SPODUMENE

Spodumene – LiAlSi2O6 – is a rare inosilicate, characteristic of lithium-bearing pegmatites. It is the main mineral in hard rock Li ores. Lithium can be mined in brines or hard rock mines, which are mines located in pegmatites exploiting spodumene, lepidolite, petalite, and other minerals containing Li.

Spodumene is classified in the Pyroxene Group, Clinopyroxene Subgroup. There are two types: the normal type, low temperature (<900ºC, α-spodumene, monoclinic) and the high temperature type (>900ºC, β-spodumene, tetragonal). Spodumene can contain Fe, Mn, Mg, Ca, Na, K, and H2O and has two varieties that are very valuable gems: hiddenite is green and kunzite is pink to lilac. Under short and long UV waves, kunzite can fluoresce in yellow, orange, and pink. Tryphane is a term used for colorless to yellow varieties.

If spodumene is massive, colorless, or white, macroscopically it can be confused with many minerals that occur in the same paragenesis: quartz, albite, orthoclase, microcline, eucryptite, petalite, amblygonite, and montebrasite.

1. Characteristics

Crystal system: Monoclinic prismatic          

Color: Colorless, white, gray, yellow, light green, emerald green, pink to violet, purple.     

Habit: Prismatic, typically flattened and striated // {100}, with {100} and {110}. Generally massive.   

Cleavage: {110} good (forms the typical pyroxene angle of 87º). Striations // “c”.

Tenacity: Brittle, very splintery.        

Twinning: Common in {100}.       

Fracture: Irregular to subconchoidal.       

Mohs Hardness: 6.5 – 7

Parting: According to {100} and {010}.         

Streak: White.         

Lustre: Vitreous, dull.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.1 a 3.2

 

2. Geology and Deposits

Spodumene is a rare clinopyroxene, common only in lithium-rich complex pegmatites, where it occurs associated with other lithium-bearing minerals, generally forming large crystals. The largest recorded crystal of spodumene measured 12.5 m and weighed 54 tons.

It also occurs in some aplites and rarely in gneisses.

 

3. Mineral Associations

It ocurrs with minerals typical of granitic pegmatites such as quartz, calcic feldspars (albite, clevelandite variety), potassic feldspars (microcline), micas (muscovite), apatite, garnet (spessartine), beryl (including aquamarine variety), tourmaline (elbaite, schorlite) and columbite-tantalite.

Since spodumene occurs in lithium-bearing pegmatites, it occurs with the typical Li minerals of these occurrences: lepidolite, petalite, amblygonite, montebrasite and eucryptite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.648 – 1.661     nβ: 1.655 – 1.670     nγ: 1.662 – 1.679

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. The varieties are pleochroic: hiddenite exhibits moderate pleochroism ranging from colorless to green, and kunzite exhibits strong pleochroism ranging from colorless to pink or purple.

Relief: Moderate.           

Cleavage: {110} good and distinct, exhibiting in the basal sections two cleavages that form angles of 87º and 93º between them. In the prismatic sections only one cleavage. This is the typical cleavage of pyroxenes.

There are also partings along {100} {010}.

Habits: Prismatic crystals, can be massive.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence between 0.014 and 0.018, corresponding to colors between gray and first-order orange. It does not reach blue.

Extinction: Oblique, between 20 and 26º.           

Elongation sign: ES(+).            

Twins: Common on {100}.         

Zoning:  No information available.            

CONVERGENT LIGHT

Character: B(+)          

2V angle: 54 – 69º        

Alterations: It can alter to cymatolite, a fibrous to lamellar mixture of muscovite and albite. Through late alkaline hydrothermal alteration, spodumene forms an intergrowth of eucryptite (LiAlSiO4) and albite.          

May be confused with: Diopside is very similar, but spodumene has a lower extinction angle. A characteristic of spodumene is its paragenesis (lithium-bearing pegmatites).        

 

Spodumene 01: In PPL, high-relief spodumene in microcline. Cleavage is well-defined and microcline is of low relief,
spodumene 01: The same section as of the previous image, now in XPL. Microcline shows the typical twinning and spodumene orange first-ordem interference colors.
Spodumene crystals are typically large in pegmatites; it is not so simple to obtain a thin section that shows spodumene in small crystals along with other minerals of the paragenesis. Muscovite and quartz are also associated here. XPL.
In this image, In XPL, two (black) holes associated with spodumene crystals can be observed. Spodumene shatters during the preparation of the thin section or polished section. The same behavior is observed with kyanite and wollastonite. Therefore, holes in the thin section are one of the diagnostic features for spodumene. XPL.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying spodumene. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with spodumene.

Sample preparation:  Polishing spodumene presents no problems. Due to the cleavage process, there will always be some imperfections and aligned starting figures. 

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray.      

Pleochroism: No.      

Reflectivity:  Low (<<10%).      

Bireflectance:  No.      

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy is possible, but difficult to see due to strong internal reflections.        

Internal reflections: Widespread in macro color, generally colorless to milky.      

May be confused with: many other transparent minerals of light colors, including many that participate in the same paragenesis. 

 

Caption.