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 A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...

s. It is used in glass and ceramic manufacturing and other industries, and has been investigated as an ore of aluminium.

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 on the

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

is 5.5–6, and its specific gravity 2.60–2.65. It is often translucent with a greasy luster. The low

index of refraction In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

and the feeble double refraction in nepheline are nearly the same as in quartz; 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. An important determinative character of nepheline is the ease with which it is decomposed by hydrochloric acid, with separation of gelatinous silica (which may be readily stained by coloring matters) and cubes of salt. For this reason, a clear crystal of nepheline becomes cloudy when immersed in

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

. The mineral is prone to alteration to zeolites (especially natrolite),

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

, 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, 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 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 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. 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 rocks. 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 Foidolite () is a rare phaneritic (coarse-grained) intrusive igneous rock in which more than 60% (by volume) of light-coloured minerals are feldspathoids. Crystals of alkali feldspar, plagioclase, biotite, amphibole, pyroxene, and/or olivine ma ...

, and

phonolite Phonolite is an uncommon extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained). Phonolite is a variation of the igneous ...

. It is often found along with leucite, sodalite, potassium feldspars, and sodium-rich plagioclase,

amphibole Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...

s, or

pyroxene 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) ...

s, but almost never in association with quartz. Notable outcrops of nepheline-bearing rocks are found on the Kola Peninsula; in Norway and South Africa; and at

Litchfield, Maine Litchfield is a town in Kennebec County, Maine, United States. The population was 3,586 at the 2020 census. Litchfield is included in the Augusta, Maine micropolitan New England City and Town Area. Litchfield was incorporated in 1795 and was or ...

; Magnet Cove, Arkansas; and

Beemerville, New Jersey Beemerville is an unincorporated community located within Wantage Township in Sussex County, New Jersey, United States. Space Farms Zoo and Museum, established in 1927, is a combination of a natural environment for animals and historical museum ...

, in the United States.

Syenite Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proport ...

s 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 ��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-free nepheline is valued for use in

glass manufacturing Glass production involves two main methods – the float glass process that produces sheet glass, and glassblowing that produces bottles and other containers. It has been done in a variety of ways during the history of glass. Glass container ...

in place of feldspar. Most of the nepheline used for this purpose comes from Ontario. Nepheline produced as a byproduct of

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...

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 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