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[sub2]O[sub3], the micas correspond to Rb- and Cs-dominant polylithionite (with representative interlayer populations of Rb[sub0.82]K[sub0.12]Cs[sub0.07] and Cs[sub0.74]Rb[sub0.12]K[sub0.08] apfu, respectively), Rb-and Cs-dominant magnesian annite (Rb[sub45]K[sub0.37]Cs[sub0.20] and Cs[sub0.67]Rb[sub0.20]K[0.12] apfu, respectively), and Cs-dominant ferroan phlogopite (Cs[sub0.92]Rb[sub0.04]K[sub0.02] apfu). [ABSTRACT FROM AUTHOR]