Eucryptite

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.

Eucryptite
Eucryptite grains in albite from the type locality (size:9.3 x 7.0 x 2.8 cm)
General
Category	Silicate mineral
Formula (repeating unit)	LiAlSiO4
IMA symbol	Ecp[1]
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	[2][3][4]

Its typical occurrence is in lithium-rich pegmatites in association with albite, spodumene, petalite, amblygonite, lepidolite and quartz.[3]

It occurs as a secondary alteration product of spodumene. It was first described in 1880 for an occurrence at its type locality, Branchville, Connecticut.[2] Its name was from the Greek for well concealed, for its typical occurrence embedded in albite.[2][3]