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

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

in the

feldspathoid The feldspathoids are a group of tectosilicate minerals which resemble feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, pota ...

groupa silica-undersaturated aluminosilicate, Na₃ K Al₄ Si₄ O₁₆, that occurs in intrusive and

volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...

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

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

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, porcela ...

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

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

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. 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 plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...

on the

Mohs scale The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by the Ger ...

is 5.5 to 6, and its

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

2.60–2.65. It is often translucent with a greasy luster. The low index of refraction and the feeble double refraction in nepheline are nearly the same as in

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

; 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 equipment, 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 ...

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

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

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

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

. For this reason, a clear crystal of nepheline becomes cloudy when immersed in

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

. The mineral is prone to alteration to zeolites (especially natrolite), sodalite, kaolin, or compact muscovite.

Structure and composition

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

tridymite Tridymite is a high-temperature polymorphism (materials science), 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 sili ...

, with aluminum substituting for every other silicon atom. This structure produces one nearly hexagonal interstitial site and three irregular interstitial sites per unit cell. In ideal nepheline, the hexagonal sites are occupied by

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

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 homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solutio ...

series with kalsilite, KAlSiO₄. At temperatures below about , there is a wide miscibility gap between nepheline and kalsilite, similar to the miscibility gap between microcline 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 tectosilicat ...

. A composition falling in this gap will experience exsolution 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 ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

s. These include nepheline syenite, foidite, and phonolite. It is often found along with leucite, sodalite, potassium feldspars, and sodium-rich

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

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

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

Kola Peninsula The Kola Peninsula (; ) is a peninsula in the extreme northwest of Russia, and one of the largest peninsulas of Europe. Constituting the bulk of the territory of Murmansk Oblast, it lies almost completely inside the Arctic Circle and is border ...

; in Norway and South Africa; and at Litchfield, Maine; Magnet Cove, Arkansas; and Beemerville, New Jersey, in the United States. Syenites found near Bancroft, Ontario contain large deposits of high-purity nepheline. Elaeolite (a name given by M. H. Klaproth 1809, from Greek words for oil ��λαιονand stone �ίθος ) 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; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

-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 the southernmost Provinces and territories of Canada, province of Canada. Located in Central Canada, Ontario is the Population of Canada by province and territory, country's most populous province. As of the 2021 Canadian census, it ...

. Nepheline produced as a byproduct of

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...

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, foam rubber, and sorbent. Nepheline of Kiya Shaltyr deposit (Kemerovo Region, Russia) is also used as a raw material 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