Nepheline

Nepheline with Schorlomite garnet Bou-Agrao Mt., Tamazeght complex, High Atlas Mts, Khenifra Province, Morocco Small Cabinet, 8.8 x 6.0 x 4.2 cm © irocks

Chemical Formula: (Na,K)AlSiO4
Locality: Magnet Cove, Magnet Cove, Ouachita Mountains, Hot Spring County, Arkansas, USA.
Name Origin: From the Greek nephele, “cloud,” because it becomes clouded when put in strong acid.

Nepheline, is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites.

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 unsymmetrical etched figures produced artificially on the prism faces indicate, however, that the crystals are hemimorphic and tetartohedral, the only element of symmetry being a polar hexad axis. It is found in compact, granular aggregates, and can be white, yellow, gray, green, or even reddish (in the eleolite variety). The hardness is 5.5 – 6, and the specific gravity 2.56 – 2.66. It is often translucent with a greasy luster.

History

Discovery date : 1801
Town of Origin : MONTE SOMMA, MT. VESUVE (VOLCAN), NAPLES, CAMPANIE
Country of Origin : ITALIE

Optical properties

Optical and misc. Properties: Transparent –  Opaque –  Translucide
Refractive Index: from 1,52 to 1,54

Physical Properties

Cleavage: {1010} Poor
Color: White, Gray, Brown, Brownish gray, Reddish white.
Density: 2.55 – 2.65, Average = 2.59
Diaphaneity: Transparent to translucent to opaque
Fracture: Sub Conchoidal – Fractures developed in brittle materials characterized by semi-curving surfaces.
Hardness: 6 – Orthoclase
Luminescence:     Non-fluorescent.
Luster: Vitreous – Greasy
Streak: white

Photos :

Nepheline syenite of Tanguá, Rio de Janeiro, Brazil. ©  Motoki et al., 2011a
Nepheline, Schorlomite Locality: Jebel Bou-Agrao, Tamazeght Mountain Range, Khénifra Province, Meknès-Tafilalet Region, Morocco Dimensions: 6 cm x 4.4 cm x 3.8 cm Photo Copyright © Rob Lavinsky & irocks
Nosean, Sodalite, Nepheline, Phlogopite, Calcite, Pyrite Locality: Nepheline occurrence, Ladjuar Medam (Lajur Madan; Lapis-lazuli Mine; Lapis-lazuli deposit), Sar-e Sang (Sar Sang; Sary Sang), Koksha Valley (Kokscha Valley; Kokcha Valley), Khash & Kuran Wa Munjan Districts, Badakhshan Province (Badakshan Province; Badahsan Province), Afghanistan Photo Copyright © Gary Ansell
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