Nephelinite
Encyclopedia
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
made up almost entirely of nepheline
Nepheline
Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
and clinopyroxene (variety augite
Augite
Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...
). If olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
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 grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
in hand specimen. However, basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
consists mostly of clinopyroxene (augite) and calcic plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate 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. Alkali basalt may contain minor nepheline and does contain nepheline in its CIPW normative mineralogy
Normative mineralogy
Normative mineralogy is a geochemical calculation of the whole rock geochemistry of a rock sample that 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, 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...
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 that 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 second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
(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 tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks....
. 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, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
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" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
. Nephelinites are also associated with carbonatite
Carbonatite
Carbonatites are intrusive or extrusive igneous rocks defined by mineralogic composition consisting of greater than 50 percent carbonate minerals. Carbonatites may be confused with marble, and may require geochemical verification....
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 eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of 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.Hamadas exist in contrast to ergs, which are large areas of...
nephelinite lava flow in southwest Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, 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 geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
age. Nephelinite is also associated with the highly alkalic volcanism of the Ol Doinyo Lengai
Ol Doinyo Lengai
Ol Doinyo Lengai is an active 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. It is unique among active volcanoes in that it produces...
volcanic field in Tanzania
Tanzania
The United Republic of Tanzania is a country in East Africa bordered by Kenya and Uganda to the north, Rwanda, Burundi, and the Democratic Republic of the Congo to the west, and Zambia, Malawi, and Mozambique to the south. The country's eastern borders lie on the Indian Ocean.Tanzania is a state...
. 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 åkermanite. A generalized formula for common melilite is 2[SiO7]. Discovered in 1793 near Rome, it has a yellowish, greenish brown color...
nephelinite. The unusual chemical makeup of this igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
may be a factor in the unusual fluidity of its lavas.
Olivine nephelinite flows also occur in the Wells Gray-Clearwater volcanic field
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. It is situated in the Cariboo Mountains of the Columbia Mountains and on the...
in east-central British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
and at Volcano Mountain in central Yukon Territory. Melilite olivine nephelinite intrusives of Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
age are found in the area around Uvalde, Texas
Uvalde, Texas
Uvalde is a city in and the county seat of Uvalde County, Texas, United States. The population was 14,929 at the 2000 census.Uvalde was founded by Reading Wood Black in 1853 as the town of Encina. In 1856, when the county was organized, the town was renamed Uvalde for Spanish governor Juan de...
.