THEORETICAL-STUDIES OF THE ALTERATION OF SPODUMENE, PETALITE, EUCRYPTITE AND POLLUCITE IN GRANITIC PEGMATITES - EXCHANGE-REACTIONS WITH ALKALI FELDSPARS

The ratios Na/Li, K/Li, Na/Cs and K/Cs have been calculated for exchan ge equilibria among the Li and Cs silicates spodumene, petalite, eucry ptite, and pollucite, and the alkali feldspars albite and K-feldspar p lus quartz, in pure water and in chloride solutions at temperatures fr om 100-degrees to 700-degrees-C and pressures from 0.5 to 4 kbar, usin g available thermodynamic data for minerals and the modified HKF equat ion of state for aqueous species. For exchange equilibria between Li-b earing aluminosilicates and the alkali feldspars, the activities of th e alkali metals in solution under most of the conditions investigated follow the order Li > Na > K, and Na/Li and K/Li decrease with decreas ing temperature. For exchange equilibria between pollucite and the alk ali feldspars the order is Na > K > Cs in solution; Na/Cs and K/Cs inc rease strongly with decreasing temperature. The absolute values of the se alkali metal ratios are in good agreement with the few available ex perimental data. The effect of chloride ion pairing on the calculated ratios is slight and does not consistently improve agreement between t heory and experiment. These results suggest that the alteration of euc ryptite, petalite or spodumene to albite and/or K-feldspar should be a normal consequence of the closed system evolution of rare element peg matites upon cooling, in agreement with the ubiquity of such phenomena worldwide. On the other hand, alteration of pollucite to albite or K- feldspar upon cooling is only likely to occur if external fluids, with very high Na/Cs and/or K/Cs ratios, gain access to the pegmatite. Owi ng to the heterogeneity of rare element pegmatites, the fluid need not be external to the entire pegmatite, but could be simply external to the particular zone containing pollucite. Fluids in equilibrium with t ypical subsolidus rare metal pegmatite assemblages will invariably hav e high Li contents, thus explaining the common occurrence of Li-metaso matic halos about pegmatites. These same fluids are predicted to have relatively low Cs contents, in apparent agreement with the lesser role of Cs relative to Li in metasomatic halos. However, preferential form ation of complexes of the alkali metals with fluoride, borate or alumi nosilicate components potentially could alter the calculated alkali me tal behaviors.