Nepheline

Nepheline
Nepheline

Hand Specimen identification: Occurrence in alkaline igneous rocks, lack of cleav-age, and greasy luster help identify nepheline. It may be confused with feldspar or quartz but is softer. Occasionally it is confused with apatite.

Chemical Composition : (Na,K)AlSiO4

Hardness : 51 2 to 6

Specific Gravity2.6

Cleavage/Fracture : poor {100}, poor (001)/sub-
conchoidal

Luster/Transparency : vitreous, greasy/transparent
to translucent

Color : colorless, turbid

Streak : white

Optical Properties : Low birefringence and relief and common alter-ation identify nepheline. It is distinguished from the feldspars by its uniaxial nature and from quartz by its optic sign. Uniaxial 1- 2, v = 1.540, e = 1.536, d = 0.004.

Crystallography : Hexagonal, a = 10.01, c = 8.41, Z = 8; space group P63; point group 6.

Habit : Massive, compact, and embedded grains are com-mon. Crystals are short and prismatic with six or twelve sides.

Structure and composition : The structure of nepheline derives from that of tridymite: Every other Si is replaced by Al, and Na occupies large sites between Al and Si tetrahedra. All natural nepheline contains some K substituting for Na, but a solvus exists between nepheline and kalsilite, KAlSiO4, at temperatures below 1,000°C (1,830 °F). Exsolution, similar to perthite (see ortho-clase), is common.

Occurrence and associations : Nepheline is characteristic of some Si-poor igneous rocks, such as syenite. It is found with feldspars, ap-atite, cancrinite, sodalite, zircon, and biotite.

Varieties :

Related Minerals : Nepheline is isostructural with tridymite. It has a high-temperature polymorph above 900°C (1,650 °F). It forms a solid solution with kalsilite, KAlSiO4, and is
chemically related to kaliophilite, KAlSiO . Cancrinite,
1 2 1 2 . 4
Na3Ca2 2CO3 Si3Al3O12 2H2O, is similar to nephe-line in many ways and occurs in the same type of

rocks.

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