The Tectosilicate Subclass
(The Framework Silicates)
A subclass of the Silicates

This subclass is often called the "Framework Silicates" because its structure is composed of interconnected tetrahedrons going outward in all directions forming an intricate framework analogous to the framework of large building. In this subclass all the oxygens are shared with other tetrahedrons giving a silicon to oxygen ratio of 1:2. In the near pure state of only silicon and oxygen the mineral is quartz (SiO2). But the tectosilicates are not that simple. It turns out that the aluminum ion can easily substitute for the silicon ion in the tetrahedrons. In other subclasses this occurs to a limited extent but in the tectosilicates it is a major basis of the varying structures. While the tetrahedron is nearly the same with an aluminum at its center, the charge is now a negative five (-5) instead of the normal negative four (-4). Since the charge in a crystal must be balanced, additional cations are needed in the structure and this is the main reason for the great variations within this subclass.

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