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Nepheline
Current inventory:  0 gems
 

Chemistry:  (Na,K)AlSiO₄  [Sodium Potassium Alumiun Silicate]

Discovered in 1801;   IMA status: Valid (pre-IMA; Grandfathered).
Nepheline is named from the Greek word nephele, meaning cloud, since it becomes cloudy when treated with strong acid.

Classification

Mineral Classification:

Silicates

Strunz 8th Ed. ID:

8/J.02-10

Nickel-Strunz 10th Ed. ID:

9.FA.05

9 : SILICATES (Germanates)
F : Tektosilicates without zeolitic H₂O
A : Tektosilicates without additional non-tetrahedral anions

Related to:

A member of the Feldspathoid Group of minerals. Nepheline Group. Nepheline - Yoshiokaite Series.

Crystal Data

Crystallography:

Hexagonal - Pyramidal

Crystal Habit:

Crystals are stout six- or twelve-sided prisms, terminated, typically poorly formed, to 10 cm. Also granular, compact, massive.

Twinning:

On [1010], [3365], and [1122]

Physical Properties

Cleavage:

[1010] Poor, [0001] Poor

Fracture:

Subconchoidal

Tenacity:

Brittle

Hardness (Mohs):

5.5 - 6.0

Density:

2.55 - 2.66 (g/cm³)

Luminescence:

None

Radioactivity:

Barely Detectable, GRapi = 95.15 (Gamma Ray American Petroleum Institute Units)

Optical Properties

Color:

Colorless, White, Gray, Yellowish; variously colored by impurities

Transparency:

Transparent to nearly Opaque, from inclusions or alteration

Luster:

Vitreous to Greasy

Refractive Index:

1.526 - 1.546  Uniaxial ( - )

Birefringence:

0.0030 - 0.0040

Dispersion:

Low

Pleochroism:

Visible; colorless to gray

Occurances

Geological Setting:

Characteristic of alkalic rocks as nepheline syenites and gneisses, alkalic gabbros; in sodium-rich hypabyssal rocks, tu®s and lavas, and pegmatites; as a product of sodium metasomatism.

Common Associations:

Augite, Diopside, Plagioclase, Leucite, Olivine, Potassic Feldspar, Sodic Pyroxenes, Sodic Amphiboles

Common Impurities:

Mg, Ca, H₂O

Type Locality:

Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy

Year Discovered:

1801

View mineral photos:

Nepheline Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Nepheline is one of the members of the Feldspathoid group of minerals that includes Cancrinite, Haüyne, Lazurite, Leucite, Nepheline and Sodalite. Minerals with chemistries that are close to those of the alkalai Feldspars, but are poor in silica (SiO2) content, are called Feldspathoids. Nepheline is an extremely rare gem because clean, facetable crystals are very rare and usually very small. Occassionally, opaque cabochons can be found in colors of red, green, brown or gray, some with minute inclusions that cause a catseye effect. Colored varieties of Nepheline are called Elaeolite. Elaelite is named from the Latin word for oil because of its greasy luster. The main source of facet grade crystals is Davis Hill, Mont Saint-Hilaire, Bancroft, Quebec, Canada.
 

  
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