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Formula: LiAlSiO4
Inosilicate (chain silicate), phenakite group
Crystal System: Trigonal
Specific gravity: 2.657 to 2.666 measured, 2.654 calculated
Hardness: 6½
Streak: white
Colour: Colourless, white, pale tan, pale grey
Luminescence: Fluoresces red under short wave UV
Solubility: gelatinises in acids
Common impurities: Na,K
Environments:
Eucryptite occurs in lithium-rich
pegmatites, often as graphic intergrowths with
albite derived from alteration of
spodumene
(Dana.
Localities
At the Tanco Mine, Bernic Lake, Lac-du-Bonnet area, Manitoba, Canada, eucryptite was found associated with
spodumene and quartz
pseudomorphs after petalite embedded in
massive quartz. Irregular nodules (up to 5 cm. across)
of eucryptite are located within the spodumene and
quartz pseudomorphs, and and veinlets of
eucryptite (up to 15 cm in length) are located in the immediately adjacent quartz.
In the spodumene and quartz
pseudomorphs, eucryptite is intimately intergrown with
cesian analcime. Where located in quartz
surrounding the pseudomorphs, eucryptite is intergrown with
cesian analcime, quartz and
cookeite; lithiophosphate with
secondary apatite was also found in
the immediate neighbourhood.
The close association of eucryptite with late hydrothermal phases like cookeite,
cesian analcime and lithiophosphate
suggests that it could belong to this late assemblage of secondary minerals.
On the other hand, the textural relationships of eucryptite nodules within the
spodumene and quartz
pseudomorphs, partly replaced by spodumene
indicate an early origin - contemporanous with either the original petalite or the early
stages of its breakdown
(CM 11.708-713).
Eucryptite from Tanco - Image
At the Bikita pegmatite, Bikita District, Masvingo, Zimbabwe, the principal dike represents one of the world's great concentrations
of lithium. The chief lithium minerals are
petalite and lepidolite, but
spodumene and amblygonite are recovered in
lesser amounts. Bikitaite is also present; it is among the major
lithium-bearing minerals at Bikita. This mineral, which elsewhere is extremely minor, is
found here in thousands of tons.
The eucryptite is mostly in granular aggregates associated with large masses of
petalite
(AM 47.557-561).
Eucryptite from Bikita - Image
Alteration
Eucryptite is a secondary mineral derived from
spodumene and associated with
albite, spodumene,
petalite, amblygonite,
lepidolite and quartz.
The co-occurrence of the lithium aluminium silicates
spodumene,
petalite and eucryptite is not common, but it
does occur in some pegmatites in northern Portugal. The
spodumene
precipitates early from the magma,
petalite later, and
eucryptite is hydrothermal and secondary. On alteration
spodumene is
mainly replaced by
albite and muscovite, and
petalite by
K-feldspar and eucryptite
(CM 39.729-746).
spodumene and Na+ to eucryptite, albite
and Li+
2LiAlSi2O6 + Na+ → LiAlSiO4 + NaAlSi3O8
+ Li+
(AM 67: 97-113
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