Leucite

Leucite is a feldspathoid mineral in group of tectosilicate. It is a rock forming mineral include potassium and aluminium tectosilicate, formula is K[AlSi2O6].Specimens can also be colorless or gray. Crystals are common and can be up to 31 ⁄2 in (9 cm) wide. More often, leucite occurs as massive or granular aggregates or as disseminated grains. It is tetragonal at temperatures below 1,155°F (625°C) and cubic with trapezohedral crystals at higher temperatures. The trapezohedral form is preserved as the mineral cools and develops tetragonal symmetry. Leucite is found in potassiumrich and silica-poor igneous rocks. It is found with nepheline, sodalite, natrolite, analcime, and sodiumand potassium-rich feldspars, and occurs worldwide.

Name: From the Greek leucos for white, in allusion to its color

Association: Potassic feldspar, nepheline, analcime, natrolite, kalsilite.

Diagnostic Features. Characterized by its trapezohedral form and infusibility. It is softer than garnet and harder than analcime; analcime, moreover, is fusible and yields water

Composition: Potassium aluminum silicate, KAlSi20 6. K20 = 21.5 per cent, A120 3 = 23.5 per cent, Si02 = 55.0 per cent.

Mineral Group: Feldspathoid

Similar Species: Pollucite, CstAFSigC^e-EFO, is a rare isometric mineral usually occurring in pegmatites.

Crystallography: Pseudo-isometric. Trapezohedral habit. Other forms rare. Strictly isometric only at temperatures of 500°C or above. On cooling below this temperature it undergoes an internal molecular rearrangement to that of some other crystal system, probably orthorhombic, but the external form does not change. It is formed in lavas at high temperatures and is then isometric in internal structure as well as outward form. Usually in distinct crystals, also in disseminated grains. (Dana, 1864)

Chemical Properties

Chemical Classification	Tectosilicates, Feldspathoid
Formula	K[AlSi2O6]
Common Impurities	Ti,Fe,Mg,Ca,Ba,Na,Rb,Cs,H2O
Crystal system	Tetragonal

Leucite Physical Properties

Color	White, grey
Streak	White
Luster	Vitreous (Glassy)
Cleavage	Poor/Indistinct Very poor om {110}
Diaphaneity	Transparent, Translucent
Mohs Hardness	5,5 – 6
Tenacity	Brittle
Density	2.45 – 2.5 g/cm3 (Measured)    2.46 g/cm3 (Calculated)
Fracture	Conchoidal

Leucite Optical Properties

Type	Isotropic
2V:	Very small.
RI values:	nω = 1.508 nε = 1.509
Twinning	Common and repeated on {110} and {101}
Optic Sign	Uniaxial (+)
Birefringence	δ = 0.001
Relief	Low
Dispersion:	Moderate

Occurrence of Leucite

Leucite is a rather rare mineral, occurring only in igneous rocks, usually in the recent lavas; rarely observed in deepseated rocks. Found in rocks in which the amount of silica in the magma was insufficient to combine with the potassium to form feldspar. Is not observed, therefore, in rocks that contain quartz.

Uses Area

Leucite is used as a fertilizer in Italy (because of its high potassium content) and as a source of commercial aluminum

Distribution

From numerous localities worldwide, typically in good crystals.

• Especially from Italy, as at Vesuvius, Monte Somma, and Roccamonna, Campania; at Villa Senni, Frascati, and Ariccia, Lazio; and on Vulcano, Lipari Islands.
• Around the Laacher See, Eifel district, Germany.
• In the USA, from the Leucite Hills, Sweetwater Co., Wyoming; the Bear Paw Mountains, Hill Co., Montana; and at Magnet Cove, Hot Spring Co., Arkansas.
• In the West Kimberley district, Western Australia.
• In Tanzania, on Kilimanjaro.
• In Uganda, found around Ruwenzori and Kariya. From Mt. Nyiragongo, Kivu Province, Congo (Zaire).

References

• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• Dana, J. D. (1864). Manual of Mineralogy… Wiley.
• Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
• Mindat.org. (2019): Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].