13.6.3: Complex Atomic Arrangements

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Silicate crystal structures may be complex. Many silicates contain anions or anionic groups other than O²⁻. Muscovite, for example, contains (OH)^– and has the formula KAl₂(AlSi₃O₁₀)(OH)₂. Other silicates, such as kyanite, and titanite, CaTi(SiO₄)O, contain O^2- ions unassociated with the (SiO₄)^4- tetrahedra. In muscovite and many other minerals, aluminum is in both tetrahedral and octahedral coordination. Still other silicates do not fit neatly into a subclass. Zoisite, Ca₂Al₃O(SiO₄)(Si₂O₇)(OH), contains both isolated tetrahedra and paired tetrahedra. (It also contains oxygen not in tetrahedra.) Some mineralogy texts and reference books separate elements and include extra parentheses in mineral formulas (as has been done in this paragraph) to emphasize the nature of the atomic arrangement, but often we omit such niceties for brevity. In shorter form, we can write muscovite’s formula as KAl₃Si₃O₁₀(OH)₂; kyanite’s becomes Al₂SiO₅; titanite’s becomes CaTiSiO₅; and zoisite’s becomes Ca₂Al₃Si₃O₁₂(OH). While being shorter and, perhaps, easier to write, these formulas give little hint of crystal structure.