RUBIDIUM-RICH FELDSPARS IN A GRANITIC PEGMATITE FROM THE KOLA-PENINSULA, RUSSIA

Analysis of minerals associated with end-member pollucite in a rare-el ement granitic pegmatite from the Kola Peninsula, in Russia, has revea led two populations of Rb-rich feldspar. A 1 mm-wide vein of non-perth itic microcline cross-cutting the pollucite has an average of 8.21 wt. % Rb2O and a unit formula of 022K0.696Rb0.256Cs0.014)(Al1.004Si2.981P0 .015)O-8, corresponding to 26 mol.% content of Rb-feldspar. Small (les s than or equal to 15 mu m) grains of a Rb-dominant feldspar dispersed throughout the pollucite have an average of 24.39 wt.% Rb2O, 0.02 Na2 O, 2.02 K2O, 0.59 Cs2O, 0.10 P2O5, 15.86 Al2O3, 56.75 SiO2, sum 99.79. The unit formula (K0.136Rb0.831Cs0.013)(Al0.990Si3.006P0.004)O-8 indi cates 82 mol.% of Rb-feldspar. Analysis of individual grains gives a m aximum of 26 wt.% Rb2O corresponding to 91 mol.% Rb-feldspar. This is the highest Rb content of any known mineral. The Cs2O content attains 1.00 wt.%, corresponding to 2.1 mol.% Cs-feldspar. Activity of Rb+ mus t have been exceptionally high during late stages of crystallization o f the parent pegmatite: pollucite has the highest recorded Rb content (2.8 wt.% Rb2O; 10 mol.% RbAlSi2O6) and lepidolite has up to 6.5 wt.% Rb2O.