POLLUCITE
Pollucite – (CsNa)2(Al2Si4O12).2H2O – is a very rare tectosilicate that occurs in lithiniferous granitic pegmatites and constitutes the most important ore of Cs. Cesium is important to the oil industry.
It is classified in the Zeolite Group and is part of the Analcime-Pollucita series, constituting the Cs analogue of analcime (of Na), with which it is isostructural. It commonly contains, as impurities, Fe, Ca, K and Rb. Crystals occur as trapezohedrons or cubes or combined forms of the two, are striated and can reach 12 cm. It is generally non-fluorescent, rarely fluoresces pale yellow under shortwave UV light.
It is easy to confuse pollucite with other minerals. When idiomorphic, in the form of trapezohedrons, it is identical to analcime and leucite. When massive, it is identical to other transparent, colorless or white and vitreous minerals that also occur in pegmatites, such as quartz, orthoclase, microcline, albite, spodumene, beryl, amblygonite, montebrasite, petalite and several others. Under the microscope, pollucite does not show relevant diagnostic features.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Almost always colorless or white. Also light colors in grey, pink, blue and violet.
Habit: Crystals are rare. Usually massive, rounded grains, fine granular.
Cleavage: In traces.
Tenacity: Very brittle.
Twinning: No.
Fracture: Sub conchoidal.
Mohs Hardness: 6.5 – 7
Parting: No.
Streak: White.
Lustre: Vitreous, sub-vitreous, resinous. Shines less than quartz.
Diaphaneity: Transparent.
Density (g/cm³): 2.9
2. Geology and Deposits
Pollucite is a characteristic and typical mineral of lithium-rich granitic pegmatites. About 82% (350,000 t) of the world’s pollucite reserves are in the Tanco Mine (Manitoba, Canada).
3. Mineral Associations
Pollucite occurs associated with the common minerals of granitic pegmatites, such as quartz (including chalcedony variety), feldspar (orthoclase, microcline (including amazonite variety), albite (including cleavelandite variety)), micas (muscovite, biotite), beryl (including aquamarine variety) ), tourmaline (including elbaite and rubellite varieties), cassiterite, columbite, fluor-apatite and others.
Its specific paragenesis includes several Li minerals such as petalite, amblygonite, montebrasite, eucryptite, lepidolite and spodumene (including kunzite variety).Also with rare minerals like hambergite and nanpingite.
4. Transmitted Light Microscopy
Refraction indices: n: 1,508 – 1,528
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: No information.
Cleavage: In traces, it is not visible under the microscope.
Habits: Massive, granular, idiomorphic crystals tending to round sections.
Pollucite occur in pegmatites and, therefore, the crystals are usually much larger than the size of the thin section, so it is difficult to see crystal forms.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Isotropic. Sometimes show faint anomalous anisotropy.
Extinction: Isotropic.
Elongation sign: Isotropic.
Twins: Isotropic.
Zoning: Isotropic.
CONVERGENT LIGHT
Character: Isotropic.
2V angle: Isotropic.
Alterations: no information available.
May be confused with: other isotropic minerals, colorless and with high relief in PPL, like garnet. A careful investigation considering paragenesis is necessary to recognize pollucite under the microscope without any other analytical methods.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of pollucite. In pegmatites, where the crystals are usually very large, much larger than the size of thin/polished sections, it is difficult to obtain samples that allow comparison between different minerals.
Sample preparation: the polishing of pollucite is simple and is of good quality.
PLANE POLARIZED LIGHT – PPL
Reflection color: Light gray.
Pleochroism: No.
Reflectivity: Low (<10%).
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy was not observed.
Internal reflections: Generalized in white.
May be confused with: it is not possible to identify pollucite with Reflected Light.
