Nepheline - Na3(Na,K)[Al4Si4O16]

Nepheline, also called nephelite (from Greek "cloud"), is a feldspathoid: a silica-undersaturated aluminosilicate with forumula (Na,K)[Al4Si4O16].

Nepheline is the most common mineral variety in a very complex group of polymorphs ranging from NaAlSiO4 (Nepheline) to KAlSiO4 (kalsilite). At high temperature (>1150°C) solid solution is complete, but at low temperature Na-rich phases (Nepheline) and K-rich phases (Kalsilite) are distinct minerals.

The Nepheline structure is based on a tridymite-type framework in which one-half the Si4+ are replaced by Al3+ and an equal number of alkali ions appear in voids in the open structure to restore charge balance. Partial solid solution with both Albite and Anorthite is possible, to about Ab33 or An35.
Several polymorph exist for both Na-rich and K-rich phases. Pure Nepheline invert to a high temperature-nepheline at 900°C and to carnegietie at 1254°C.

Nepheline appears only in quarz-free alkaline rocks in association with other alkali-rich or silica-poor minerals (other feldspathoid, Na-Anphiboles, Na-pyroxenes). It forms as a primary magmatic minerals through the crystallization of a Na-rich Si-poor magmas; it may result from the assimilation of limestone by basic or intermediate magmas.

Optical properties
Color: Colorless or rich in inclusions (clouded aspect).
Form: Hexagonal or stubby prismatic section.
Interference colors: Very weak.
Relief: Low.

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Colorless hexagonal prisms of nepheline. Caspar quarry, Bellerberg volcano, Ettringen, Germany..



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Colorless hexagonal prisms oh nepheline. Capo di Bova, Via Appia Antica, Rome, Italy





Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.


Photo
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Aegirine and nepheline (I order gray) crystals. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)
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Aegirine and nepheline (I order gray) crystals. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)
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Aegirine and nepheline (I order gray) crystals. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)
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Aegirine and nepheline (I order gray) crystals. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)
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Aegirine and nepheline (I order gray) crystals. The crystals with high interference colors are Cancrinite. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)
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Aegirine and nepheline (I order gray) crystals. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)
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Nepheline crystals (colorless and rich in inclusions) in a fine grained groundmass made of plagioclase and aegirine. Mt. Flora, Lovozero, Kola, Russia. PPL image, 2x (Field of view = 7mm)
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Nepheline crystals (low interference colors) in a fine grained groundmass made of plagioclase and aegirine. Mt. Flora, Lovozero, Kola, Russia. XPL image, 2x (Field of view = 7mm)
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Nepheline crystals (colorless and rich in inclusions) in a fine grained groundmass made of plagioclase and aegirine. Mt. Flora, Lovozero, Kola, Russia. PPL image, 2x (Field of view = 7mm)
nefelinsienite(4).jpg

Nepheline crystals (low interference colors) in a fine grained groundmass made of plagioclase and aegirine. Mt. Flora, Lovozero, Kola, Russia. XPL image, 2x (Field of view = 7mm)
nefelinsienite(5).jpg

Nepheline crystals (colorless and rich in inclusions) in a fine grained groundmass made of plagioclase and aegirine. Mt. Flora, Lovozero, Kola, Russia. PPL image, 2x (Field of view = 7mm)
nefelinsienite(6).jpg

Nepheline crystals (colorless and rich in inclusions) in a fine grained groundmass made of plagioclase and aegirine. Mt. Flora, Lovozero, Kola, Russia. PPL image, 2x (Field of view = 7mm)
nefelinsienite(7).jpg

Nepheline crystals (low interference colors) in a fine grained groundmass made of plagioclase and aegirine. Mt. Flora, Lovozero, Kola, Russia. XPL image, 2x (Field of view = 7mm)
nefelinsienite(8).jpg

Nepheline crystals (colorless and rich in inclusions) in a fine grained groundmass made of plagioclase and aegirine. Mt. Flora, Lovozero, Kola, Russia. PPL image, 2x (Field of view = 7mm)
nefelinsienite(9).jpg

Nepheline crystals (colorless and rich in inclusions) in a fine grained groundmass made of plagioclase and aegirine. Mt. Flora, Lovozero, Kola, Russia. PPL image, 2x (Field of view = 7mm)
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Nepheline (colorless) and augite crystals in a nepheline foidolite from Cape Verde island. PPL image, 2x (Field of view = 7mm)
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Nepheline (colorless) and augite crystals in a nepheline foidolite from Cape Verde island. PPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)
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Nepheline (colorless) and augite crystals in a nepheline foidolite from Cape Verde island. PPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)
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Nepheline (colorless) and augite crystals in a nepheline foidolite from Cape Verde island. PPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)
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Nepheline (colorless) and augite crystals in a nepheline foidolite from Cape Verde island. PPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)