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2.4 Silicate Minerals
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Alexa Actionable Analytics for the Web. Shopbop Designer Fashion Brands. The top diagram shows the atomic arrangement, the bottom diagram is a polyhedral representation, and the middle view shows how the two are related.
Just like any other mineral containing radicals, the negatively-charged radicals are held together by intervening cations in red, at left. Because the radicals are small and surrounded by cations, nesosilicates and sorosilicates tend to have fairly uniform properties in all directions.
Many have no or weak cleavage, and those with good cleavage tend to have only one direction. Crystals are fairly equidimensional or not overly elongated. Hexagonal symmetry is conspicuously absent, and two of the most common tetragonal minerals, zircon and vesuvianite, are nesosilicates. Another important tetragonal mineral, melilite, is a sorosilicate. You might wonder why, with only isolated silica tetrahedra in the nesosilicates, they aren't mostly isometric. One answer is that in many of them, octahedrally coordinated aluminum cations link to neighbors to form sheets or chains of aluminum octahedra, which are then cross-linked by the silica units.
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