Nepheline, also called nephelite (), is a rock-forming

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

in the

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 ...

groupa silica-undersaturated

aluminosilicate Aluminosilicate minerals ( IMA symbol: Als) are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals. Andalusite, kyanite, and sillimanite are naturall ...

, Na₃ K Al₄ Si₄ O₁₆, that occurs in intrusive and

volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...

s with low silica, and in their associated pegmatites. It is used in

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...

and

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...

manufacturing and other industries, and has been investigated as an ore of

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

Description and properties

Nepheline crystals are rare and belong to the hexagonal system, usually having the form of a short, six-sided prism terminated by the

basal plane In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns t ...

. The crystals appear to have more symmetry than they actually possess, but unsymmetrical etched figures produced artificially on the prism faces indicate that the crystals are hemimorphic and tetartohedral, the only element of symmetry being a polar hexad axis. Nepheline is found in compact, granular aggregates, and can be white, yellow, gray, green, or reddish. Its

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...

on the Mohs scale is 5.5–6, and its

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...

2.60–2.65. It is often translucent with a greasy luster. The low index of refraction and the feeble

double refraction Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefring ...

in nepheline are nearly the same as in

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 ...

; but since the sign of the double refraction is negative in nepheline, while it is positive in quartz, the two minerals are readily distinguished under the

microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...

. An important determinative character of nepheline is the ease with which it is decomposed by

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

, with separation of gelatinous silica (which may be readily stained by coloring matters) and cubes of

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

. For this reason, a clear crystal of nepheline becomes cloudy when immersed in acid. The mineral is prone to alteration to

zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H+. These p ...

s (especially

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. It was named natrolite by Martin Heinrich Klapro ...

sodalite Sodalite ( ) is a tectosilicate mineral with the formula , with royal blue varieties widely used as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. Sodalite is a member of t ...

kaolin Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...

, or compact

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavag ...

Structure and composition

The

backbone of nepheline has a fairly open structure of interlocked six-member rings. This resembles the structure of

tridymite Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is Si O2. Tridymite was first describe ...

, with aluminum substituting for every other silicon atom. This structure produces one nearly hexagonal

interstitial site In crystallography, interstitial sites, holes or voids are the empty space that exists between the packing of atoms (spheres) in the crystal structure. The holes are easy to see if you try to pack circles together; no matter how close you get ...

and three irregular interstitial sites per unit cell. In ideal nepheline, the hexagonal sites are occupied by

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...

ions and the irregular sites by smaller sodium ions, yielding an atomic ratio of sodium to potassium of (3:1). This corresponds to an ideal weight percentage of K₂O of 8.1% The range of compositions seen in natural nepheline is 3% to 12% K₂O. Small amounts of calcium may be present as well. At elevated temperature, nepheline forms a complete

solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...

series with

kalsilite Kalsilite ( K Al Si O4) is a vitreous white to grey feldspathoidal mineral that is found in some potassium-rich lavas, such as from Chamengo Crater in Uganda. It has a relative hardness In materials science, hardness (antonym: softness) is a ...

, KAlSiO₄. At temperatures below about , there is a wide

miscibility gap A miscibility gap is a region in a phase diagram for a mixture of components where the mixture exists as two or more phases – any region of composition of mixtures where the constituents are not completely miscible. The IUPAC Gold Book defines ...

between nepheline and kalsilite, similar to the miscibility gap between

microcline Microcline (KAlSi3O8) 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 slo ...

and

albite Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate ...

. A composition falling in this gap will experience

exsolution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...

as it cools, where nepheline and kalsilite separate into separate microscopic layers (lamellae).

Occurrence

Nephelinite is a rock-forming mineral found in silica-poor

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...

s. These include

nepheline syenite Nepheline 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 al ...

, foidite, and phonolite. It is often found along with

leucite Leucite is a rock-forming mineral of the feldspathoid group, silica-undersaturated and composed of potassium and aluminium tectosilicate KAlSi2O6. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in ...

, sodalite, potassium feldspars, and sodium-rich

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 pro ...

, amphiboles, or pyroxenes, but almost never in association with

. Notable outcrops of nepheline-bearing rocks are found on the

Kola Peninsula sjd, Куэлнэгк нёа̄ррк , image_name= Kola peninsula.png , image_caption= Kola Peninsula as a part of Murmansk Oblast , image_size= 300px , image_alt= , map_image= Murmansk in Russia.svg , map_caption = Location of Murmansk Oblas ...

; in Norway and South Africa; and at Litchfield, Maine;

Magnet Cove, Arkansas Magnet Cove is a census-designated place (CDP) and former town in Hot Spring County, Arkansas, United States. It is located in the Ouachita Mountains southeast of Hot Springs, on Arkansas Highway 51 north of U.S. Highway 270. As of the 2020 censu ...

; and Beemerville, New Jersey, in the United States. Syenites found near

Bancroft, Ontario Bancroft () is a town located on the York River in Hastings County in the Canadian province of Ontario. It was first settled in the 1850s by United Empire Loyalists and Irish immigrants. From the mid-1950s to about 1982, mining was the primary ...

contain large deposits of high-purity nepheline. Elaeolite (a name given by M. H. Klaproth 1809, from

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

words for oil ��λαίονand stone �ίθος german: Fettstein) is a massive form of translucent nepheline with a darker color and greasy luster.

Uses

Because of its high alumina content,

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

-free nepheline is valued for use in glass manufacturing in place of feldspar. Most of the nepheline used for this purpose comes from

Ontario Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central C ...

. Nepheline produced as a byproduct of apatite mining in the Kola Peninsula has found uses in ceramics, leather, rubber, textiles, wood, and the oil industry. It can be used as a filler in paints, plastics, and foam rubber, and has been investigated as an alternative to

bauxite Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO ...

for aluminium manufacturing.

References

External links

* {{Authority control Feldspathoid Sodium minerals Potassium minerals Aluminium minerals Hexagonal minerals Minerals in space group 173 Gemstones Industrial minerals