Alkali rock
As alkali rock in which Petrologie an igneous rock with a high proportion of alkali metals (especially sodium and potassium ) and low SiO 2 content. Petrographically , this is expressed in the appearance of foids next to feldspar and the lack of quartz . In addition, numerous other minerals can occur in alkali stones : pyroxene (often aegirine , augite , melilite ), amphibole (including Richterite , kaersutite ), olivine , mica , garnet and others. Alkaline rocks can form volcanic rocks , sub- volcanic rocks or dike rocks and plutonites . Due to the very variable mineralogical composition, there is an abundance of different rock names. The better-known and widespread representatives include basanite , tephrite , essexite , phonolite , foid syenite , melilithite , shoshonite , nephelinite , hawaiite , alkali basalt , ankaramite , lamproit , lamprophyr and kimberlite .
Occurrence
Alkaline rocks occur both in the oceanic (e.g. Hawaii and Iceland ) and in the continental area. Today they are extracted from numerous active volcanic areas , but are only involved in the formation of the earth's crust to a minor extent . In the continental area they are spatially and temporally connected with the formation of rift systems such as the East African Rift . Alkali rocks often appear together with carbonatites and form so-called alkali rock-carbonatite complexes . An example of this in Germany is the Kaiserstuhl in the southern Upper Rhine Graben .
Alkali rocks or alkali basalts are of particular interest due to the fact that their magma sources are very deep in the earth's mantle, they rise very quickly from this depth and often bring out the earth's mantle xenolites . These xenolites are very important rocks for science, as they give a direct insight into the chemical and mineralogical composition of the upper mantle.
Chemism
With a low SiO 2 content, alkali stones can contain well over 10 percent by weight of alkali metal oxides (mainly K 2 O and Na 2 O) and are therefore in the upper left area of the TAS diagram . Based on the alkali ratio (K 2 O: Na 2 O) , a distinction is made within the group of alkali stones into a Na series, a K series and a high K series. While the oceanic representatives have comparatively low alkali contents and a relatively constant alkali ratio, the continental alkali rocks show a very large variability.
literature
- Winter JD (2001): An introduction to igneous and metamorphic petrology. Prentice Hall. ISBN 0132403420
- Coltorti, M., Gregoire, M. (2008): Metasomatism in Oceanic and Continental Lithospheric Mantle. Geological Society, Special Publication 293.
- Vinx, Roland; Schumann, Andreas (1991) Weathering and Alteration of Alkaline Rocks. The Earth Sciences; 9, 1; 1-11; doi : 10.2312 / geosciences . 1991.9.1 .