COMPOSITIONAL AND TEXTURAL EVOLUTION OF POLLUCITE IN PEGMATITES OF THE MOLDANUBICUM

Pollucite is found in five pegmatites in the Moldanubicum, all of them within the Czech Republic: Susice III (elbaite subtype) and Nova Ves near Cesky Krumlov (petalite subtype) in southern Bohemia, Jeclov I an d Puklice II(lepidolite subtypes) and Vezna I(beryl-columbite subtype) in western Moravia. Pollucite displays a broad range of primary compo sitions at these localities, with CRK = 100(Cs + Rb + K)/Sigma cations variable from 72 to 85, and Si/Al from 2.43 to 2.60. Exsolution to Cs ,Al-rich and Na,Si-rich components is not widespread: the maximum CRK is 92 at Si/Al of 2.30. However, Na-exchanged compositions and local v einlets of analcime are much more common; they reach CRK of 5 and 7, r espectively. Three occurrences are comparable to the usual pollucite-b earing assemblages, except for the geochemically rather primitive pegm atites at Susice and Vezna At the latter locality, an extremely steep fractionation gradient generates minor amounts of a highly evolved -,C s-,Be-,Ca-,Sr-,Ba-,Mn-,Bi-,REE-,Zr-,Sn-,Th-,W-, B-,P-,F-rich assemblag e, with pollucite closely associated with chabazite and harmotome.