Eucryptite

inosilicate

lithium mineral

Formula: LiAlSiO4 inosilicate, phenakite group
Specific gravity: 2.657 to 2.666
Hardness: 6½
Streak: white
Colour: Colourless, white, pale tan, pale grey
Solubility: gelatinises in acids
Environments:

Pegmatites

Eucryptite occurs in lithium-rich pegmatites, often as graphic intergrowths with albite derived from alteration of spodumene (Dana.

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 (The Canadian Mineralogist Vol. 39, pp. 729-746 (2001).

spodumene and Na+ to eucryptite, albite and Li+
2LiAlSi2O6 + Na+ → LiAlSiO4 + NaAlSi3O8 + Li+ (AM 67: 97-113

Common impurities: Na,K

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