Rubidium- and cesium-dominant micas in granitic pegmatites

Abstract

The mode of occurrence and chemical composition of five types of micas with Rb- or Cs-dominant populations of interlayer cations, collected from the Red Cross Lake rare-element pegmatites in north-central Manitoba, are described here. All five micas are candidates for new mineral species but crystal-structural data and Li contents could not be determined to date because of extremely small particle size, restricted to the margins of strongly zoned microcrystals. Based on electron-microprobe analyses, on Li contents estimated from Li/F (at.) = 1.0, and on bulk analysis of ferromagnesian micas for FeO and Fe2O3, the micas correspond to Rb- and Cs-dominant polylithionite (with representative interlayer populations of Rb0.82K0.12Cs0.07 and Cs0.74Rb0.12K0.08 apfu, respectively), Rb and Cs-dominant magnesian annite (Rb45K0.37Cs0.20 and Cs0.67Rb0.20K0.12 apfu, respectively), and Cs-dominant ferroan phlogopite (Cs0.92Rb0.04K0.02 apfu).