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Eucryptite grains in albite from the type locality (size:9.3 x 7.0 x 2.8 cm)
Category Silicate mineral
(repeating unit)
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
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 [1][2][3]

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.[2]

It occurs as a secondary alteration product of spodumene. It was first described in 1880 for an occurrence at its type locality, Branchville, Connecticut.[1] Its name was from the Greek for well concealed, for its typical occurrence embedded in albite.[1][2]