Melilithite
Melilithite is a dark gray to black ultramafic rock that belongs to the group of igneous rocks and occurs in the form of volcanic rocks , diatrems and dike rocks . The name is derived from the mineral Melilith , which by definition must make up more than 10% of the mineral inventory. Due to the mineral stock and the geochemical characteristics, melilithite is one of the undersaturated alkaline rocks . The plutonic counterpart is melilitolite .
Mineral composition and chemistry
Melilithites typically have a porphyry structure . Olivine , clinopyroxene and melilite occur as insects , which are in a fine-grained to dense base mass of melilite, clinopyroxene, spinel , perovskite , nepheline , apatite and the like. a. exist. Increasing levels of nepheline result in a smooth transition to melilite-containing nephelinites .
Melilithite is characterized by low levels of SiO 2 (<40 percent by weight ) and aluminum (Al 2 O 3 often <10 percent by weight) and high alkaline earth metal contents (CaO and MgO each 10 to> 15 percent by weight).
Occurrence
Melilithit is a product of intraplate volcano and is provided with island volcanoes (z. B. Hawaii ) and continental Riftsystemen associated (z. B. East African grave fraction ). He often performs together with basaltic rocks in the broader sense of, but can also be the sole volcanic rock of a volcanic province to be. In Germany, melilithites occur in the Urach volcanic area on the Swabian Alb , in Hegau and in the area of the Upper Rhine Graben .
literature
- Mitchell RH (1996): The Melilitite Clan. In Mitchell RH (Ed.) Undersaturated Alkaline Rocks: Mineralogy, Petrogenesis, and Economic Potential . Mineralogical Association of Canada, Short Course Volume 24, Winnipeg, pp. 123-151.
Individual evidence
- ↑ RW Le Maitre (Ed.): Igneous Rocks. A Classification and Glossary of Terms . 2nd Edition. Cambridge University Press, Cambridge 2004, ISBN 0-521-61948-3 , pp. 11 .