Leucite - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Tetragonal
    Chemistry : KAlSi2O6

    Rarity : Uncommon

Leucite is an accessory feldspathoid, characteristic of alkaline volcanic rocks rich in potassium and deficient in silica (tephrites), sometimes also present in potassium basalts, basanites and phonolites. Its name comes from the Greek leucos (white) in reference to its color. The crystalline form is constant and characteristic : we almost always observe well-formed trapezoidal crystals, more precisely tetragonotrioactahedra (or leucitohedra), consisting in fact of twinned tetragonal crystals ; the striation of the faces betraying the organization into polysynthetic twins of the mineral. Leucite is sometimes also grainy or massive. Translucent, rarely transparent, it is colorless, greyish, whitish or yellowish. It easily deteriorates in meteoritic conditions, becoming opacified and releasing potassium salts, resulting in the exceptional fertility of the soils derived from it.

Leucite in the World

The Italian volcanoes of Latium and Campania have given many remarkable samples, among the most beautiful in the world : Mount Somma and Vesuvius, for example, have delivered exceptional white crystals of 4 cm, with perfect shapes. In Germany, the Laacher See (Eifel) and the Kaiserstuhl (Baden-Württemberg) also gave spectacular groups. But the largest known crystals are 12 cm trapezohedra, perfectly formed, from Kaman (Anatolia, Turkey); they are usually in the process of pseudomorph by a mixture of orthoclase, nepheline and analcime. Among the leucite deposits in the USA, Leucite Hills (Wyoming) bears its name particularly well. Finally, let us mention the blocks of leucitite rejected by the Nyiragongo volcano (Kivu, Democratic Republic of Congo).

Leucite in France

In France, leucite is reported in microcrystals in certain volcanic rocks of Auvergne such as the Devès Plateau (Haute-Loire) or tSt-Angel, St-Georges de Mons or Menat (Puy-de-Dôme) but also in Tahiti, at Lapanouse-de-Sévérac in the slag heap of a former bituminous shale mine, and near a coal mine at Mines de Saint-Maime (Alpes-de-Haute-Provence).

Twinning

Twins are common and polysynthetic on {001} and {110}.

Fakes and treatments

No fake inventories for this mineral species. 



Hardness : 5 to 5.5
Density : 2.24 to 2.29
Fracture : Irregular to sub-conchoidal
Trace : White



TP : Translucent to transparent
RI : 1.479 to 1.494
Birefringence : 0.001
Optical character : Biaxial -
Pleochroism : None
Fluorescence : Yellow, white, green


Solubility : Hydrochloric acid

Magnetism : None
Radioactivity : None