The mineralogy of pollucite and beryl from the Tanco pegmatite at Bernic Lake, Manitoba

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Simpson, Frederick Muir
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Pollucite and beryl are two of the economically and petrologically Important mineraLs in the Tanco pegmatite,, Bernic Lake, Manitoba. One hundred and thirty six pollucite samples were collected from the eastern pollucite body and the isolated pods of pollucite 1n the western portion of the pegmatite. The refractive index (n) was measured on all samples. Samples were selected for partial or full chemical analysis on the basis of n and location. Three sampleswere chemically analysed in full and their density and unit cell demensions were determined. The pollucite from the Tanco pegmatite was found to fit the previously establilshed positive correlations between n, density and wt.% Cs2O. It was noted that the samples from the western isolated pods of pollucite had, in general, a higher n than those from the eastern pollucite body indicating they are richer ln Cs20. Alteration products of pollucite occur as thin veins and sphericaL masses, and consist of spodumene, mica and clay minerals. One hundred and thirteen beryl samples were collected from the wall zone, lower intermediate zone, central intermediate zone and the lepidolite zone of the pegmatite. The refractive index (w) of each sample was measured, and on the basis of w and location samples were selected for full or partial chemlcal analysis. Densities and unlt cell dimensions were measured for the three samples which were chemically analysed in full..