Eucryptite

(repeating unit)LiAlSiO₄IMA symbolEcp^[1]Crystal systemTrigonalCrystal classRhombohedral (3)
H-M symbol: (3)Space groupR3Unit cella = 13.48, c = 9.01 [Å]; Z = 18IdentificationColorBrown, colorless, whiteCrystal habitRare as euhedral crystals, coarse crystalline aggregates and massiveCleavageIndistinct on {1010} and {0001}FractureConchoidalTenacityVery brittleMohs scale hardness6.5LusterVitreousStreakWhiteDiaphaneityTransparent to translucentDensity2.67Optical propertiesUniaxial (+)Refractive indexn_ω = 1.570 – 1.573 n_ε = 1.583 – 1.587Birefringenceδ = 0.013Other characteristicsFluoresces pink to red or orange under SW UVReferences^[2][3][4]

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.^[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]

References