In metasomatosis ( Greek μετα (meta) 'mit-, nach-, um-'; and σῶμα (sóma) 'body') or displacement , the material of a rock or certain rock components is replaced by another material. In this way, pseudomorphoses of minerals can arise. Metasomatosis is a borderline case of metamorphosis , as it is generally not isochemical. In contrast to isochemical metamorphosis, metasomatosis changes the elementary chemical composition of the rock (allochemical metamorphosis).
Metasomatic processes primarily take place in the end phases of the differentiation of magmas , which are characterized by pneumatolytic and hydrothermal processes . The rock is attacked here by more or less hot, aggressive fluids . The minerals present react with the substances dissolved in the fluids to form other minerals.
A typical metasomatic rock is the skarn , a typical process alkali metasomatosis, in which potassium and sodium are added to the formation of potassium feldspar and albite . The alkali metasomatism plays an important role in Granitisation origin named by granite -like rocks from non-granites as feldspar greywacke or paragneisses , in which the emerging feldspar crystals displace the original rock more or less. This process leads, for example, to the formation of eye gneiss .
Also fossils can be caused by metasomatism. Examples are silicified wood or hard parts whose original material (mostly calcite or aragonite ) has been dissolved and replaced by another, e.g. B. by silicon dioxide such as quartz , chalcedony and opal ( silicification ) or sulfides such as pyrite or marcasite ( silicification ).