| Eucryptite | |
| Category: | Silicate mineral |
| Formula: | LiAlSiO4 |
| Imasymbol: | Ecp[1] |
| System: | Trigonal |
| Class: | Rhombohedral H-M symbol: |
| Symmetry: | R |
| Unit Cell: | a = 13.48, c = 9.01 [Å]; Z = 18 |
| Color: | Brown, colorless, white |
| Habit: | Rare as euhedral crystals, coarse crystalline aggregates and massive |
| Cleavage: | Indistinct on and |
| Fracture: | Conchoidal |
| Tenacity: | Very brittle |
| Mohs: | 6.5 |
| Luster: | Vitreous |
| Refractive: | nω = 1.570 – 1.573 nε = 1.583 – 1.587 |
| Opticalprop: | Uniaxial (+) |
| Birefringence: | δ = 0.013 |
| Streak: | White |
| Density: | 2.67 |
| Diaphaneity: | Transparent to translucent |
| Other: | Fluoresces pink to red or orange under SW UV |
| References: | [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]