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The Aqueous Chemistry of Oxides$
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Bruce C. Bunker and William H. Casey

Print publication date: 2016

Print ISBN-13: 9780199384259

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780199384259.001.0001

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Aqueous Polymerization of Silicates and Aluminosilicates

Aqueous Polymerization of Silicates and Aluminosilicates

(p.405) 14 Aqueous Polymerization of Silicates and Aluminosilicates
The Aqueous Chemistry of Oxides

Bruce C. Bunker

William H. Casey

Oxford University Press

Part Five of this book is devoted to silicates for several important reasons. First, silicates represent critical components of our planet and our lives. Silicon is the second most abundant element in Earth’s crust after oxygen, representing about 28% of the atoms present. As such, transformations of silicate minerals dominate much of the aqueous geochemistry of Earth. Every day, each of us encounters materials and objects the primary constituents of which are silicon oxides and related phases such as aluminosilicates. Granite facings on buildings, bricks, glass, pottery, ceramics, engineered materials used in water purification, catalysis, electronics, and even the optical fibers used in our most advanced communication systems are all silica based. Aluminosilicate minerals are even used as food additives. A key attribute of silicates that distinguishes them from most of the oxides highlighted in Parts One through Four of this book is that the Si(IV) cation is almost always present in a tetrahedral rather than in an octahedral coordination geometry. Exceptions include a few high-pressure phases such as stishovite (see Chapter 2) and a limited number of chelated Si(IV) complexes (see Section 14.3). The authors know of no stable compounds where Si(IV) is coordinated to only three oxygen atoms. The pathways for both forming and destroying silicate bonds are substantially different than for octahedral metal ions. Ligand-exchange pathways for silicate ions are via nucleophilic attack, where the coordination number increases in a transition state from four to five or even six (see Section 14.3 and Chapters 4 and 5). These contrast with pathways for octahedral metal ions, such as Al(III), where it is easier to decrease the coordination number from six to five or four in dissociative ligand exchange reactions. Of course, Si(IV) is not the only common element capable of forming tetrahedral oxide species. As outlined in Chapters 2 and 4, any cation with an ionic radius between roughly 0.03 nm and 0.055 nm can fit within the tetrahedral void between four close-packed oxygen anions, as expressed by Linus Pauling’s First Rule of coordination chemistry (see Chapter 2).

Keywords:   Marianas Trench, aluminosilicate clusters, diatoms, gel crystallization, hydrothermal conditions, silicatein, tetrahedral coordination geometry

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