FIRST NATURAL OCCURRENCES OF END-MEMBER POLLUCITE - A PRODUCT OF LOW-TEMPERATURE REEQUILIBRATION

Sodium-poor pollucite extremely close to the ideal end-member composit ion CsAlSi2O6 was discovered on a microscopic scale in five specimens from localities in Europe, Africa and North America. It crystallized a t subsolidus temperatures of about 300-200 degrees C under hydrotherma l conditions and is closely associated with the latest minerals in a t ypical sequence of alteration of pollucite. Wide veins of coarse-grain ed lepidolite + quartz +/- spodumene cut the pollucite, followed by th in veins of fine-grained muscovite and spodumene; both types of veins are overgrown by granular aggregates of (K,Rb)-feldspar and albite. En d-member CsA1Si(2)O(6) occurs with these feldspars. The CRK = 100(Cs+R b+K)/Sigma cations of the latest pollucite ranges from 97.2-99.9 with Si/Al near 2.0. A mechanism of dissolution and reprecipitation is sugg ested for its origin, which could remarkably increase its Cs,AI conten t relative to its Na,Si-rich precursor. Later events (at less than 150 -100 degrees C) caused only analcimization and replacement of pollucit e by clay minerals.