Nepheline - Na₃(Na,K)[Al₄Si₄O₁₆]

Nepheline, from the Greek "nephele" for "cloud" (since it turns cloudy when treated with strong acid) is a minerals of the feldspathoid group. 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.
• Cleavage: poor basal (001).
• Interference colors: very weak.
• Relief: low.

Colorless hexagonal prisms of nepheline. Capo di Bova, Via Appia Antica, Rome, Italy. Immagine tratta da RRUFF

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
Altered Nepheline crystals in a foidite from Cape Verde. PPL image , 10x (Field of view = 2mm)	Altered Nepheline crystals in a foidite from Cape Verde. PPL image , 10x (Field of view = 2mm)	Altered Nepheline crystals in a foidite from Cape Verde. PPL image , 10x (Field of view = 2mm)
Nepheline crystal in a foidite from Cape Verde. PPL image , 10x (Field of view = 2mm)	Nepheline crystal in a foidite from Cape Verde. XPL image , 10x (Field of view = 2mm)	Basal section of Nepheline in a foidite from Cape Verde. PPL image , 10x (Field of view = 2mm)
Nepheline crystals in a foidite from Cape Verde. XPL image , 10x (Field of view = 2mm)	Nepheline crystals in a foidite from Cape Verde. XPL image , 2x (Field of view = 7mm)	Nepheline crystals in a foidite from Cape Verde. XPL image , 2x (Field of view = 7mm)
Nepheline crystals in a foidite from Cape Verde. XPL image , 2x (Field of view = 7mm)	Nepheline crystals in a foidite from Cape Verde. XPL image , 2x (Field of view = 7mm)	Nepheline crystals in a foidite from Cape Verde. XPL image , 2x (Field of view = 7mm)
Nepheline crystals in a foidite from Cape Verde. PPL image , 2x (Field of view = 7mm)	Nepheline crystals in a foidite from Cape Verde. XPL image , 2x (Field of view = 7mm)	Nepheline crystals in a foidite from Cape Verde. PPL image , 2x (Field of view = 7mm)
Nepheline crystals in a foidite from Cape Verde. XPL image , 2x (Field of view = 7mm)	Nepheline crystals in a foidite from Cape Verde. XPL image , 2x (Field of view = 7mm)	Nepheline crystals in a foidite from Cape Verde. XPL image , 2x (Field of view = 7mm)
Nepheline crystal in a foidite from Cape Verde. XPL image , 10x (Field of view = 2mm)	Nepheline crystal in a foidite from Cape Verde. XPL image , 10x (Field of view = 2mm)	Nepheline crystal in a foidite from Cape Verde. XPL image , 10x (Field of view = 2mm)
Nepheline crystals in a foidite from Cape Verde. XPL image , 10x (Field of view = 2mm)	Nepheline crystal in a foidite from Cape Verde. XPL image , 10x (Field of view = 2mm)