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.

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°.



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



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Leucite crystals.



Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


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