Leucite

Hand Specimen identification: Occurrence in Si-poor, K-rich volcanic rocks, crystal habit and pseudocubic form (if euhedral), and color help identify leucite. It may be confused with anal-cime, but leucite typically forms as a matrix mineral whereas analcime forms in cavities.

Chemical Composition : KAlSi2O6

Hardness : 5 1 2 to 6

Specific Gravity2.48

Cleavage/Fracture : poor {100}, poor (001)/

Luster/Transparency : vitreous/transparent to
translucent

Color : white, gray

Streak : white

Optical Properties : Low relief, gray interference colors, and lamellar or concentric twins help identify leucite. Uniaxial 1+ 2, v = 1.508, e = 1.509, d = 0.001.

Crystallography : Tetragonal, a = 13.04, c = 13.85, Z = 16; space group I41/a; point group 4/m.

Habit : Although tetragonal at low temperatures, leucite normally has the form of its high-temperature cubic polymorph. Trapezohedral crystals are typical. Polysynthetic twinning may give faces fine striations.

Structure and composition : Leucite’s structure consists of a framework of 4-, 6-, and 8-membered rings of AlO4 and SiO4 tetrahedra. K+ ions occupy half the available sites between the rings. Leucite is generally close to end member com-position, although small amounts of Fe, Na, and other alkalis may be present.

Occurrence and associations : Leucite is a rare mineral found in Si-poor, K-rich vol-canic rocks. It is never found with quartz.

Varieties :

Related Minerals : Leucite is isostructural with pollucite,1Cas,Na2
1AlSi2O622 . H2O. Chemically, it is closely related to
orthoclase, KAlSi3O8;. kaliophilite, KAlSiO4; and to analcime, NaAlSi2O6 H2O. A cubic polymorph of leucite exists above 605°C (1,120 °F).

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