Nepheline
Nepheline, Na3K(Al4Si4O16) on the IMA list, is a feldspathoid: hexagonal, hardness 5.5 to 6, density 2.55 to 2.66. How much potassium a real crystal holds.
Nepheline is a sodium potassium aluminosilicate, Na3K(Al4Si4O16) on the IMA list and (Na,K)AlSiO4 on the Handbook sheet: hexagonal, hardness 5.5 to 6, density 2.55 to 2.66, uniaxial negative. It is a feldspathoid, not a feldspar. Worked on the sheet’s Monte Somma, Italy analysis, potassium is 21.0% of the alkalis, against the IMA formula’s 25%.
In short
- One species, two formulas. The September 2026 IMA-CNMNC Master List lists nepheline as Na3K(Al4Si4O16), Rd, 2018 s.p., Italy. The Handbook of Mineralogy sheet writes (Na,K)AlSiO4. Read at either end, that shorthand names another species: NaAlSiO4 is trinepheline (A, 2012-024, Myanmar) and KAlSiO4 is kalsilite, kaliophilite and megakalsilite on the IMA list, by script.
- The Na3K check. The IMA formula puts one potassium in four alkali positions: 25%. Recalculated from the sheet’s Monte Somma oxides on 16 oxygens, the crystal holds Na 2.23, K 0.59 and Ca 0.45: potassium is 21.0% of the sodium plus potassium, and 3.28 of the 4 positions are filled. The sheet’s own reading, (Na0.56K0.15Ca0.11)Σ0.82, agrees.
- A feldspathoid. The Alex Strekeisen petrography atlas defines feldspathoids as aluminosilicates of sodium, potassium or calcium having too little silica to form feldspar, and names nepheline the most common. Coombs et al., Canadian Mineralogist 35 (1997), the IMA zeolite report, names nepheline among framework structures that are not zeolites. Why it is not a feldspar is set out on is nepheline a feldspar.
- Documented failure: the balance. A 20.00 g piece reads 12.16 to 12.48 g in water, a window that overlaps 7 of the 10 Handbook feldspar sheets and the kalsilite sheet’s 12.28 to 12.37 g. Optics separate nepheline from feldspar, uniaxial against biaxial, but not from kalsilite, which the sheets also give as uniaxial negative.
- Is nepheline fluorescent? The nepheline sheet prints no fluorescence, by a search that also matches the stray glyph pdftotext leaves for the fl ligature; the same search finds it on the hemimorphite and willemite sheets. This page gives no fluorescence colour; the general picture is on fluorescent minerals explained.
| Material | Formula (source) | K share of Na + K | System and optic class (sheet) | Density; 20.00 g piece in water | IMA row |
|---|---|---|---|---|---|
| Nepheline, end-member | Na3K(Al4Si4O16) (IMA) | 25.0% | Hexagonal; uniaxial (−) | 2.55 to 2.66; 12.16 to 12.48 g | Rd, 2018 s.p., Italy |
| Monte Somma, Italy | (Na0.56K0.15Ca0.11)Σ0.82Al0.97Si1.04O4 (sheet analysis 1) | 21.0%, worked | - | - | As nepheline |
| Synthetic nepheline | (Na0.67K0.33)AlSiO4 (sheet, powder-pattern material) | 33% | - | - | As nepheline |
| Trinepheline | NaAlSiO4 (IMA) | 0% | No sheet consulted | - | A, 2012-024, Myanmar |
| Trikalsilite | K2Na(AlSiO4)3 (IMA) | 66.7% | Hexagonal; [uniaxial] | Not determined | G, 1957 |
| Panunzite | K3Na(AlSiO4)4 (IMA) | 75.0% | No sheet consulted | - | A, 1978-050, Italy |
| Kalsilite | KAlSiO4 (IMA); Mafuru, Uganda analysis | 100%; Mafuru 89.2%, worked | Orthorhombic, probably, pseudohexagonal; uniaxial (−) | 2.59 to 2.62; 12.28 to 12.37 g | G, 1942, Uganda |
| Albite, a feldspar | Na(AlSi3O8) (IMA) | 0% | Triclinic; biaxial | 2.60 to 2.65; 12.31 to 12.45 g | G, 1815, Sweden |
What is nepheline? Two formulas, one species
Nepheline is the sodium potassium aluminosilicate of alkalic rocks, written Na3K(Al4Si4O16) by the IMA and (Na,K)AlSiO4 by the Handbook. The September 2026 IMA-CNMNC Master List gives it status Rd, redefined, 2018 s.p., Italy. On standard atomic weights the IMA formula has a mass of 584.32 and works out at 41.13% SiO2, 34.90% Al2O3, 15.91% Na2O and 8.06% K2O. The Handbook of Mineralogy sheet’s formula is the shorthand: it says sodium and potassium share a site, not in what proportion.
A formula is not a species. A script over the IMA list finds NaAlSiO4 as trinepheline, A, 2012-024, Myanmar, and KAlSiO4 three times, as kalsilite, kaliophilite and megakalsilite. The kalsilite sheet calls kalsilite polymorphous with kaliophilite, panunzite and trikalsilite. A label reading (Na,K)AlSiO4 is nepheline only between those ends; the general point is on crystal vs mineral.
What the sheet records. Hexagonal, point group 6; stout six- or twelve-sided prisms terminated by a dominant {0001}, typically poorly formed, to 10 cm, and granular, compact or massive. Twinning on {10-10}, {33-65} and {11-22}. Cleavage poor on {10-10} and {0001}; fracture subconchoidal; brittle. Hardness 5.5 to 6; density 2.55 to 2.66 measured, 2.64 calculated. Transparent to nearly opaque, from inclusions or alteration; colorless, white, gray, yellowish, variously colored by impurities; lustre vitreous to greasy. Uniaxial (−), ω = 1.529 to 1.546, ε = 1.526 to 1.542.
How much potassium is in nepheline? The Na3K check
The Na3K check: in the sheet’s analysed nepheline, potassium is 21.0% of the alkalis, a little under the one in four of the IMA formula. The Monte Somma, Italy analysis prints 43.55% SiO2, 34.66% Al2O3, 0.05% MgO, 4.44% CaO, 12.09% Na2O and 4.87% K2O, with 0.50% water. Recalculated without the water on 4 oxygens it gives Na 0.558, K 0.148, Ca 0.113, Al 0.972 and Si 1.037, matching the printed (Na0.56K0.15Ca0.11)Σ0.82Al0.97Si1.04O4.
On the IMA’s scale. Multiplied to the IMA formula’s 16 oxygens, the same analysis reads Na 2.23, K 0.59, Ca 0.45, Al 3.89 and Si 4.15. K/(Na + K) is 21.0%, against 25.0% for Na3K. The four alkali positions hold 3.28 cations, calcium among them; each calcium carries two charges, so Na + K + 2Ca + 2Mg comes to 3.74 against 3.89 aluminium. Silicon slightly exceeds aluminium, at a ratio of 1.066.
What the check does and does not show. It shows the analysed crystal sits on the sodium side of the IMA end-member, with calcium and empty positions making up the difference. It is one analysis; the sheet gives no range across localities, and this page does not extend the figure to other Monte Somma or Vesuvius material. The sheet’s synthetic powder-pattern material, (Na0.67K0.33)AlSiO4, sits on the potassium side at 33%.
Is nepheline a feldspar? A feldspathoid
Nepheline is a feldspathoid, a family the Alex Strekeisen petrography atlas defines as aluminosilicates of sodium, potassium or calcium having too little silica to form feldspar. The same atlas calls it not a true group, because the structures vary, and names nepheline, leucite, sodalite and cancrinite as members. Its nepheline page describes a tridymite-type framework in which one-half the silicon is replaced by aluminium. The IMA list has no line containing feldspathoid or feldspar, by script: those are family names, not species.
Where the two meet. The nepheline sheet lists potassic feldspar and plagioclase first among its associates, and no quartz. USGS Mineral Commodity Summaries 2026 treats feldspar and nepheline syenite in one chapter and says that in glass and pottery both function as a flux. The full comparison, with a worked silica sum and the optical test, is on is nepheline a feldspar; feldspar identification itself is on how to identify feldspar.
Its company. The cancrinite sheet records cancrinite as an alteration product of nepheline; the sodalite and haüyne pages list nepheline among their associates. Coombs et al., Canadian Mineralogist 35 (1997) say framework structures such as feldspars, nepheline, sodalites and scapolites have channels too restricted for zeolitic behaviour and are not zeolites; the zeolites are on which minerals are zeolites.
Nepheline, kalsilite and the look-alikes
Against the feldspars, the balance fails and the microscope works. A 20.00 g piece of nepheline reads 12.16 to 12.48 g in water. Albite reads 12.31 to 12.45 g, orthoclase 12.16 to 12.40 g and microcline 12.13 to 12.22 g, all overlapping. Every one of the 10 Handbook feldspar sheets gives a biaxial optic class, by script; nepheline is uniaxial. Hardness, 5.5 to 6 against the feldspars’ 6 to 6.5, meets at 6. Feldspar cleavage is perfect; nepheline’s is poor.
Against kalsilite, nothing simple works. The kalsilite sheet gives KAlSiO4 as anhedral grains and patchy to fine intergrowths with nepheline or potassic feldspar, hardness 6, density 2.59 to 2.62, 12.28 to 12.37 g in water, uniaxial (−), ω = 1.535 to 1.554, ε = 1.530 to 1.539. Every one of those figures falls inside or against nepheline’s. Trikalsilite, on its sheet, is extremely fine-grained, in parallel growth with nepheline and kalsilite. Telling them apart takes an analysis.
Against quartz. quartz is 7 in hardness, 12.45 g in water for 20.00 g, uniaxial (+) with ω = 1.544 and ε = 1.553: the optic sign is opposite and quartz’s lowest index sits above nepheline’s ε range. The International Gem Society calls a red, green, brown or gray variety with a cat’s-eye sheen elaeolite; the IMA list has no elaeolite row, by script.
Where nepheline is found
The Occurrence field: characteristic of alkalic rocks as nepheline syenites and gneisses, alkalic gabbros; in sodium-rich hypabyssal rocks, tuffs and lavas, and pegmatites; as a product of sodium metasomatism. Associates are potassic feldspar, plagioclase, sodic pyroxenes, sodic amphiboles, leucite, olivine, augite and diopside.
The Distribution field calls nepheline widespread and names only a few prominent localities: Vesuvius and Monte Somma, Campania, Italy; the Katzenbuckel, Odenwald, Baden-Württemberg, Germany; the Langesundsfjord area, Norway; the Sierra de Monchique, Portugal; around the Tunugdliarfik Fjord and on the Kangerdluarssuk Plateau, in the Ilímaussaq intrusion, southern Greenland; the Lovozero massif, Kola Peninsula, Russia; the Nyiragongo volcano, Kivu Province, Congo (Zaire); Litchfield, Maine; near Magnet Cove and on Granite Mountain, near Little Rock, Arkansas; Iron Hill, Colorado; Point of Rocks, New Mexico; the Bancroft district, Ontario, especially large crystals from Davis Hill; and Mont Saint-Hilaire, Quebec.
Name and type material. The sheet derives the name from the Greek for cloud, as it becomes cloudy when treated with strong acid; the International Gem Society says a Latin word. The type material is in the Natural History Museum, Paris, Haüy collection. The gem society’s other localities are not on the sheet and we add none. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.