Nephelinite

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Nephelinite

Nephelinite is a fine-grained or aphanitic igneous rock

Igneous rock

Igneous rock is one of the three main Rock types . Igneous rock is formed by magma being cooled and becoming solid . They may form with or without crystallization, either below the surface as Intrusion rocks or on the surface as extrusive rocks....
made up almost entirely of nepheline

Nepheline

Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Sodium3PotassiumAluminium4silicon4Oxygen16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites....
and clinopyroxene (variety augite

Augite

Augite is a Silicate_minerals#Single_chain_inosilicates: mineral described chemically as SiO3 or calcium magnesium iron silicate. The crystals are monoclinic and prismatic....
). If olivine

Olivine

The mineral olivine is a magnesium iron Silicate minerals with the formula 2siliconoxygen4. It is one of the most common minerals on Earth, and has also been identified in meteorites and on the Moon, Mars, and comet Wild 2....
is present, the rock may be classified as an olivine nephelinite. Nephelinite is dark in color and may resemble basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet....
in hand specimen. However, basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet....
consists mostly of clinopyroxene (augite) and calcic plagioclase
Plagioclase

Plagioclase is a very important series of Silicate minerals minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series ....
.

Basalt, alkali basalt, basanite, tephritic nephelinite, and nephelinite differ partly in the relative proportions of plagioclase and nepheline.

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Encyclopedia

Nephelinite is a fine-grained or aphanitic igneous rock

Igneous rock

Nepheline

Augite

Augite is a Silicate_minerals#Single_chain_inosilicates: mineral described chemically as SiO3 or calcium magnesium iron silicate. The crystals are monoclinic and prismatic....
). If olivine

Olivine

Basalt

Basalt is a common extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet....
in hand specimen. However, basalt

Basalt

Normative mineralogy

Normative mineralogy is a geochemical calculation of the whole rock geochemistry of a rock sample which estimates the idealised mineralogy of a rock according to the principles of geochemistry....
. A critical ratio in the classification of these rocks is the ratio nepheline/(nepheline plus plagioclase). Basanite

Basanite

Basanite is an igneous rock, volcanic rock with aphanitic to porphyritic texture. The mineral assembly is usually abundant feldspathoids , plagioclase, and augite, together with olivine and lesser iron-titanium oxides such as ilmenite and magnetite-ulvospinel; minor alkali feldspar may be present, as illustrated by the position of the fiel...
has a value of this ratio between 0.1 and 0.6 and also contains more than 10% olivine. Tephritic nephelinite has a value between 0.6 and 0.9. Nephelinite has a value greater than 0.9. Le Maitre (2002) defines and discusses these and other criteria in the classification of igneous rocks.

Nephelinite is an example of a silica-undersaturated igneous rock. The degree of silica saturation can be evaluated with normative mineralogy

Normative mineralogy

Normative mineralogy is a geochemical calculation of the whole rock geochemistry of a rock sample which estimates the idealised mineralogy of a rock according to the principles of geochemistry....
calculated from chemical analyses, or with actual mineralogy for completely crystallized igneous rocks with equilibrated assemblages. Silica-oversaturated rocks contain quartz

Quartz

Quartz is the most abundant mineral in the Earth's continental crust . It is made up of a Crystal structure of silica tetrahedra. Quartz has a hardness of 7 on the Mohs scale and a density of 2.65 g/cm?....
(or another silica polymorph

Polymorphism (materials science)

Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements....
). Silica-undersaturated mafic igneous rocks contain magnesian olivine but not magnesian orthopyroxene, and/or a feldspathoid

Feldspathoid

The feldspathoids are a group of Silicate minerals minerals which resemble feldspars but have a different structure and much lower silica content....
. Silica-saturated igneous rocks fall in between these two classes.

Silica-undersaturated, mafic igneous rocks are much less abundant than silica-saturated and oversaturated basalts. Genesis of the less common mafic rocks such as nephelinite is usually ascribed to more than one of the following three causes:

relatively high pressure of melting;
relatively low degree of fractional melting in a mantle source;
relatively high dissolved carbon dioxide in the melt.

Nephelinites and similar rocks typically contain relatively high concentrations of elements such as the light rare earths, as consistent with a low degree of melting of mantle peridotite

Peridotite

A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic and ultrabasic, as the rock contains less than 45% silica....
at depths sufficient to stabilize garnet

Garnet

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" comes from the Latin language granatus , possibly a reference to the Punica granatum , a plant with red seeds similar in shape, size, and color to some garnet crystals....
. Nephelinites are also associated with carbonatite

Carbonatite

Carbonatites are intrusive or extrusive igneous Rock defined by mineralogic composition consisting of greater than 70 volume percent carbonate minerals....
in some occurrences, consistent with source rocks relatively rich in carbon dioxide.

Nephelinite is found on ocean islands such as Oahu, although the rock type is very rare in the Hawaiian islands

Hawaiian Islands

The Hawaiian Islands are an archipelago of 19 islands and atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the Hawaii in the south to northernmost Kure Atoll....
. It is found in a variety of continental settings. An example is the Hamada

Hamada

A hamada is a type of desert landscape consisting of largely barren, hard, rocky plateaus, with very little sand. A hamada may sometimes also be called a reg , though this more properly refers to a stony plain rather than a highland....
nephelinite lava flow in southwest Japan

Japan

Japan is an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south....
which occurred in the late Miocene

Miocene

The Miocene is a Geologic time scale of the Neogene period and extends from about 23.03 to 5.33 million years before the present. As with other older geologic periods, the rock beds that define the start and end are well identified but the exact dates of the start and end of the period are uncertain....
age. Nephelinite is also associated with the highly alkalic volcanism of the Ol Doinyo Lengai

Ol Doinyo Lengai

Ol Doinyo Lengai is a volcano located in the north of Tanzania and is part of the volcanic system of the Great Rift Valley in Eastern Africa. It is located in the eastern Rift Valley, south of both Lake Natron and Kenya....
volcanic field in Tanzania

Tanzania

Tanzania , officially the United Republic of Tanzania , is a country in East Africa that is bordered by Kenya and Uganda on the north, Rwanda, Burundi and the Democratic Republic of the Congo on the west, and Zambia, Malawi and Mozambique on the south....
. Nyiragongo, another African volcano known for its semipermanent lava lake activity, erupts lava made of melilite

Melilite

Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and akermanite....
nephelinite. The unusual chemical makeup of this igneous rock

Igneous rock

Wells Gray-Clearwater volcanic field

The Wells Gray-Clearwater volcanic field, also called the Clearwater Cone Group, is a potentially active monogenetic volcanic field in east-central British Columbia, Canada, located approximately north of Kamloops, British Columbia....
in east-central British Columbia

British Columbia

British Columbia is the westernmost of Canada's Provinces and territories of Canada and is famed for its natural beauty, as reflected in its Latin motto, Splendor sine occasu ....
and at Volcano Mountain in central Yukon Territory. Melilite olivine nephelinite intrusives of Cretaceous

Cretaceous

The Cretaceous , usually abbreviated K for its German translation Kreide, is a geologic period from circa to million years ago . In the geologic timescale, the Cretaceous follows on the Jurassic period and is followed by the Paleogene period....
age are found in the area around Uvalde, Texas

Uvalde, Texas

Uvalde is a city in and the county seat of Uvalde County, Texas, Texas, United States. The population was 14,929 at the 2000 United States Census....
.

Nephelinite

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