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Eucryptite grains in albite from the type locality (size:9.3 x 7.0 x 2.8 cm)
CategorySilicate mineral
(repeating unit)
IMA symbolEcp[1]
Crystal systemTrigonal
Crystal classRhombohedral (3)
H-M symbol: (3)
Space groupR3
Unit cella = 13.48, c = 9.01 [Å]; Z = 18
ColorBrown, colorless, white
Crystal habitRare as euhedral crystals, coarse crystalline aggregates and massive
CleavageIndistinct on {1010} and {0001}
TenacityVery brittle
Mohs scale hardness6.5
DiaphaneityTransparent to translucent
Optical propertiesUniaxial (+)
Refractive indexnω = 1.570 - 1.573 nε = 1.583 - 1.587
Birefringenceδ = 0.013
Other characteristicsFluoresces pink to red or orange under SW UV

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]


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c
  3. ^ a b c Handbook of Mineralogy
  4. ^ Webmineral data