Eucryptite

Eucryptite
Eucryptite grains in albite from the type locality (size: 9.3 × 7.0 × 2.8 cm)
General
Category	Silicate mineral
Formula	LiAlSiO4
IMA symbol	Ecp
Crystal system	Trigonal
Crystal class	Rhombohedral (3) H-M symbol: (3)
Space group	R3
Unit cell	a = 13.48, c = 9.01 [Å]; Z = 18
Identification
Color	Brown, colorless, white
Crystal habit	Rare as euhedral crystals, coarse crystalline aggregates and massive
Cleavage	Indistinct on {1010} and {0001}
Fracture	Conchoidal
Tenacity	Very brittle
Mohs scale hardness	6.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Density	2.67
Optical properties	Uniaxial (+)
Refractive index	nω = 1.570 – 1.573 nε = 1.583 – 1.587
Birefringence	δ = 0.013
Other characteristics	Fluoresces pink to red or orange under SW UV
References

Eucryptite is a lithium-bearing aluminium silicate mineral with formula LiAlSiO₄. It crystallizes in the trigonal - rhombohedral crystal system. It typically occurs as granular to massive in form and may pseudomorphically replace spodumene. 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.

Its typical occurrence is in lithium-rich pegmatites in association with albite, spodumene, petalite, amblygonite, lepidolite and quartz.

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.