Nepheline syenite
Encyclopedia
Nephelene syenite is a holocrystalline plutonic rock
that consists largely of nepheline
and alkali
feldspar
. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granite
s, but dark green varieties are also known. Phonolite
is the fine-grained extrusive equivalent.
(terms discussed in igneous rock
). Nepheline is a feldspathoid
, a solid-solution mineral, that does not coexist with quartz
; rather, nepheline would react with quartz to produce alkali feldspar.
They are distinguished from ordinary syenite
s not only by the presence of nepheline but also by the occurrence of many other minerals rich in alkalis and in rare earths and other incompatible element
s. Alkali feldspar dominates, commonly represented by orthoclase
and the exsolved lamellar albite
, form perthite
. In some rocks the potash feldspar
, in others the soda feldspar predominates. Fresh clear microcline
is very characteristic of some types of nepheline syenite.
Sodalite
, colorless and transparent in thin section, but frequently pale blue in the hand specimens, is the principal feldspathoid
mineral in addition to nepheline. Reddish-brown to black triclinic aenigmatite occurs also in these rocks. Extremely iron-rich olivine
is rare, but is present in some nepheline syenite. Other minerals common in minor amounts include sodium-rich pyroxene
, biotite
, titanite
, zircon
, iron oxide
s, apatite
, fluorite
, melanite garnet
, and zircon
. Cancrinite
occurs in several nepheline-syenites. A great number of interesting and rare minerals have been recorded from nepheline syenites and the pegmatite
veins which intersect them.
. The presence of nepheline
and absence of quartz
are the fundamental difference. Biotite
is generally of low content and the main mafic
minerals are clinopyroxene (±) and amphibole
(±). The macroscopic colour is grey, being little darker than granite. There is high-grade metamorphic rock
originated from nepheline syenite
that is characterized by gneiss
texture of very rare occurrence. It is called nepheline syenite gneiss or litchfieldite
. An example is found at Canaã village, State of Rio de Janeiro
, Brazil
.
, equidirectional, and gross with grain size of 2 mm to 5 mm. In certain rare cases, the rock contains alkaline feldspar phenocrysts of 2 cm to 5 cm in length and 5 mm to 2 cm in thickness. The phenocrysts demonstrate orientation and eventually show cumulative texture.
, nepheline
, clinopyroxene (±), amphibole
(±), and biotite
(±). Nepheline
is the main feldspathoid
. Quartz
and orthopyroxene are absent. According to the IUGS classification nomenclature (International Union of Geological Sciences, Streckeisen, 1978), nepheline syenite has 10%Phonolite
is the fine-grained equivalent. In case nepheline is less than 10%, the rock is called alkaline syenite with nepheline or pulaskite. The similar rock without quartz and nepheline is denominated alkaline syenite or syenite. Because of the presence of feldspathoid
s, nepheline syenite is classified to be a typical alkaline rock.
The alkaline feldspar is not potassic, but generally sodic-potassic, which is characterized by interlocking perthite
, called anorthoclase
. In the alkali feldspar almost pure albite
domains are observed. Nepheline
generally shows partial alteration into natrolite
and cancrinite
. The clinopyroxene is sodic whose composition varies from hedenbergite
to aegirine-augite. This mineral eventually presents resorption
shape. The reaction rim constituted by amphibole
and/or biotite
is commonly observed. The amphibole is of high alkali, such as alkaline hornblende
and riebeckite
. The alkaline clinopyroxene and amphibole are characteristics of typical alkaline rocks. Biotite is annite, with high Fe/Mg ratio.
The accessory minerals are magnetite
, ilmenite
, apatite
, and titanite
. Eventually, sodalite
is found along hydrothermal fractures. Different from granite, zircon
is rare and if present it is as xenocrysts. On the other hand, nepheline syenite gneiss contains abundant and large zircon crystals.
zones. Nepheline syenite and phonolite may be derived by crystal fractionation from more mafic silica-undersaturated mantle-derived melts, or as partial melts of such rocks. Igneous rocks with nepheline in their normative mineralogy
commonly are associated with other unusual igneous rocks such as carbonatite
.
, Norway
, Greenland
, Sweden
, the Ural Mountains
, the Pyrenees
, Italy
, Brazil
, China
, the Transvaal
region, and Magnet Cove igneous complex
of Arkansas
, as well as on oceanic islands.
Phonolite lavas formed in the East African rift in particularly large quantity, and the volume there may exceed the volume of all other phonolite occurrences combined, as discussed by Barker (1983).
Nepheline-normative rocks occur in close association with the Bushveld Igneous Complex
, possibly formed from partial melting of the wall rocks to that large ultramafic layered intrusion.
Nepheline syenites are rare; there is only one occurrence in Great Britain
and one in France
and Portugal
. They are known also in Bohemia
and in several places in Norway
, Sweden
and Finland
. In the Americas
these rocks have been found in Texas
, Arkansas
and Massachusetts
, also in Ontario
, British Columbia
and Brazil
. South Africa
, Madagascar
, India
, Tasmania
, Timor
and Turkestan
are other localities for the rocks of this series.
Rocks of this class also occur in Brazil
(Serra de Tingua) containing sodalite and often much augite, in the western Sahara
and Cape Verde Islands; also at Zwarte Koppies in the Transvaal
, Madagascar
, São Paulo
in Brazil
, Paisano Pass in West Texas
and Montreal
, Canada
. The rock of Salem, Massachusetts
, United States
, is a mica
-foyaite rich in albite
and aegirine
: it accompanies granite
and essexite
. Litchfieldite
is another well-marked type of nepheline-syenite, in which albite is the dominant feldspar
. It is named after Litchfield, Maine
, United States, where it occurs in scattered blocks. Biotite, cancrinite and sodalite are characteristic of this rock. A similar nepheline-syenite is known from Hastings County, Ontario
, and contains hardly any orthoclase
, but only albite feldspar. Nepheline is very abundant and there is also cancrinite, sodalite, scapolite, calcite, biotite and hornblende. The lujaurites are distinguished from the rocks above described by their dark color, which is due to the abundance of minerals such as augite, aegirine, arfvedsonite and other kinds of amphibole
. Typical examples are known near Lujaur on the White Sea
, where they occur with umptekites and other very peculiar rocks. Other localities for this group are at Julianehaab in Greenland
with sodalite-syenite; at their margins they contain pseudomorph
s after leucite
. The lujaurites frequently have a parallel-banding or gneissose structure. Sodalite-syenites in which sodalite very largely or completely takes the place of nepheline occur in Greenland, where they contain also microcline-perthite, aegirine, arfvedsonite and eudialyte.
Cancrinite
syenite
, with a large percentage of cancrinite, has been described from Dalekarlia, Sweden
and from Finland
. We may also mention urtite from Lujaur Urt on the White Sea
, which consists very largely of nepheline, with aegirine and apatite, but no feldspar. Jacupirangite (from Jacupiranga in Brazil) is a blackish rock composed of titaniferous augite, magnetite
, ilmenite
, perofskite and nepheline, with secondary biotite.
Foyaite: foyaites are named after Foya in the Serra de Monchique
, in southern Portugal
. These are K-feldspar-nepheline syenites containing <10% ferromagnesian minerals, usually pyroxene
, hornblende
and biotite
.
Laurdalite: The laurdalites, from Laurdal in Norway
, are grey or pinkish, and in many ways closely resemble the larvikite
s of southern Norway, with which they occur. They contain anorthoclase
feldspars, biotite or greenish augite, much apatite and in some cases, olivine.
Ditroite: Ditroite derives is name from Ditrau, Transylvania
, Romania
. It is essentially a microcline
, sodalite
and cancrinite
variety of nepheline syenite. It contains also orthoclase
, nepheline
, biotite, aegirine
, acmite.
, magnesia
and iron
are never present in great quantity, though somewhat more variable than the other components. A worldwide average of the major elements in nepheline syenite tabulated by Barker (1983) is listed below, expressed as weight percent oxides.
Nepheline syenite is characterized by high ratio of (Na2O+K2O)/SiO2 and (Na2O+K2O)/Al2O3, which are represented respectively by the existence of nepheline and alkaline mafic
minerals. Therefore, it is classified geochemically as alkaline rock. This rock has low Fe and Mg contents, in total about 3wt%, and in this sense it is classified to be felsic rock. However, the SiO2 content is not so high, being 53% to 62wt%, which is equivalent to andesite
and diorite
. In this sense, it corresponds to intermediate rock. Light rare earth element
s are highly concentrated, indicating that the magma is highly differentiated.
The normative mineralogy
of this average composition contains about 22 percent nepheline
and 66 percent feldspar
.
Because nepheline syenite lacks quartz
and is rich in feldspar and nepheline, it is used in the manufacturing of glass and ceramics.
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
that consists largely 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 alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
s, but dark green varieties are also known. Phonolite
Phonolite
Phonolite is a rare igneous, volcanic rock of intermediate composition, with aphanitic to porphyritic texture....
is the fine-grained extrusive equivalent.
Petrology
Nepheline syenites are silica-undersaturated and some are peralkalinePeralkaline
Peralkaline rocks include those igneous rocks which have a deficiency of aluminium such that sodium and potassium are in excess of that needed for feldspar. The presence of aegerine and riebeckite are indicative of peralkaline conditions....
(terms discussed in 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...
). Nepheline is 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....
, a solid-solution mineral, that does not coexist with 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,...
; rather, nepheline would react with quartz to produce alkali feldspar.
They are distinguished from ordinary syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
s not only by the presence of nepheline but also by the occurrence of many other minerals rich in alkalis and in rare earths and other incompatible element
Incompatible element
An incompatible element is a term used in petrology and geochemistry to describe an element that is unsuitable in size and/or charge to the cation sites of the minerals, and is defined by the partition coefficient between rock-forming minerals and melt being much smaller than 1..During the...
s. Alkali feldspar dominates, commonly represented by orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
and the exsolved lamellar albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
, form perthite
Perthite
Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar . Typically the host grain is orthoclase or microcline, and the lamellae are albite...
. In some rocks the potash feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, in others the soda feldspar predominates. Fresh clear microcline
Microcline
Microcline is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures...
is very characteristic of some types of nepheline syenite.
Sodalite
Sodalite
Sodalite is a rich royal blue mineral widely enjoyed as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent...
, colorless and transparent in thin section, but frequently pale blue in the hand specimens, is the principal 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....
mineral in addition to nepheline. Reddish-brown to black triclinic aenigmatite occurs also in these rocks. Extremely iron-rich 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 rare, but is present in some nepheline syenite. Other minerals common in minor amounts include sodium-rich pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
, titanite
Titanite
Titanite, or sphene , is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are typically present...
, zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
, iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
s, apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
, fluorite
Fluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
, melanite 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...
, and zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
. Cancrinite
Cancrinite
Cancrinite is a complex carbonate and silicate of sodium, calcium and aluminium with the formula Na6Ca2[2|Al6Si6O24]·2H2O. It is classed as a member of the feldspathoid group of minerals; the alkali feldspars that are poor in silica. Yellow, orange, pink, white or even blue, it has a vitreous or...
occurs in several nepheline-syenites. A great number of interesting and rare minerals have been recorded from nepheline syenites and the pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
veins which intersect them.
Macroscopic aspects
Macroscopic aspects of nepheline syenite are similar to those of graniteGranite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
. The presence 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 absence of 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,...
are the fundamental difference. Biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
is generally of low content and the main mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
minerals are clinopyroxene (±) and amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
(±). The macroscopic colour is grey, being little darker than granite. There is high-grade metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
originated from nepheline syenite
Nepheline syenite
Nephelene syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known...
that is characterized by gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
texture of very rare occurrence. It is called nepheline syenite gneiss or litchfieldite
Litchfieldite
Litchfieldite is a rare igneous rock. It is a variety of nepheline syenite and is composed of two near pure phases of feldspar, albite and microcline, with predominance of the first one, plus nepheline, sodalite, cancrinite and calcite...
. An example is found at Canaã village, State of Rio de Janeiro
Rio de Janeiro
Rio de Janeiro , commonly referred to simply as Rio, is the capital city of the State of Rio de Janeiro, the second largest city of Brazil, and the third largest metropolitan area and agglomeration in South America, boasting approximately 6.3 million people within the city proper, making it the 6th...
, Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
.
Texture
The rock is holocrystalline, generally equigranularEquigranular
An equigranular material is composed chiefly of crystals of similar orders of magnitude to one another. Basalt and gabbro commonly exhibit an equigranular texture....
, equidirectional, and gross with grain size of 2 mm to 5 mm. In certain rare cases, the rock contains alkaline feldspar phenocrysts of 2 cm to 5 cm in length and 5 mm to 2 cm in thickness. The phenocrysts demonstrate orientation and eventually show cumulative texture.
Mineral composition
The main minerals are alkali feldsparAlkali feldspar
The alkali feldspar group are those feldspar minerals rich in the alkali elements like potassium. The alkali feldspars include: anorthoclase, microcline, orthoclase and sanidine....
, 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...
, clinopyroxene (±), amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
(±), and biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
(±). 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...
is the main 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....
. 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,...
and orthopyroxene are absent. According to the IUGS classification nomenclature (International Union of Geological Sciences, Streckeisen, 1978), nepheline syenite has 10%
Phonolite
Phonolite is a rare igneous, volcanic rock of intermediate composition, with aphanitic to porphyritic texture....
is the fine-grained equivalent. In case nepheline is less than 10%, the rock is called alkaline syenite with nepheline or pulaskite. The similar rock without quartz and nepheline is denominated alkaline syenite or syenite. Because of the presence of 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....
s, nepheline syenite is classified to be a typical alkaline rock.
The alkaline feldspar is not potassic, but generally sodic-potassic, which is characterized by interlocking perthite
Perthite
Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar . Typically the host grain is orthoclase or microcline, and the lamellae are albite...
, called anorthoclase
Anorthoclase
The mineral anorthoclase is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member exists in larger proportion. It typically consists of between 10 and 36 percent of KAlSi3O8 and between 64 and 90 percent of NaAlSi3O8....
. In the alkali feldspar almost pure albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
domains are observed. 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...
generally shows partial alteration into natrolite
Natrolite
Natrolite is a tectosilicate mineral species belonging to the zeolite group. It is a hydrated sodium and aluminium silicate with the formula . The type locality is Hohentwiel, Hegau, Germany....
and cancrinite
Cancrinite
Cancrinite is a complex carbonate and silicate of sodium, calcium and aluminium with the formula Na6Ca2[2|Al6Si6O24]·2H2O. It is classed as a member of the feldspathoid group of minerals; the alkali feldspars that are poor in silica. Yellow, orange, pink, white or even blue, it has a vitreous or...
. The clinopyroxene is sodic whose composition varies from hedenbergite
Hedenbergite
Hedenbergite, CaFeSi2O6, is the iron rich end member of the pyroxene group having a monoclinic crystal system. The mineral is extremely rarely found as a pure substance, and usually has to be synthesized in a lab. It was named in 1819 after M.A. Ludwig Hedenberg, who was the first to define...
to aegirine-augite. This mineral eventually presents resorption
Resorption
Resorption is a process by which a structure is remodeled.Types include:* Bone resorption* Root resorption...
shape. The reaction rim constituted by amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
and/or biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
is commonly observed. The amphibole is of high alkali, such as alkaline hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
and riebeckite
Riebeckite
Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula [][Na2][32][2|Si8O22]. It forms a series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in...
. The alkaline clinopyroxene and amphibole are characteristics of typical alkaline rocks. Biotite is annite, with high Fe/Mg ratio.
The accessory minerals are magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
, ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
, apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
, and titanite
Titanite
Titanite, or sphene , is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are typically present...
. Eventually, sodalite
Sodalite
Sodalite is a rich royal blue mineral widely enjoyed as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent...
is found along hydrothermal fractures. Different from granite, zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
is rare and if present it is as xenocrysts. On the other hand, nepheline syenite gneiss contains abundant and large zircon crystals.
Genesis
Silica-undersaturated igneous rocks typically are formed by low degrees of partial melting in the Earth's mantle. Carbon dioxide may dominate over water in source regions. Magmas of such rocks are formed in a variety of environments, including continental rifts, ocean islands, and supra-subduction positions in subductionSubduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
zones. Nepheline syenite and phonolite may be derived by crystal fractionation from more mafic silica-undersaturated mantle-derived melts, or as partial melts of such rocks. Igneous rocks with nepheline in their 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....
commonly are associated with other unusual igneous rocks such as 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....
.
Distribution
Nepheline syenites and phonolites occur in CanadaCanada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
, Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
, the Ural Mountains
Ural Mountains
The Ural Mountains , or simply the Urals, are a mountain range that runs approximately from north to south through western Russia, from the coast of the Arctic Ocean to the Ural River and northwestern Kazakhstan. Their eastern side is usually considered the natural boundary between Europe and Asia...
, the Pyrenees
Pyrenees
The Pyrenees is a range of mountains in southwest Europe that forms a natural border between France and Spain...
, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
, Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, the Transvaal
Transvaal Province
Transvaal Province was a province of the Union of South Africa from 1910 to 1961, and of its successor, the Republic of South Africa, from 1961 until the end of apartheid in 1994 when a new constitution subdivided it.-History:...
region, and Magnet Cove igneous complex
Magnet Cove igneous complex
The Magnet Cove igneous complex is a small alkalic ring complex lying to the west of the town of Magnet Cove in Hot Spring County, Arkansas. It and the adjacent town are so named due to the existence of magnetite and the terrain being a cove, a basin-shaped valley.The complex is of Mesozoic age,...
of Arkansas
Arkansas
Arkansas is a state located in the southern region of the United States. Its name is an Algonquian name of the Quapaw Indians. Arkansas shares borders with six states , and its eastern border is largely defined by the Mississippi River...
, as well as on oceanic islands.
Phonolite lavas formed in the East African rift in particularly large quantity, and the volume there may exceed the volume of all other phonolite occurrences combined, as discussed by Barker (1983).
Nepheline-normative rocks occur in close association with the Bushveld Igneous Complex
Bushveld igneous complex
The Bushveld Igneous Complex is a large layered igneous intrusion within the Earth's crust which has been tilted and eroded and now outcrops around what appears to be the edge of a great geological basin, the Transvaal Basin. Located in South Africa, the BIC contains some of the richest ore...
, possibly formed from partial melting of the wall rocks to that large ultramafic layered intrusion.
Nepheline syenites are rare; there is only one occurrence in Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
and one in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
and Portugal
Portugal
Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...
. They are known also in Bohemia
Bohemia
Bohemia is a historical region in central Europe, occupying the western two-thirds of the traditional Czech Lands. It is located in the contemporary Czech Republic with its capital in Prague...
and in several places in Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
and Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
. In the Americas
Americas
The Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
these rocks have been found in Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
, Arkansas
Arkansas
Arkansas is a state located in the southern region of the United States. Its name is an Algonquian name of the Quapaw Indians. Arkansas shares borders with six states , and its eastern border is largely defined by the Mississippi River...
and Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
, also in Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
, 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 Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, Tasmania
Tasmania
Tasmania is an Australian island and state. It is south of the continent, separated by Bass Strait. The state includes the island of Tasmania—the 26th largest island in the world—and the surrounding islands. The state has a population of 507,626 , of whom almost half reside in the greater Hobart...
, Timor
Timor
Timor is an island at the southern end of Maritime Southeast Asia, north of the Timor Sea. It is divided between the independent state of East Timor, and West Timor, belonging to the Indonesian province of East Nusa Tenggara. The island's surface is 30,777 square kilometres...
and Turkestan
Turkestan
Turkestan, spelled also as Turkistan, literally means "Land of the Turks".The term Turkestan is of Persian origin and has never been in use to denote a single nation. It was first used by Persian geographers to describe the place of Turkish peoples...
are other localities for the rocks of this series.
Rocks of this class also occur in Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
(Serra de Tingua) containing sodalite and often much augite, in the western Sahara
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...
and Cape Verde Islands; also at Zwarte Koppies in the Transvaal
Transvaal Province
Transvaal Province was a province of the Union of South Africa from 1910 to 1961, and of its successor, the Republic of South Africa, from 1961 until the end of apartheid in 1994 when a new constitution subdivided it.-History:...
, Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
, São Paulo
São Paulo
São Paulo is the largest city in Brazil, the largest city in the southern hemisphere and South America, and the world's seventh largest city by population. The metropolis is anchor to the São Paulo metropolitan area, ranked as the second-most populous metropolitan area in the Americas and among...
in Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
, Paisano Pass in West Texas
West Texas
West Texas is a vernacular term applied to a region in the southwestern quadrant of the United States that primarily encompasses the arid and semi-arid lands in the western portion of the state of Texas....
and Montreal
Montreal
Montreal is a city in Canada. It is the largest city in the province of Quebec, the second-largest city in Canada and the seventh largest in North America...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. The rock of Salem, Massachusetts
Salem, Massachusetts
Salem is a city in Essex County, Massachusetts, United States. The population was 40,407 at the 2000 census. It and Lawrence are the county seats of Essex County...
, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, is a mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
-foyaite rich in albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
and aegirine
Aegirine
Aegirine is a member of the clinopyroxene group of inosilicates. Aegirine is the sodium endmember of the aegirine-augite series. Aegirine has the chemical formula NaFeSi2O6 in which the iron is present as Fe3+. In the aegirine-augite series the sodium is variably replaced by calcium with iron and...
: it accompanies granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
and essexite
Essexite
Essexite , also called nepheline monzogabbro , is a dark gray or black holocrystalline plutonic igneous rock. Its name is derived from the type locality in Essex County, MA.Modern petrology identifies rocks according to mineralogical criteria...
. Litchfieldite
Litchfieldite
Litchfieldite is a rare igneous rock. It is a variety of nepheline syenite and is composed of two near pure phases of feldspar, albite and microcline, with predominance of the first one, plus nepheline, sodalite, cancrinite and calcite...
is another well-marked type of nepheline-syenite, in which albite is the dominant feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. It is named after Litchfield, Maine
Litchfield, Maine
Litchfield is a town in Kennebec County, Maine, United States. The population was 3,110 at the 2000 census.-Geography:According to the United States Census Bureau, the town has a total area of , of which, of it is land and of it is water....
, United States, where it occurs in scattered blocks. Biotite, cancrinite and sodalite are characteristic of this rock. A similar nepheline-syenite is known from Hastings County, Ontario
Hastings County, Ontario
Hastings County is located in the province of Ontario, Canada. It is The Cheese Capital of Canada. Geographically, it is located on the border of Eastern Ontario and Central Ontario. The population was 125,915 in 2001 and grew to 130,474 in the 2006 Canada Census...
, and contains hardly any orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
, but only albite feldspar. Nepheline is very abundant and there is also cancrinite, sodalite, scapolite, calcite, biotite and hornblende. The lujaurites are distinguished from the rocks above described by their dark color, which is due to the abundance of minerals such as augite, aegirine, arfvedsonite and other kinds of amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
. Typical examples are known near Lujaur on the White Sea
White Sea
The White Sea is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is surrounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the northeast. The whole of the White Sea is under Russian sovereignty and considered to be part of...
, where they occur with umptekites and other very peculiar rocks. Other localities for this group are at Julianehaab in Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
with sodalite-syenite; at their margins they contain pseudomorph
Pseudomorph
In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form , resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another...
s after leucite
Leucite
Leucite is a rock-forming mineral composed of potassium and aluminium tectosilicate K[AlSi2O6]. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in 1821, they are not optically isotropic, and are therefore pseudo-cubic. Goniometric measurements made by...
. The lujaurites frequently have a parallel-banding or gneissose structure. Sodalite-syenites in which sodalite very largely or completely takes the place of nepheline occur in Greenland, where they contain also microcline-perthite, aegirine, arfvedsonite and eudialyte.
Cancrinite
Cancrinite
Cancrinite is a complex carbonate and silicate of sodium, calcium and aluminium with the formula Na6Ca2[2|Al6Si6O24]·2H2O. It is classed as a member of the feldspathoid group of minerals; the alkali feldspars that are poor in silica. Yellow, orange, pink, white or even blue, it has a vitreous or...
syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
, with a large percentage of cancrinite, has been described from Dalekarlia, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
and from Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
. We may also mention urtite from Lujaur Urt on the White Sea
White Sea
The White Sea is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is surrounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the northeast. The whole of the White Sea is under Russian sovereignty and considered to be part of...
, which consists very largely of nepheline, with aegirine and apatite, but no feldspar. Jacupirangite (from Jacupiranga in Brazil) is a blackish rock composed of titaniferous augite, magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
, ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
, perofskite and nepheline, with secondary biotite.
Nomenclature
There is a wide variety of silica-undersaturated and peralkaline igneous rocks, including many informal place-name varieties named after the locations in which they were first discovered. In many cases these are plain nepheline syenites containing one or more rare minerals or mineraloids, which do not warrant a new formal classification. These include;Foyaite: foyaites are named after Foya in the Serra de Monchique
Serra de Monchique
The Serra de Monchique is a chain of mountains in the western part of the Algarve region of Portugal. The chain's highest point is the peak of Fóia, at 902 meters.The town of Monchique is located in this area....
, in southern Portugal
Portugal
Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...
. These are K-feldspar-nepheline syenites containing <10% ferromagnesian minerals, usually pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
and biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
.
Laurdalite: The laurdalites, from Laurdal in Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
, are grey or pinkish, and in many ways closely resemble the larvikite
Larvikite
Larvikite is a variety of monzonite, notable for the presence of handsome, thumbnail-sized crystals of feldspar. These feldspars are known as ternary because they contain significant components of all three end-member feldspars...
s of southern Norway, with which they occur. They contain anorthoclase
Anorthoclase
The mineral anorthoclase is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member exists in larger proportion. It typically consists of between 10 and 36 percent of KAlSi3O8 and between 64 and 90 percent of NaAlSi3O8....
feldspars, biotite or greenish augite, much apatite and in some cases, olivine.
Ditroite: Ditroite derives is name from Ditrau, Transylvania
Transylvania
Transylvania is a historical region in the central part of Romania. Bounded on the east and south by the Carpathian mountain range, historical Transylvania extended in the west to the Apuseni Mountains; however, the term sometimes encompasses not only Transylvania proper, but also the historical...
, Romania
Romania
Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...
. It is essentially a microcline
Microcline
Microcline is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures...
, sodalite
Sodalite
Sodalite is a rich royal blue mineral widely enjoyed as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent...
and cancrinite
Cancrinite
Cancrinite is a complex carbonate and silicate of sodium, calcium and aluminium with the formula Na6Ca2[2|Al6Si6O24]·2H2O. It is classed as a member of the feldspathoid group of minerals; the alkali feldspars that are poor in silica. Yellow, orange, pink, white or even blue, it has a vitreous or...
variety of nepheline syenite. It contains also orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
, 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...
, biotite, aegirine
Aegirine
Aegirine is a member of the clinopyroxene group of inosilicates. Aegirine is the sodium endmember of the aegirine-augite series. Aegirine has the chemical formula NaFeSi2O6 in which the iron is present as Fe3+. In the aegirine-augite series the sodium is variably replaced by calcium with iron and...
, acmite.
Chemical composition
The chemical peculiarities of the nepheline-syenites are well marked. They are exceedingly rich in alkalis and in alumina (hence the abundance of felspathoids and alkali feldspars) with silica varying from 50 to 56%, while limeLime (mineral)
Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
, magnesia
Magnesium oxide
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
are never present in great quantity, though somewhat more variable than the other components. A worldwide average of the major elements in nepheline syenite tabulated by Barker (1983) is listed below, expressed as weight percent oxides.
Nepheline syenite is characterized by high ratio of (Na2O+K2O)/SiO2 and (Na2O+K2O)/Al2O3, which are represented respectively by the existence of nepheline and alkaline mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
minerals. Therefore, it is classified geochemically as alkaline rock. This rock has low Fe and Mg contents, in total about 3wt%, and in this sense it is classified to be felsic rock. However, the SiO2 content is not so high, being 53% to 62wt%, which is equivalent to andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
and diorite
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
. In this sense, it corresponds to intermediate rock. Light rare earth element
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
s are highly concentrated, indicating that the magma is highly differentiated.
- SiO2Silicon dioxideThe chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
— 54.99% - TiO2Titanium dioxideTitanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
— 0.60% - Al2O3Aluminium oxideAluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
— 20.96% - Fe2O3Iron(III) oxideIron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...
— 2.25% - FeOIron(II) oxideIron oxide, also known as ferrous oxide, is one of the iron oxides. It is a black-colored powder with the chemical formula . It consists of the chemical element iron in the oxidation state of 2 bonded to oxygen. Its mineral form is known as wüstite. Iron oxide should not be confused with rust,...
— 2.05% - MnOManganese oxideManganese oxide is a generic term used to describe a variety of manganese oxides and hydroxides. It may refer to:* Manganese oxide, MnO* Manganese oxide, Mn3O4* Manganese oxide, Mn2O3* Manganese dioxide, , MnO2...
— 0.15% - MgOMagnesium oxideMagnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
— 0.77% - CaOCalcium oxideCalcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
— 2.31% - Na2OSodium oxideSodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses, though not in a raw form. Treatment with water affords sodium hydroxide....
— 8.23% - K2OPotassium oxidePotassium oxide is an ionic compound of potassium and oxygen. This pale yellow solid, the simplest oxide of potassium, is a rarely encountered, highly reactive compound...
— 5.58% - H2O — 1.47%
- P2O5Phosphorus pentoxidePhosphorus pentoxide is a chemical compound with molecular formula P4O10 . This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant.-Structure:...
— 0.13%
The 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....
of this average composition contains about 22 percent 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 66 percent feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
.
Because nepheline syenite lacks 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,...
and is rich in feldspar and nepheline, it is used in the manufacturing of glass and ceramics.