Fundamental Minerals

Sialic Minerals

Feldspar

Feldspathoid

Quartz

Femic Minerals

Accessory Minerals

Very Rare Minerals

Secondary Minerals

Texture

Alteration Products

Glass and related Texture

Volcanic Rocks

Komatiites

Lamprophyres

Leucite - KAlSi₂O₆

Leucite is usually included as a feldspathoid by petrologists, as it is a common primary rock-forming mineral in some high-potassium rocks but, despite being anhydrous, structurally it also has a topological relation to zeolites, especially analcime, and thus is typically included in that family also. The name leucite derives From a Greek word meaning "white"; Leucite structure is similar to that of analcime and pollucite, and it is based on a cubic lattice containing rings of four and six tetrahedra oriented normal to the fourfold and threefold symmetry axis. The leucite structure is characterized by two distinct sites, available for large cations; the larger sites contain K⁺ ions and the smaller sites are vacant, yielding a very open, low-density structure. Leucite is common in some volcanic rocks in which it crystallises with a cubic crystal structure at high temperature (ca. 900°C), forming isometric trapezohedral crystals. Upon cooling to 700-600°C, it transforms into a tetragonal modification which is stable at room temperature, and forms characteristic polysynthetic twin lamellae. The transformation is reversible.

Leucite in confined to volcanic or shallow dike rocks that are Si-poor, Na-poor and K-rich. Lucite occurrences are sufficiently rare to make practical the actual listing of specific localities. Is common in Italy (Vulsini, Vesuvius), in the Lucite hills of Wyoming, and in some areas of Africa.

Fig.1: Characteristics forms of leucite. a) concentric inclusions, b) radial inclusions, c) euhedral deltoid-icositetrahedron with typical laminar twins, d) and e) schematic representations of real skeletal crystals from vulsini volcano, Rome, Italy.

Optical properties

• Color: Colorless
• Form: Normally forms trapezohedron crystals, with octahedral to roughly circular outline in thin section
• Interference colors: Very low, no more than dark first-order gray
• Relief: Low
• Twinning: Several set of diffuse, parallel twin lamellae resemble those of microcline but tend to appears in three set, intersecting at about 60°.
• Inclusions: Leucite may have radial inclusions (with complex pattern) of other minerals or glass, evident in plane-polarized light (Fig.1).

leucite2017(2).jpg

Leucite crystals. Poggio Nibbio, Vico Lake, Viterbo Province, Latium, Italy. From: Wikipedia

Colorless pseudocubic crystal showing trapezohedral faces. Alban Hills, Rome Province, Latium, Italy. From: 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.

Leucite crystals in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite crystal in a tephrite. Vulsini volcano, Italy. PPL image, 10x (Field of view = 2mm)	Leucite crystals in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)
Leucite crystals in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite and pyroxene crystals in a tephrite..Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite and pyroxene crystals in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)
Euhedral leucite phenocryst. Vulsini volcano, Italy. PPL image, 10x (Field of view = 2mm)	Euhedral leucite phenocryst. Vulsini volcano, Italy. PPL image, 10x (Field of view = 2mm)	Euhedral leucite phenocryst. Vulsini volcano, Italy. PPL image, 10x (Field of view = 2mm)
Basaltic glass enclosed by leucite crystals. Note the small skeletal plagioclase crystals. Vulsini volcano, Italy. PPL image, 20x (Field of view = 1mm)	Basaltic glass enclosed by leucite crystals. Note the small skeletal plagioclase crystals. Vulsini volcano, Italy. PPL image, 20x (Field of view = 1mm)	Basaltic glass enclosed by leucite crystals. Vulsini volcano, Italy. PPL image, 20x (Field of view = 1mm)
Basaltic glass enclosed by leucite crystals. Note the small skeletal plagioclase crystals. Vulsini volcano, Italy. PPL image, 20x (Field of view = 1mm)	Basaltic glass enclosed by leucite crystals. Note the small skeletal plagioclase crystals. Vulsini volcano, Italy. PPL image, 20x (Field of view = 1mm)	Twin lamellae in leucite. Vulsini volcano, Italy. XPL image, 20x (Field of view = 1mm)
Twin lamellae in leucite. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)	Twin lamellae in leucite. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)	Twin lamellae in leucite. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)
Twin lamellae in leucite. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)	Twin lamellae in leucite. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)	Twin lamellae in leucite. Vulsini volcano, Italy. XPL image, 20x (Field of view = 1mm)
Twin lamellae in leucite. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)	Twin lamellae in leucite. Vulsini volcano, Italy. XPL image, 40x (Field of view = 0.2mm)	Twin lamellae in leucite. Vulsini volcano, Italy. XPL image, 40x (Field of view = 0.2mm)
Fluid inclusions in leucite. Vulsini volcano, Italy. PPL image, 40x (Field of view = 0.2mm)	Leucite phenocrysts. Vulsini volcano, Italy. PPL image, 10x (Field of view = 2mm)	Leucite phenocrysts. Vulsini volcano, Italy. PPL image, 10x (Field of view = 2mm)
Leucite phenocryst. Vulsini volcano, Italy. PPL image, 10x (Field of view = 2mm)	Leucite phenocryst. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)	Leucite phenocryst. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)
Leucite phenocryst. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)	Leucite phenocryst. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)	Leucite phenocryst. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)
Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 1x (Field of view = 9mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 1x (Field of view = 9mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 1x (Field of view = 9mm)
Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 1x (Field of view = 9mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 1x (Field of view = 9mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 1x (Field of view = 9mm)
Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)
Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)
Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)
Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)	Leucite phenocryst in a tephrite. Vulsini volcano, Italy. PPL image, 2x (Field of view = 7mm)