nephelinite
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Nephelinite is a fine-grained or
aphanitic Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This geologic ...
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
made up almost entirely of
nepheline Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatite ...
and
clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
(variety
augite Augite is a common rock-forming pyroxene mineral with formula . 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 The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
is present, the rock may be classified as an olivine nephelinite. Nephelinite is dark in color and may resemble
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
in hand specimen. However,
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
consists mostly of clinopyroxene (augite) and calcic
plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
. Basalt,
alkali basalt Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields. Alkali basalt ...
,
basanite Basanite () is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and plagioclase and forms from magma low in silica and enriched in alkali metal oxides that s ...
, 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 is a calculation of the composition of a rock sample that estimates the ''idealised mineralogy'' of a rock based on a quantitative chemical analysis according to the principles of geochemistry. Normative mineral calculations ...
. A critical ratio in the classification of these rocks is the ratio nepheline/(nepheline plus plagioclase).
Basanite Basanite () is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and plagioclase and forms from magma low in silica and enriched in alkali metal oxides that s ...
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 is a calculation of the composition of a rock sample that estimates the ''idealised mineralogy'' of a rock based on a quantitative chemical analysis according to the principles of geochemistry. Normative mineral calculations ...
calculated from chemical analyses, or with actual mineralogy for completely crystallized igneous rocks with equilibrated assemblages. Silica-oversaturated rocks contain
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
(or another silica polymorph). Silica-undersaturated mafic igneous rocks contain magnesian olivine but not magnesian orthopyroxene, and/or a
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, and are usually not found in rocks containing prim ...
. 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 ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
at depths sufficient to stabilize
garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...
. Nephelinites are also associated with
carbonatite Carbonatite () is a type of intrusive rock, intrusive or extrusive rock, extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geoche ...
in some occurrences, consistent with source rocks relatively rich in carbon dioxide. Nephelinite is found on ocean islands such as
Oʻahu Oahu () ( Hawaiian: ''Oʻahu'' ()), also known as "The Gathering Place", is the third-largest of the Hawaiian Islands. It is home to roughly one million people—over two-thirds of the population of the U.S. state of Hawaii. The island of O’ ...
, although the rock type is very rare in the
Hawaiian Islands The Hawaiian Islands ( haw, Nā Mokupuni o Hawai‘i) are an archipelago of eight major islands, several atolls, and numerous smaller islets in the North Pacific Ocean, extending some from the island of Hawaii in the south to northernmost Kur ...
. It is found in a variety of continental settings. An example is the
Hamada A hamada ( ar, حمادة, ) is a type of desert landscape consisting of high, largely barren, hard rocky plateaus, where most of the sand has been removed by deflation. The majority of the Sahara is in fact hamada. Other examples are Negev dese ...
nephelinite lava flow in southwest
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
which occurred in the late
Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
age. Nephelinite is also associated with the highly alkalic volcanism of the
Ol Doinyo Lengai Ol Doinyo Lengai (Oldoinyo Lengai), "Mountain of God" in the Maasai language, is an active volcano located in the Gregory Rift, south of Lake Natron within the Arusha Region of Tanzania, Africa. Part of the volcanic system of the East African Ri ...
volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...
field in
Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands and ...
.
Nyiragongo Mount Nyiragongo ( ) is an active stratovolcano with an elevation of in the Virunga Mountains associated with the Albertine Rift. It is located inside Virunga National Park, in the Democratic Republic of the Congo, about north of the town of ...
, another African volcano known for its semipermanent lava lake activity, erupts lava made of
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 ( Ca, Na)2( Al, Mg, ...
nephelinite. The unusual chemical makeup of this
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
may be a factor in the unusual fluidity of its lavas. Olivine nephelinite flows also occur in the
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 Carib ...
in east-central
British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...
and at
Volcano Mountain Volcano Mountain is a cinder cone in central Yukon, Yukon Territory, Canada, located a short distance north of Fort Selkirk, near the confluence of the Pelly River, Pelly and Yukon Rivers. Volcano Mountain is called Nelrúna in the Northern Tutc ...
in central
Yukon Territory Yukon (; ; formerly called Yukon Territory and also referred to as the Yukon) is the smallest and westernmost of Canada's three territories. It also is the second-least populated province or territory in Canada, with a population of 43,964 as ...
. Melilite olivine nephelinite intrusives of
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
age are found in the area around
Uvalde, Texas Uvalde is a city and the county seat of Uvalde County, Texas, United States. The population was 15,217 at the 2020 census. Uvalde is located in the Texas Hill Country, west of downtown San Antonio and east of the Mexico–United States bord ...
.


References

* 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).
Hamada nephelinite, SW Japan


volcano, Eastern Rift Valley, North Tanzania * * Daniel P. Miggins, Charles D. Blome, and David V. Smith
Preliminary Argon40/Argon39 geochronology of igneous intrusions from Uvalde County, Texas: Defining a more precise eruption history for the southern Balcones Volcanic Province
U.S. Geological Survey Open-File Report 2004-1031. In ''usgs.gov''


Bibliography


External links

*
Nefelinita
in ''- Atlas de rocas ígneas'' {{Volcanoes Igneous petrology Ultramafic rocks Volcanic rocks