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Different arrangements of metal oxides, featured silica and its forms. (A) Examples of coordination geometries that MxOy can assume, from top to bottom: quadratic planar, tetrahedral, octahedral and square antiprismatic. (B) SiO4 tetrahedra in silica (SiO2). (C) Amorphous SiO2. (D) Examples of different polymorphs of crystalline SiO2: cristobalite, tridymite and quartz. Note that further crystalline polymorphs exist. Photographs of minerals are reproduced from www.mindat.org.
