Nephelinite
As nephelinite a volcanic rock of the group of is Foidite denotes that as predominantly occurring Foidmineral nepheline contains. Viewed macroscopically, a nephelinite is a fine-grained or aphanitic volcanic rock , which can essentially consist of clinopyroxene and nepheline. Leucite and / or representatives of the sodalite group Sodalite , Haüyn and Nosean can also occur as further, felsic parts of the mixture, but they are less than nepheline in terms of quantity. If olivine is present, then the rock is called olivine nephelinite. Nephelinite is dark in color and can be similar to basalt . Nepheline can occur as a minor component in basalt, but the most important rock mineral there is plagioclase . If the proportion of nepheline (or of the totality of foid minerals) in the felsic components falls below 90%, it is referred to as tephritic or phonolithic nephelinite, with less than 60% the designation after the foid mineral is given up (cf. in the route iron diagram ).
Nephelinite is an example of a silicon dioxide - under saturated volcanic rock (see silicon saturation ). Olivine nephelinite is considered to be the most abundant representative of volcanites with a foiditic composition.
Occurrence
In Germany, nephelinite is found in the volcanic areas of the Eifel , in the Vogelsberg , at Katzenbuckel in the Odenwald , in the Urach volcanic area , in the Hegau and in the northern Hessian volcanic area .
Nephelinite is found on oceanic islands such as Oahu , but is otherwise very rare on the Hawaiian island chain . It has been found in a variety of continental subsidence areas; an example of this is the nephelinite lava flow from Hamada in southwestern Japan , which originated in the late Miocene . The highly alkaline volcanism of the Ol Doinyo Lengai volcanic field in Tanzania is also associated with nephelinite. The Nyiragongo , an African volcano, which for its almost constant lava lake is known -Tätigkeit, throwing lava from Melilith-nephelinite. The unusual chemical composition of this igneous rock can be one of the reasons for the unusually low viscosity and thus high flowability of these lavas.
Olivine nephelinite occurs in the Wells-Gray-Clearwater volcanic field in east-central British Columbia and at Volcano Mountain in the central Yukon Territory . Melilith olivine Nephelinit- intrusions from the Cretaceous period were in the area around Uvalde in Texas found.
literature
- Roger W. Le Maitre (Editor): Igneous Rocks: A Classification and Glossary of Terms. (Recommendations of the International Union of Geological Sciences Subcommission of the Systematics of Igneous Rocks). Cambridge University Press (2002). ISBN 0-521-66215-X
Web links
- N. Abe, S. Arai: Petrology of the ultramafic xenoliths with K-feldspher veinlet from Hamada nephelinite, SW Japan arc. Goldschmidt Conference Abstracts 2003, p. A5 (PDF; 13.4 MB)
- Oldoinyo Lengai - a unique volcano. St. Lawrence University Blogs
- Demant, A. et al .: Volcanological and petrological evolution of Nyiragongo volcano, Virunga volcanic field, Zaire. (now Democratic Republic of the Congo ) Bulletin of Volcanology, Vol. 56, Issue 1, pp. 47-60, 1994, bibcode : 1994BVol ... 56 ... 47D
- Miggins, DP et al .: Preliminary 40Ar / 39Ar geochronology of igneous intrusions from Uvalde County, Texas: Defining a more precise eruption history for the southern Balcones Volcanic Province. USGS Open-File Report 2004-1031 (pdf; 7.94 MB)
- Mineral Atlas: Nephelinite
Individual evidence
- ↑ Roland Vinx: Rock determination in the field . 3. Edition. Spectrum, Heidelberg 2011, ISBN 978-3-8274-2748-9 , p. 246 .