LEPIDOLITE

Lepidolite is a rare phyllosilicate, restricted to litiniferous granitic pegmatites. May constitute an ore of Li, Cs and Rb.

The name “lepidolite” as a mineral was discredited in 1989 and today it is just a generic name used for lithiniferous micas that make up a Series composed of Polylithionite (KLi2Al(Si4O10)(F,OH)2) – Trilithionite – (K(Li1,5 Al1,5 )(AlSi3O10)(F,OH)12). Some lithiniferous muscovites are also called “lepidolite”.

The typical pink to purple color of lepidolite and muscovite-Li is not due to the presence of Li, but due to Mn levels. Trilithionite has three varieties (with Cs, with Rb or massive). Polylithionite has a variety with Cs. Regular fluorescence from cream white to pale yellow or yellow may occur.

The data below refer to “lepidolites” in general.

1. Characteristics

Crystal system: Monoclinic.

Color: Pink, light purple, light pink-red. Rarely other colors.

May be colorless if there is no Mn

Habit: Micaceous. Tabular to prismatic, can be pseudohexagonal. Massive or in scaly aggregates.

Cleavage: {001} perfect, like all micas.

Tenacity: Elastic.

Twinning: No.

Fracture: Micaceous.

Mohs Hardness: 2.5 – 3.5

Parting: No.

Streak: White.

Lustre: Subvitreous, pearly, resinous.

Diaphaneity: Transparent.

Density (g/cm³): 2.8 – 2.9

 

2. Geology and Deposits

Lepidolite is a very rare mica that usually occurs in aplites associated with Li-rich granitic pegmatites. It also occurs in gresens.

It rarely occurs in hydrothermal veins and in very isolated cases in granite contact zones.

 

3. Mineral Associations

Lepidolites are generally associated with quartz, muscovite, feldspar (albite, cleavelandite, microcline), tourmalines (rubellite, elbaite), beryl (morganite), spodumene, cassiterite, amblygonite and topaz.

Polylithionite is associated with quartz, feldspar (albite, microcline), aegirin, rhodochrosite, leucophanite, analcime, tugtupite and serandite.

Trilithionite is associated with quartz, lithiophilite, feldspar (albite, microcline), fluorite, beryl (aquamarine), topaz, tourmaline (elbaite) and montebrasite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.530    nβ: 1.551 – 1.556     nγ: 1.555 – 1.559  (polylithionite)

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. In “thick” thin sections it is pale pink with discreet pleochroism.

Relief: Low. Diagnostic!

Cleavage: {001} perfect, typical of the micas.

Habits: Lamellar, pseudohexagonal, like micas in general. Also in flaky or massive masses. May be interspersed with muscovite.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence between 0.018 and 0.038, corresponding to intense interference colors of up to 2nd order.

Extinction: Parallel to the cleavage and mottled (“birds-eye” extinction).

Elongation sign: ES(+), as all micas.

Twins: Rare.

Zoning: No.

CONVERGENT LIGHT

Character: B(-).Can simulate being uniaxial!

2V angle: 0 – 43º.

Alterations: alters under hydrothermal conditions or weathering into other phyllosilicates and secondary lithium-bearing minerals, like cookeite, muscovite/lithian muscovite, kaolinite, montmorilonite/smectite and jarosite.

May be confused with: muscovite and pyrophyllite are very similar, but have higher relief (moderate) and more intense colors (higher birefringence) in CPL. Pyrophyllite occurs in other paragenesis.

 

Lepidolite 01: in PPL lepidolite is colorless (left), even the cleavage may not be visible. At right, quartz with the same low relief as lepidolite (diagnostic!)
Lepidolite 01: the same section as in the previous image, now in XPL. Lepidolite is colorful, with many different colors.
In XPL, quartz (in gray and white) and lepidolite (coloured). There are two lepidolite grains side by side: the one on the left is in the maximum illumination position and the one on the right is in the extinction position (mottled!).

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of lepidolite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with lepidolite.

Sample preparation: lepidolite, due to its low hardness and excellent cleavage, is very difficult to polish. While quartz, feldspar and other associated minerals are already well polished, lepidolite remains poorly polished, with many polishing scratches and very dark colors, like all micas.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, much darker than quartz and feldspar.

Pleochroism: No.

Reflectivity: Very low (<<4%?)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy was not observed.

Internal reflections: Widespread in a dark tone with violet reflections. They are much more discreet shades than the color in hand specimen. They can be quite dark, in which case the violet tone is difficult to perceive.

May be confused with: other micas that, however, present internal reflections with other colors.

Lepidolite 01: In PPL, lepidolite is very dark, with a horrible polish. Quartz (top) is much lighter and better polished.
Lepidolite 01: now in XPL, the same section as in the previous figure. Quartz show clear and gray internal reflections. Lepidolite with dark reflections, with a slight violet hue, difficult to perceive.
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