silicate

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One of a group of minerals containing silicon and oxygen in tetrahedral units of SiO₄, bound together in various ways to form specific structural types. Silicates are the chief rock-forming minerals. Most rocks are composed, wholly or in part, of silicates (the main exception being limestones). Glass is a manufactured complex polysilicate material in which other elements (boron in borosilicate glass) have been incorporated.

Generally, additional cations are present in the structure, especially Al³⁺, Fe²⁺, Mg²⁺, Ca²⁺, Na⁺, K⁺, but quartz and other polymorphs of SiO₂ are also considered to be silicates; stishovite (a high-pressure form of SiO₂) is a rare exception to the usual tetrahedral coordination of silica and oxygen.

In orthosilicates, the oxygens are all ionically bonded to cations such as Mg²⁺ or Fe²⁺ (as olivines), and are not shared between tetrahedra. All other silicate structures involve some degree of oxygen sharing between adjacent tetrahedra. For example, beryl is a ring silicate based on tetrahedra linked by sharing oxygens to form a circle. Pyroxenes are single chain silicates, with chains of linked tetrahedra extending in one direction through the structure; amphiboles are similar but have double chains of tetrahedra. In micas, which are sheet silicates, the tetrahedra are joined to form continuous sheets that are stacked upon one another. Framework silicates, such as feldspars and quartz, are based on three-dimensional frameworks of tetrahedra in which all oxygens are shared.

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