Fundamental Minerals

Sialic Minerals

Feldspars

Feldspathoids

Quartz

Femic Minerals

Spodumene

Accessory Minerals

Rare Accessory

Alteration Products

Microstructures

Rocks

Jacupiranga complex

Nepheline - Na₃(Na,K)[Al₄Si₄O₁₆]

Nepheline, also called nephelite (from Greek "cloud"), is a feldspathoid: a silica-undersaturated aluminosilicate. 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.

nepheline2017(1).jpg

Colorless hexagonal prisms of nepheline. Caspar quarry, Bellerberg volcano, Ettringen, Germany. Immage from Mindat. Picture by Stephan Wolfsried..

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Aegirine and nepheline (I order gray) crystals. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)	Aegirine and nepheline (I order gray) crystals. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)	Aegirine and nepheline (I order gray) crystals. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)
Aegirine and nepheline (I order gray) crystals. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)	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)	Aegirine and nepheline (I order gray) crystals. Nepheline pegmatite from mount st hilaire. XPL image, 2x (Field of view = 7mm)
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)	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)	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)
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)	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)	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)
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)	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)	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)
Nepheline (colorless) and augite crystals in a nepheline foidolite from Cape Verde island. PPL image, 2x (Field of view = 7mm)	Nepheline (colorless) and augite crystals in a nepheline foidolite from Cape Verde island. PPL image, 2x (Field of view = 7mm)	Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)
Nepheline (colorless) and augite crystals in a nepheline foidolite from Cape Verde island. PPL image, 2x (Field of view = 7mm)	Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)	Nepheline (colorless) and augite crystals in a nepheline foidolite from Cape Verde island. PPL image, 2x (Field of view = 7mm)
Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)	Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)	Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)
Nepheline (colorless) and augite crystals in a nepheline foidolite from Cape Verde island. PPL image, 2x (Field of view = 7mm)	Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)	Nepheline (I order gray) and augite crystals in a nepheline foidolite from Cape Verde island. XPL image, 2x (Field of view = 7mm)