Rubidium- and cesium-dominant micas in granitic pegmatites

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 Fe 2 O 3 , the micas correspond to Rb- and Cs-dominant polylithionite (with representative interlayer populations of Rb 0.82 K 0.12 Cs 0.07 and Cs 0.74 Rb 0.12 K 0.08 apfu, respectively), Rb and Cs-dominant magnesian annite (Rb 45 K 0.37 Cs 0.20 and Cs 0.67 Rb 0.20 K 0.12 apfu, respectively), and Cs-dominant ferroan phlogopite (Cs 0.92 Rb 0.04 K 0.02 apfu).