Eucryptite is lithium bearing aluminum silicate mineral with formula LiAlSiO4. It is one of the few silicate minerals that have a trigonal symmetry. This symmetry is far more common among carbonates than among silicates. It crystallizes in the trigonal – rhombohedral crystal system. It typically occurs as granular to massive in form and may pseudomorphically replace spodumene.
- Category: Silicate mineral
- Formula: LiAlSiO4
- Crystal system: Trigonal
- Crystal class: Rhombohedral (3)
It has a brittle to conchoidal fracture and indistinct cleavage. It is transparent to translucent and varies from colorless to white to brown. It has a Mohs hardness of 6.5 and a specific gravity of 2.67. Optically it is uniaxial positive with refractive index values of nω = 1.570 – 1.573 and nε = 1.583 – 1.587.
- Color: Brown, colorless, white
- Crystal habit: Rare as euhedral crystals, coarse crystalline aggregates and massive
- Fracture: Conchoidal
- Tenacity: Very brittle
- Mohs scale hardness: 6.5
- Luster: Vitreous
- Streak: White
- Diaphaneity: Transparent to translucent
- Density: 2.67
- Optical properties: Uniaxial (+)
It occurs as a secondary alteration product of spodumene. It was first described in 1880 for an occurrence at its type locality, Branchville, Connecticut. Its name was from the Greek for well concealed, for its typical occurrence embedded in albite.
Its typical occurrence is in lithium-rich pegmatites in association with albite, spodumene, petalite, amblygonite, lepidolite, and quartz.