Crystallography and chemistry of Li-Rb-Cs bearing micas from the Tanco (Chemalloy) pegmatite, Bernic Lake, Manitoba

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Rinaldi, Romano
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The Tanco pegmatite at Bernic Lake is located about 125 miles northeast of Winnipeg in the Lac du Bonnet Mining Division of southeastern Manitoba. Previous works on the same pegmatite may refer to it as the Montgary or Chemalloy pegmatite, according to the names of the companies that in different times were mining the deposit. At present the property is owned by Tanco. It is a complex zoned pegmatite which contains the western world's largest known reserve of pollucite, the chief ore mineral of cesium, besides several lithium-bearing units. Other minerals of economic importance are Ta oxides, spodumene and beryl, the first ones being the main ore minerals presently extracted. The pegmatite is located near the margin of a granitic intrusive rock, and it is bordered by amphibolite; the earlier rocks in the area and the pegmatite itself are all of Precambrian age... The mineralogy is typical of that of cesium-lithium deposits found in various parts of the world, the principal minerals being: quartz, alkali and plagioclase feldspars, lithium nicas, spodumene, pollucite, and amblygonite-montebasite; accessory minerals include beryl,lithiophilite, apatite, tourmaline, wodginite-tantalite-pseudoixiolite, tapiolite, cassiterite, microlite, ilmenite, rhodochrosite and some sulphides... The chemical and physical full definition of the micas from Bernic Lake was one of two objectives of the present study, and the variation of the physical properties and crystal structure with the chemistry, especially with respect to the Li, Rb and Cs contents, was the other objective... All studies should lead to a better understanding of the paragenesis of the different minerals in the pegmatite.