XII. Class of Minerals
SILICATES
The largest and most important class of minerals is silicates-minerals whose fundamental building unit of the crystal lattice is the (SiO4)4- tetrahedron. Silicates include the most important petrogenic minerals, such as feldspars, quartz, mica, amphiboles, pyroxenes, olivine, and others, and they form about 90% of the Earth's crust. A great variety of silicate minerals occurs on Earth as a result of processes of formation and modification of the lithosphere over billions of years. Many silicate minerals have great practical value as raw materials in the glass, ceramics, and construction industries, some are important ore minerals, while others are characterized by features of gemstones.

Depending on the way the (SiO4)4- tetrahedra are connected in the structure of silicates, we distinguish between nesosilicates, sorosilicates, cyclosilicates, inosilicates, phyllosilicates, and tectosilicates. The structure of nesosilicates is characterized by isolated (SiO4)4- tetrahedra. In sorosilicates, two (SiO4)4- tetrahedra are connected, while in cyclosilicates, three, four, or six (SiO4)4- tetrahedra form a ring. The structure of inosilicates is characterized by (SiO4)4- tetrahedra connected in infinite single or double chains, while phyllosilicates are connected in layers. The crystal lattice of tectosilicates is such that all (SiO4)4- tetrahedra are connected at all four corners to form a three-dimensional structural framework.