tectoszilikátok

Tectosilicates, also known as framework silicates, represent the largest and most important group of minerals in the Earth's crust. In these minerals, each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is shared by two silicon atoms. This creates a three-dimensional framework structure where the silicon-oxygen tetrahedra (SiO4) are linked in all directions. The general formula for tectosilicates is Si2O4, or more simply SiO2, but this is often modified by the substitution of aluminum for silicon in the tetrahedral sites.

When aluminum substitutes for silicon, the overall charge of the framework becomes negative, requiring cations to balance the charge. Common cations found in tectosilicates include sodium (Na+), potassium (K+), calcium (Ca2+), and barium (Ba2+). These cations are typically located in the interstitial spaces within the framework structure. The size and charge of these cations influence the specific mineral species formed and can lead to variations in crystal structure and properties.

The most prominent examples of tectosilicates include quartz, feldspars, and feldspathoids. Quartz (SiO2) is a pure tectosilicate with a simple, highly stable framework. Feldspars are the most abundant minerals in the Earth's crust and include potassium feldspar (orthoclase, microcline) and plagioclase feldspars, which are solid solutions between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Feldspathoids, such as nepheline and leucite, are also tectosilicates but are found in silica-undersaturated magmas. The structural characteristics of tectosilicates contribute to their hardness, cleavage, and general chemical stability.