EUCRYPTITE

Eucryptite – LiAlSiO4 – is a rare nesosilicate found exclusively in lithium-bearing granitic pegmatites. It constitutes a secondary lithium ore; it is not the primary lithium ore mineral in the deposit.

It is classified within the Phenakite Group and may contain Na and K.

Identifying eucryptite macroscopically without the aid of an ultraviolet lamp is virtually impossible, as the mineral lacks easily recognizable diagnostic features and closely resembles at least half a dozen other minerals found in the same paragenesis.

Its name derives from the Greek expression for “well hidden,”reflecting its occurrence in intergrowth with albite. Its fluorescence—appearing magenta-red or orange under short-wave ultraviolet light—is a significant and diagnostic feature in this context.

1. Characteristics

Crystal system: Trigonal rhombohedral. 

Color: Colorless, white, pale bronze, pale gray. 

Habit: Generally massive or granular, rarely as equidimensional euhedral crystals. 

Cleavage: {10-10} poor, {0001} poor.    

Tenacity: Brittle.        

Twinning: No.       

Fracture: Irregular, conchoidal.       

Mohs Hardness: 6.5

Parting: No.         

Streak: White.         

Lustre: Sub-vitreous to resinous. 

Diaphaneity: Transparent.           

Density (g/cm³):  2.6

          

2. Geology and Deposits

Eucryptite is characteristic of—and exclusive to—lithium-rich granitic pegmatites, where it occurs in graphic intergrowths with albite derived from the alteration of spodumene.

Consequently, it can form pseudomorphs after spodumene.

 

3. Mineral Associations

It is associated with minerals typical of granitic pegmatites: quartz, sodic-calcic feldspars (albite, cleavelandite variety), potassium feldspars (microcline, orthoclase), micas (muscovite, biotite, lepidolite), fluorite, beryl, topaz, tourmaline, and others.

Furthermore, it is associated with the lithium-bearing minerals found in these pegmatites: lepidolite, amblygonite, montebrasite, spodumene (kunzite), petalite, cookeite, and cymatolite (a mixture of muscovite and albite resulting from the replacement of spodumene).

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 1.570 – 1.573      nε: 1.583 – 1.587                       

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. 

Relief: Low.           

Cleavage: No.           

Habits: Massive, granular, rarely in rhombohedral crystals            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: maximum birefringence of 0.013: colors from gray to white to yellow, not reaching 1st order orange. 

Extinction:  No information available, probably paralell.          

Elongation sign: No information available.          

Twins: No.         

Zoning: No.             

CONVERGENT LIGHT

Character: U(+)          

2V angle: No.         

Alterations: No information available.          

May be confused with: Quartz exhibits the same characteristics under the microscope.         

Hand specimen with eucryptite in normal light. It look exactly like quartz, feldspar and many other minerals of the same paragenesis.
The same hand specimen of the former image, now seen under short-wave UV light. The orange fluorescence is extremely strong and diagnostic.

5. Reflected Light Microscopy

It is not possible to identify eucryptite in Reflected Light.

Sample preparation:        

PLANE POLARIZED LIGHT – PPL

Reflection color:        

Pleochroism:       

Reflectivity:         

Bireflectance:        

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:         

Internal reflections:       

May be confused with:        

General Characteristics: 

Caption.