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polytetrahedron In crystallography, a tetrahedron in which negative and positive forms are developed to different degrees in the same crystal, together with the possibility of raised secondary faces on each principal tetrahedron face. In mineralogy, the term may refer to the ability of SiO4 tetrahedra in silicate structures to polymerize, giving a variety of structures as a result of oxygen-sharing. These may result in chains (pyroxenes), double chains (amphiboles), sheets (micas), and three-dimensional framework silicates (quartz and feldspars).