Mixed crystal
A mixed crystal (Mk) is a crystal or crystallite which consists of at least two different chemical elements , the foreign atoms or ions being randomly distributed. These can be stored either in the interstitial sites ( solid solution or interstitial solution) or an atom of the other element by substituting replace ( substitution solid solution ). Mixed crystals are therefore solid solutions which, if they have metallic properties, are also called alloys .
Mixed crystal formation
There are two types of mixed crystal formation:
- In the homeotypic mixed crystal formation, the substances involved have the same type of crystal structure as the mixed crystal consequently also has; Compare two-component systems with complete solubility in the solid state .
- In heterotypic mixed crystal formation, however, one of the substances involved forces its crystal structure on the other during installation. Compare two-component systems with limited solubility in the solid state .
If the atoms of the elements involved are no longer randomly distributed in the lattice, one speaks of intermediate crystals or intermetallic compounds whose crystal structure does not correspond to any of the basic elements.
Mixed crystals in minerals
In the realm of minerals , mixed crystals are rarely formed from pure substances , but mostly from chemical compounds . Two minerals with a defined composition form a so-called mixed crystal row (mixed row for short), the end members of which they represent and whose mixed crystals are often given names that do not allow any conclusions to be drawn about their composition.
For example, labradorite , a mixed crystal from the albite - anorthite series , consists of 50–70% anorthite.
The minerals forsterite (Mg 2 SiO 4 ) and fayalite (Fe 2 SiO 4 ) form a complete mixed crystal series, the olivine series ([Mg, Fe] 2 SiO 4 ). Further examples are the biotite series between phlogopite and annite and the melilite series between akermanite and gehlenite .
Some solid solution rows contain miscibility gaps . This means that with certain relationships between the two end links, two phases exist. One example is the plagioclase series albite-anorthite. Many rows of mixed crystals are complete at high temperatures, but have such miscibility gaps at low temperatures. Some crystals can dissolve small amounts of other elements. This is known as diadochia . One example is the incorporation of magnesium in calcite (CaCO 3 ). There is no complete miscibility with dolomite [CaMg (CO 3 ) 2 ] or with magnesite (MgCO 3 ).
literature
- Jürgen Ruge, Helmut Wohlfahrt: Technology of materials. For students of mechanical engineering and civil engineering, process engineering and materials science . 7th revised and expanded edition. Vieweg + Teubner, Braunschweig et al. 2002, ISBN 3-528-63021-3 , pp. 47, 48 . (Google Books) .
See also
Web links
- Mineral Atlas: Mixed Crystal (Wiki)