primary mineral

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

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

Common impurities: Ti,Fe,Mg,Ca,Ba,Na,Rb,Cs,H2O

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