Leucite

minerals

orthoclase

kalsilite

nepheline

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Formula: K(AlSi2O6)
Tectosilicate (framework silicate), feldspathoid
Specific gravity: 2.45 to 2.5
Hardness: 5½ to 6
Streak: White
Colour: White, colourless
Solubility: Slightly soluble in hydrochloric acid
Common impurities: Ti,Fe,Mg,Ca,Ba,Na,Rb,Cs,H2O
Environments:

Plutonic igneous environments (rarely)
Volcanic igneous environments
Hydrothermal environments

Leucite is very rare in plutonic masses. In volcanic environments leucite is characteristic of potassium-rich mafic and ultramafic lavas, where it forms directly from cooling lava in low silica environments with high potassium content.
Leucite is a primary rock-forming mineral.
It may be found in andesite, basalt, diorite, gabbro, syenite and trachyte.
Associated minerals include K-feldspar, nepheline, analcime, natrolite and kalsilite (HOM).

Localities

At the Nyiragongo volcano, Congo, leucite is associated with pyroxene, olivine and magnetite (DHZ 4 p284).

At Oberwiesenthal, Erzgebirge, Germany, orthoclase pseudomorphs after leucite have been found (KL p262).

At Ariccia, near Rome, Italy, leucite occurs with andradite on tuff (R&M 94.5.436).

Alteration

If sodium is abundant, nepheline occurs rather than leucite.
Leucite never occurs together with quartz; it reacts with free quartz to form K-feldspar.
In the oxidation zone it often transforms to pseudoleucite, which is a mixture of nepheline and orthoclase; further oxidation may break it down into kaolinite or clay.
In pre-tertiary rocks (older than 65 million years) leucite readily decomposes to zeolites, analcime and other secondary minerals.

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