe²⁺, Mg²⁺, or Al³⁺, and to triangular (BO₃)^– groups. Ca²⁺, Na⁺, and K⁺ occupy large sites centered in the rings and coordinated to the borate groups and silica tetrahedra. Thus, tourmaline contains several different kinds of sites and may incorporate just about any element in its structure. And tourmaline comes in many different colors because of its highly variable chemistry. The photo below (Figure 13.41) shows a black variety called schorl. Figure 6.90 (Chapter 6) shows a different example of schorl. Tourmaline is one of the few minerals that commonly exhibits color zonation. See Figure 4.13 (Chapter 4) and 6.22 (Chapter 6) for examples. So, tourmaline is a popular and valuable gemstone.

13.40.png — Figure 13.40: The atomic arrangement in tourmaline

13.41.png — Figure 13.41: Tourmaline crystals from Minas Gerais, Brazil. The specimen is 8 cm tall.