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Perfect Leucite Crystal in Matrix

The Mineral leucite




Leucite is a member of the feldspathoid group, forming directly from cooling lava in volcanic rock. It forms in low-silica environments where little Quartz is present. When Leucite forms at high temperatures, it crystallizes as isometric crystals in trapezohedral crystal form. As it cools, its crystal structure becomes unstable, and transforms into tetragonal form while retaining the original isometric shape. Though Leucite crystals are usually equant, the crystal faces may be slightly uneven due to the alteration. When in the oxidized zone, Leucite often alters to Pseudoleucite, which is a chalky-white combination of Orthoclase, Nepheline, and Analcime. Extensive oxidation may further break it down into Kaolinite or clay.


Chemical Formula KAlSi2O6
Composition Potassium aluminum silicate
Color White to gray, sometimes with a yellow or brown tinge. Rarely colorless.
Streak White
Hardness 5.5 - 6
Crystal System Tetragonal
3D Crystal Atlas
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Crystal Forms
and Aggregates		 Crystals are pseudo-isometric, forming as trapezohedrons. May be in single individual crystals, groups of crystals, or in grainy masses. Single crystals may be very well-formed, and may be floater crystals when smoothly popping out of a matrix. Crystals may also be intergrown.
Transparency Transparent to opaque
Specific Gravity 2.4 - 2.5
Luster Vitreous to dull
Cleavage Indiscernible
Fracture Conchoidal, uneven
Tenacity Brittle
In Group Silicates; Tectosilicates; Feldspathoid Group
Striking Features Crystal formation and mode of occurrence
Environment In young, low-silica igneous basalt formed from direct lava cooling.
Rock Type Igneous
Popularity (1-4) 3
Prevalence (1-3) 3
Demand (1-3) 3


Leucite AUCTIONS



USES
Leucite is an rare and somewhat odd collectors mineral. Good crystals, especially those from the Italian localities, are in demand by collectors. Leucite was once used as a fertilizer for its potassium content.

NOTEWORTHY LOCALITIES
The best occurrences for Leucite are in Italy. Several Italian deposits have produced fairly large and often individual crystals. Important localities include Roccamonfina, in Campania; Monte Somma, at Vesuvius; and Ariccia, in the Alban Hills near Rome. Colorless micro-crystals are found in Eifel Mountains, Germany, at Üdersdorf and Ettrigen; and large single crystals from Cascata, Poços de Caldas, São Paulo, Brazil.

In the U.S., dark single and large crystals were once found in Magnet Cove, Garland Co., Arkansas; and small crystals from the Leucite Hills District, Sweetwater Co., Wyoming.

COMMON MINERAL ASSOCIATIONS
Olivine, Nepheline, Augite, Biotite

DISTINGUISHING SIMILAR MINERALS
Analcime - Usually has more reflective luster and is more often associated with zeolites; otherwise can be extremely difficult to distinguish.
Garnet - Harder, usually a different color.


leucite PHOTOS
 
ADDITIONAL RESOURCES
 
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