PETALITE
Petalite – LiAl(Si4O10) – is a fairly rare phyllosilicate, a lithium ore. In this case, it is “hard rock lithium ore”, which are lithium ores from pegmatites. The other type of lithium ore is “brines,” which are brines mined in salt flats, such as those in the Andes, for example.
Petalite forms crystals up to two meters long and can contain Mg, Fe, Na, Ca, K, and H2O. It is thermoluminescent.
Macroscopically, it can easily be confused with other transparent and light-colored minerals typical of pegmatites, such as milky quartz, microcline, orthoclase, albite, amblygonite, montebrasite, spodumene, beryl, pollucite, and others.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Colorless, white, gray, rarely reddish or greenish.
Habit: Tabular on {010}, elongated // {100}, clivable, massive.
Cleavage: {001} perfect, {201} poor.
Tenacity: Brittle.
Twinning: Common on {001}, lamellar.
Fracture: Conchoidal imperfect.
Mohs Hardness: 6.5
Parting: No.
Streak: White.
Lustre: Vitreous. Pearly on cleavage planes.
Diaphaneity: Transparent.
Density (g/cm³): 2.412 to 2.422
2. Geology and Deposits
It occurs together with other lithium minerals in lithium-bearing granitic pegmatites.
3. Mineral Associations
It occurs associated with common constituent minerals of pegmatites in general, such as quartz, feldspars (microcline, orthoclase, albite), micas (muscovite, biotite), tourmaline (elabaite), amphiboles from the Holmquistite Group, sulfides, Nb-Ta minerals (columbite, tantalite), apatite, topaz and garnet (spesartine).
In the lithiniferous pegmatites where it occurs, there are other Li minerals such as lepidolite, spodumene, montebrasite, eucryptite, lithiophilite and bikitaite.
It also occurs with pollucite, phenacite, montmorillonite and stilbite-Ca.
In these Li-rich pegmatites there is usually a defined mineral zoning with up to 12 or 15 different zones. Around 1% of pegmatite may be made up of rare minerals of little economic importance, but which may comprise more than 50 different mineral species.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.504 nβ: 1.510 nγ: 1.516
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Low.
Cleavage: {001} perfect and {201} poor.
Habits: Tabular, massive, cleavable masses.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.012: first-order colors from gray to white to straw yellow.
Extinction: It must be oblique, since petalite is monoclinic.
Elongation sign: No information available.
Twins: Common in {001}, lamellar.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 82 – 84º
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying petalite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with petalite.
Sample preparation: Polishing petalite is simple and results in good quality polishing.
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium gray.
Pleochroism: No.
Reflectivity: Low (<10%)
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy could not be observed.
Internal reflections: Generally colorless to white, with significant multicolored internal reflections.
May be confused with: No information availavle.
General Characteristics:
Cleavages, depending on the section, may become visible and generate polishing pits.
