View source for Nepheline

{{short description|Silica-undersaturated aluminosilicate mineral}}
{{distinguish|Nephrite|Nepheline syenite}}
{{Infobox mineral
| name        = Nepheline
| category    = [[Tectosilicate]] [[minerals]], [[feldspathoid]] [[Mineral group|group]] 
| boxwidth    = 
| boxbgcolor  = 
| image       = Nepheline3.jpg
| caption     = 
| formula     = (Na,K)AlSiO<sub>4</sub>
| IMAsymbol   = Nph<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref>
| molweight   = 146.08 g/mol
| strunz      = 9.FA.05
| system      = [[Hexagonal]]
| class       = Pyramidal (6) <br/>[[H–M symbol]]: (6)
| symmetry    = ''P''6<sub>3</sub>
| color       = White, gray, brown, brownish gray, or reddish white
| habit       = Massive granular to prismatic
| twinning    = On [1010], [3365], and [1122]
| cleavage    = [1010] Poor
| fracture    = Subconchoidal
| mohs        = 6
| luster      = Vitreous to greasy
| refractive  = ''n''<sub>ω</sub> = 1.529–1.546 <br> ''n''<sub>ε</sub> = 1.526–1.542
| opticalprop = Uniaxial (−)
| birefringence = ''δ'' = 0.003–0.004
| pleochroism = 
| streak      = White
| gravity     = 2.55–2.65 (average: 2.59)
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| diaphaneity = 
| other       = non-radioactive, non-magnetic, non-fluorescent
| references  = <ref name=HoM>{{cite web |last1=Anthony |first1=John W. |last2=Bideaux |first2=Richard A. |last3=Bladh |first3=Kenneth W. |last4=Nichols |first4=Monte C. |title=Nepheline |url=http://www.handbookofmineralogy.org/pdfs/nepheline.pdf |website=Handbook of Mineralogy |publisher=Mineral Data Publishing |access-date=13 August 2022 |date=2005}}</ref><ref name=mindat>{{Mindat |id=2880 |name=Nepheline}}</ref><ref name=webmin>{{Cite web|last1=Barthelmy|first1=David|year=2014|url=http://www.webmineral.com/data/Nepheline.shtml|access-date=13 August 2022|title = Nepheline Mineral Data|website=Webmineral.com}}</ref>
}}

'''Nepheline''', also called '''nephelite''' ({{etymology|grc|{{lang|grc|νεφέλη}} (nephélē)|cloud}}), is a rock-forming [[mineral]] in the [[feldspathoid]] group{{spaced en dash}}a silica-undersaturated [[aluminosilicate]], [[sodium|Na]]<sub>3</sub>[[potassium|K]][[aluminium|Al]]<sub>4</sub>[[silicon|Si]]<sub>4</sub>[[oxygen|O]]<sub>16</sub>, that occurs in [[intrusive rock|intrusive]] and [[volcanic rock]]s with low silica, and in their associated [[pegmatite]]s. It is used in [[glass]] and [[ceramic]] manufacturing and other industries, and has been investigated as an ore of [[aluminium]].

== Description and properties ==
[[File:Nepheline-Schorlomite-250211.jpg|thumb|left|Grayish-white nepheline crystals with dark [[schorlomite]] from Bou-Agrao Mount, Tamazeght complex, High Atlas Mountains, Morocco. Size: {{cvt|6.0|x|4.4|x|3.8|cm|1}}|204x204px]]

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]].<ref name="klein-hurlbut-1993-546-547">{{cite book |last1=Klein |first1=Cornelis |last2=Hurlbut |first2=Cornelius S. Jr. |title=Manual of Mineralogy (after James D. Dana) |date=1993 |publisher=Wiley |location=New York |isbn=047157452X |pages=546–547 |edition=21st}}</ref> The crystals appear to have more symmetry than they actually possess,<ref>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=9780195106916 |page=35}}</ref> but unsymmetrical etched figures produced artificially on the prism faces indicate that the crystals are [[wiktionary:hemimorphic|hemimorphic]] and [[wiktionary:tetartohedral|tetartohedral]], the only element of symmetry being a polar hexad axis.<ref>{{cite book |last1=Borchardt-Ott |first1=Walter |title=Crystallography |date=1995 |publisher=Springer |location=Berlin |isbn=3540594787 |page=159 |edition=2nd}}</ref> Nepheline is found in compact, granular aggregates, and can be white, yellow, gray, green, or reddish. Its [[hardness]] on the [[Mohs scale of mineral hardness|Mohs scale]] is 5.5 to 6, and its [[specific gravity]] 2.60–2.65. It is often translucent with a greasy [[lustre (mineralogy)|luster]].<ref name="klein-hurlbut-1993-546-547"/>

The low [[refractive index|index of refraction]] and the feeble [[birefringence|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]].<ref name="klein-hurlbut-1993-546-547"/> 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 [[Halite|salt]].<ref name="eb11">{{cite EB1911|wstitle=Nepheline |volume= 19 | pages = 383 |last1=Spencer |first1=Leonard James}}</ref> For this reason, a clear crystal of nepheline becomes cloudy when immersed in [[acid]].<ref name="klein-hurlbut-1993-546-547"/>

The mineral is prone to alteration to [[zeolite]]s (especially [[natrolite]]), [[sodalite]], [[kaolin]], or compact [[muscovite]].<ref name="eb11"/>{{sfn|Nesse|2000|p=226}}

==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 K<sub>2</sub>O of 8.1% The range of compositions seen in natural nepheline is 3% to 12% K<sub>2</sub>O.{{sfn|Klein|Hurlbut|1993|pp=544-545}} Small amounts of calcium may be present as well.{{sfn|Nesse|2000|p=226}}

At elevated temperature, nepheline forms a complete [[solid solution]] series with [[kalsilite]], KAlSiO<sub>4</sub>. At temperatures below about {{convert|1000|C|F|sp=us}}, 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).<ref name="klein-hurlbut-1993-546-547"/>

==Occurrence==
Nephelinite is a rock-forming mineral found in silica-poor [[igneous rock]]s. These include [[nepheline syenite]], [[foidite]], and [[phonolite]]. It is often found along with [[leucite]], sodalite, potassium feldspars, and sodium-rich [[plagioclase]], [[amphibole]]s, or [[pyroxene]]s, but almost never in association with [[quartz]].{{sfn|Nesse|2000|p=226}} Notable outcrops of nepheline-bearing rocks are found on the [[Kola Peninsula]]; in Norway and South Africa; and at [[Litchfield, Maine]]; [[Magnet Cove, Arkansas]]; and [[Beemerville, New Jersey]], in the United States.  [[Syenite]]s found near [[Bancroft, Ontario]] contain large deposits of high-purity nepheline.{{sfn|Klein|Hurlbut|1993|p=547}}

Elaeolite (a name given by M. H. Klaproth 1809, from [[Greek language|Greek]] words for oil [ἔλαιον] and stone [λίθος]; {{langx|de|Fettstein}})<ref name="eb11"/> is a massive form of translucent nepheline with a darker color and greasy luster.<ref>{{cite book |date=2009 |pages=298 |doi=10.1007/978-3-540-72816-0_7337|isbn=978-3-540-72795-8 |chapter=Eleolite |title=Dictionary of Gems and Gemology }}</ref>

==Uses==
Because of its high [[alumina]] content, [[iron]]-free nepheline is valued for use in [[glass manufacturing]] in place of feldspar. Most of the nepheline used for this purpose comes from [[Ontario]]. 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.{{sfn|Klein|Hurlbut|1993|p=547}} It can be used as a filler in paints, plastics, foam rubber,{{sfn|Nesse|2000|p=226}} and sorbent.<ref>{{Cite journal |last1=Zosin |first1=A. P. |last2=Priimak |first2=T. I. |last3=Koshkina |first3=L. B. |last4=Masloboev |first4=V. A. |date=2005 |title=Sorption-Active Materials Based on an Acid-Nonresistant Aluminosilicate Nepheline, Used for Defluorination of Industrial Sewage |url=http://link.springer.com/10.1007/s11167-005-0453-2 |journal=[[Russian Journal of Applied Chemistry]] |language=en |volume=78 |issue=7 |pages=1077–1083 |doi=10.1007/s11167-005-0453-2 |s2cid=91524499 |issn=1070-4272|url-access=subscription }}</ref> Nepheline of Kiya Shaltyr deposit (Kemerovo Region, Russia) is also used as a raw material for aluminium manufacturing.<ref>{{Cite book |url=https://www.mnr.gov.ru/docs/o_sostoyanii_i_ispolzovanii_mineralno_syrevykh_resursov_rossiyskoy_federatsii/gosudarstvennyy_doklad_o_sostyanii_i_ispolzovanii_mineralno_syrevykh_resursov_rossiyskoy_federatsii/ |title=Government report on the state and use of mineral and raw resources of the Russian Federation in 2019 |publisher=Ministry of natural resources and environment of the Russian Federation |year=2020 |editor-last=Kiselev |editor-first=E.A. |location=Moscow |pages=139–145 |language=RU |access-date=2023-04-05 |archive-date=2021-09-24 |archive-url=https://web.archive.org/web/20210924131359/https://www.mnr.gov.ru/docs/o_sostoyanii_i_ispolzovanii_mineralno_syrevykh_resursov_rossiyskoy_federatsii/gosudarstvennyy_doklad_o_sostyanii_i_ispolzovanii_mineralno_syrevykh_resursov_rossiyskoy_federatsii/ |url-status=dead }}</ref>

==References==
{{Reflist}}

* {{EB1911|wstitle=Nepheline|author=[[Leonard James Spencer]]}}

==External links==
*{{commons category-inline}}

{{Authority control}}

[[Category:Feldspathoid]]
[[Category:Sodium minerals]]
[[Category:Potassium minerals]]
[[Category:Aluminium minerals]]
[[Category:Hexagonal minerals]]
[[Category:Minerals in space group 173]]
[[Category:Gemstones]]
[[Category:Industrial minerals]]