Nephelinite

Nephelinite is a fine-grained or aphanitisches volcanic rock that ( a feldspar ) consists essentially of clinopyroxene and nepheline. Another, felsisches Gemengteil also leucite may occur. If olivine is present, then the rock is referred to as olivine nephelinite. Nephelinite is dark in color and may resemble basalt. In the basalt may occur as a minor component nepheline basalt consists mainly of clinopyroxene and plagioclase, however.

Basalt, alkali basalt, basanite, nephelinite tephritic and nephelinite differ in the relative proportions of plagioclase and nepheline. A relevant parameter for the classification of such rocks is the ratio to the sum of nepheline nepheline plagioclase. Basanit has values ​​between 0.1 and 0.6 and containing more than 10 % olivine; tephritic nephelinite has values ​​between 0.6 and 0.9; Nephelinite those of about 0.9. Le Maitre (2002) define and discuss these and other criteria for the classification of igneous rocks.

Nephelinite is an example of a silica - undersaturated volcanic rock. The degree of saturation with silica can be evaluated according to the normative mineral composition and calculated from chemical analysis, or can be determined for fully crystallized volcanic rock after the modal mineralogical composition of representative samples. Silica - oversaturated rocks contain free quartz or other forms of SiO2 Minerals ( polymorphism ). Silica - undersaturated, mafic volcanic rocks often contain other hand, mainly forsterite ( in the olivine group), but neither magnesium nor orthopyroxene feldspars. Silica - saturated igneous rocks lie between these two classes.

Occurrence

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 reduction continental areas; An example of this is the nephelinite lava flow of Hamada in southwestern Japan, which originated in the late Miocene. The high - alkaline volcanism of Ol Doinyo Lengai volcano field in Tanzania is also associated with nephelinite. The Nyiragongo, an African volcano, which is known for its almost constant lava lake activity, throwing lava from melilite nephelinite. The unusual chemical composition of igneous rock can be a contributing cause for the unusually low viscosity and therefore high flowability of these lavas.

Olivine nephelinite occurs with the Wells Gray - Clearwater volcanic field in east-central British Columbia and at the Volcano Mountain in the central Yukon Territory. Melilite - olivine nephelinite intrusions from the Cretaceous period were found in the area around Uvalde, Texas.