Pollucite

pollucite

perhamite

montebrasite

lepidolite

Images

Formula: Cs(Si2Al)O6.nH2O
Tectosilicate (framework silicate), zeolite, cesium-bearing mineral, forms solid solutions with analcime and with leucite (R&M 94.5.420-425).
Crystal System: Isometric
Specific gravity: 2.9 measured, 2.96 calculated
Hardness: 6½ to 7
Streak: White
Colour: Colourless, white, grey, pink, blue, violet
Solubility: Slightly soluble in hot hydrochloric acid; readily soluble in HF
Common impurities: Fe,Ca,K,Rb
Environments:

Pegmatites

Pollucite occurs only in the latest stages of crystallisation in the most evolved of the lithium pegmatites. It is very rare because of the paucity of cesium, Cs, in the Earth’s crust (4.5 ppm). It appears to crystallise near 400oC in most of its occurrences (R&M 94.5.420-425).
Associated minerals include quartz, spodumene, petalite, amblygonite, lepidolite, elbaite, cassiterite, columbite, apatite, eucryptite, muscovite, K-feldspar, albite and microcline (HOM, Mindat).

Localities

The Tanco pegmatite at Bernic Lake, Manitoba, Canada, contains an estimated 2,640 ppm Cs (compared with average 4.5 ppm in the Earth's crust), and pollucite in vast deposits occurs here with beryl, lepidolite and elbaite (R&M 94.5.420-425).
The assemblages of minerals in the pegmatite were deposited in succession from petalite + quartz to spodumene + quartz to eucryptite + quartz (R&M 92.2.153).
Pollucite from the Tanco Mine - Image

The Koktokay number 3 pegmatite, Altai, northwest China, is a strongly zoned spodumene-subtype pegmatite. Different types of pollucite occur here. First is pollucite consisting of sodium- and cesium-enriched phases, which resulted from local exsolution of primary pollucite. Next is nearly end-member pollucite that occurs as an aureole of primary pollucite in contact with lepidolite or feldspar, and is formed by dissolution and re-precipitation of primary pollucite. Then pollucite associated with quartz occurs in sub-parallel veinlets penetrating surrounding albite crystals; this pollucite resulted from replacement of albite by cesium-rich fluids. Lastly in oscillatory-zoned pollucite, typically restricted to the contact of pollucite with small miarolitic cavities, the zonation compositionally oscillates about the cesium/sodium variations, and is simply related to locally changing fluid composition. (AM 91.729-739).

The type locality is La Speranza, San Piero in Campo, Campo nell'Elba, Elba Island, Livorno Province, Tuscany, Italy.
Pollucite from La Speranza - Image

At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, pollucite masses over one metre across occur in the core-margin zone, cut by thin veinlets of lepidolite or muscovite. Some veins have a core of analcime, and thinner veins near the margin of the pollucite mass have perhamite, fluorapatite and quartz. Pollucite is usually associated with beryl, montebrasite and spodumene. The Emmons pegmatite is an example of a highly evolved boron-lithium-cesium-tantalum enriched pegmatite (R&M 94.6.513).
Pollucite from the Emmons Quarry - Image

Alteration

quartz pseudomorphs after pollucite have been found. Pollucite also alters to analcime, K-feldspar variety adularia, lepidolite, spodumene and clay.

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