Aluminosilicates
Alumosilicates or aluminosilicates , also known as aluminosilicates or aluminosilicates (both names are often used synonymously in German and English, as they are derived from the British-English name for aluminum - aluminum - or the corresponding American-English name - aluminum - derive), are the collective names for minerals and chemical compounds from the group of silicates , which are made up of the basic building blocks SiO 4 - tetrahedron and AlO 4 - tetrahedron . They are among the most important components of the earth's crust .
A silicon atom or an aluminum atom is therefore surrounded by four oxygen atoms. These tetrahedra always share the oxygen atoms (i.e. Si-O-Al bonds). Mathematically, aluminosilicates contain SiO 2 and Al 2 O 3 . The Al: Si ratio cannot exceed 1. The Löwenstein rule applies to aluminosilicates , according to which AlO 4 tetrahedra are never directly linked to one another. The Al-O-Al assembly is thus avoided. Pure aluminates are extremely rare in nature. Here, aluminum can behave in a chemically similar manner to silicon . In installation of aluminum (Al 3+ instead of Si 4+ charge balance must be in the mineral lattice) by incorporation of other positively charged ions ( cations ) take place. Alumosilicates are z. B. feldspar , feldspar representatives and zeolites such as zeolite A or sodalite .
In contrast, silicates with octahedrally coordinated aluminum are called aluminum silicates . Examples are andalusite , kyanite and sillimanite .
Aluminosilicate glass
Depending on its chemical composition, glass can be divided into different types of glass, which have different properties such as fusibility with metals and chemical resistance. Borosilicate glasses are roughly divided into 6 different groups based on the composition of oxides . One group is that of aluminosilicate glasses. In this group one can again distinguish between two categories:
Alkaline earth aluminosilicate glass
Characteristic for this type of glass is the freedom from alkali oxides and an SiO 2 content of 15-25% with an alkaline earth content of 15%. Very high transformation temperatures and softening points are typical features. The main fields of application are glass bulbs for halogen lamps , high-temperature glass thermometers and thermally and electrically highly resilient resistors.
Alkali aluminosilicate glass
The Al 2 O 3 content of alkaline earth aluminosilicate glass is typically 10-35% and the alkali content is over 10%. The high alkali content prepares the glass for an ion exchange with larger alkali ions in order to improve the compressive strength of the surface. Because of this property, the glass is particularly suitable for use in touch displays, solar cells and coated safety glass. High transformation temperatures and excellent mechanical properties, such as hardness and resistance to scratches, are characteristic features of this type of glass.