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Formula: K(AlSi2O6)
Tectosilicate (framework silicate), feldspathoid
Crystal System: Tetragonal
Specific gravity: 2.45 to 2.5 measured, 2.46 calculated
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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