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Leucite
Current inventory:  1 gem
 

Chemistry:  KAlSi₂O₆

Discovered in 1791;   IMA status: Valid (pre-IMA; Grandfathered).
The name Leucite is from the Greek  leukos, meaning white.

Classification

Mineral Classification:

Silicates

Strunz 8th Ed. ID:

8/J.05-10

8 : Silicates
J : Tectosilicates (network) without anions unfamiliar to the tetraheders
05 : Leucite - Ammonioleucite series

Crystal Data

Crystallography:

Tetragonal - Dipyramidal

Crystal Habit:

Commonly euhedral, pseudocubic crystals, to 9 cm, typically showing fine twin striae. As disseminated grains; rarely granular, massive.

Twinning:

Common and repeated on [110], [101], and other orientations, from two complex displacive phase transformations during cooling

Physical Properties

Cleavage:

[110] Indistinct

Fracture:

Conchoidal

Tenacity:

Brittle

Hardness (Mohs):

5.5 - 6.0

Density:

2.45 - 2.50 (g/cm³)

Luminescence:

None

Radioactivity:

Barely Detectable; GRapi = 255.81 (Gamma Ray American Petroleum Institute Units)

Optical Properties

Color:

Colorless, Gray, Yellowish Gray, White

Transparency:

Transparent to Translucent

Luster:

Vitreous

Refractive Index:

1.508 - 1.509  Uniaxial ( + ); may be anomalously Biaxial

Birefringence:

0.001

Dispersion:

Moderate

Pleochroism:

None

Occurances

Geological Setting:

With potassium-rich mafic and ultramafic lavas and hypabyssal rocks, and may almost entirely compose them.

Type Locality:

Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy

Year Discovered:

1791

View mineral photos:

Leucite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Leucite is one of the members of the Feldspathoid group of minerals that includes Cancrinite, Haüyne, Lazurite, Leucite, Nepheline and Sodalite. Minerals with chemistries that are close to those of the alkalai Feldspars, but are poor in silica (SiO2) content, are called Feldspathoids. Leucite is abundant in various lava rocks but is extemely rare in facetable crystals. Gems are usually small and cloudy or included.

The only source of transparent Leucite crystals large enough for faceting is the Alban Hills, near Rome, Italy.
 

  
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