Pollucite from the Proterozoic petalite-bearing pegmatites of Utö, Stockholm archipelago, Sweden

Abstract

Primary pollucite has been discovered in one of the two petalite-bearing pegmatites transecting the iron formation of Nyköpingsgruvan, Utö, Stockholm archipelago, which have been the first lithium pegmatites ever discovered. The pollucite is associated with albite, K-feldspar, rubellite, indicolite, apatite, and quartz. It contains numerous microinclusions of acicular apatite arranged in a three-dimensional rectangular grid. Late veining of the pollucite carries sericite, albite, adularia and a calcium carbonate. The pollucite is isotropic with n=1.521(1), a=13.686(1) Å, and D=2.92(3) g/cm³. Chemical analysis gives Poll_76.7Anal_23.3 to Poll_78.5Anal_21.5, and Si/Al from 2.43 to 2.49. The mineral assemblage of the parent pegmatite, examined for almost two centuries, is not particularly indicative of the presence of pollucite. However, the geochemical features of the dikes, virtually unexplored to date, indicate high fractionation levels that match those of other pollucite-bearing pegmatites: e.g. average K/Rb of 7.5, K/Cs 185 and K/Ba 1713 in K-feldspar, K/Rb 3.3 and K/Cs 13.3 in lithian micas, Fe/Mn < 1.0 in columbitetantalite and Nb/Ta <0.6 in the bulk of the oxide minerals.

Key Words:

• Pollucite
• pegmatites
• fractionation
• Utö
• south central Sweden