STABILITY AND SOLUBILITY OF POLLUCITE IN THE GRANITE SYSTEM AT 200 MPA H2O

The chemistry and stability of pollucite (Pol, CsAlSi2O6) - leucite (L ct, KAlSi2O6) - analcime (Anl, NaAlSi2O6 . H2O) solid-solutions and th e effects of added pollucite on phase relations in the haplogranite sy stem were studied experimentally between 450 degrees and 850 degrees C at 200 MPa H2O. Addition of Cs via dissolution of pollucite lowers th e haplogranite solidus by similar to 40 degrees C (to similar to 640 d egrees C) and displaces the minimum melt composition slightly reward t he Qtz apex. The Cs content of melt saturated in pollucite solid-solut ion near the minimum is similar to 5 wt.% Cs2O. There is substantial m iscibility among Pol and Lct components at magmatic temperatures, but the Let content of pollucite decreases linearly with decreasing temper ature to zero Lct component at similar to 385 degrees C. The Lct and A nl components vary inversely in pollucite solid-solutions as a functio n of temperature, such that the Anl and maximum Pol components of the pollucite solid-solution increase with decreasing temperature. Natural and synthetic pollucite equilibrates readily with melt or with hydrot hermal solutions (and coexisting alkali feldspars) down to temperature s below similar to 400 degrees C. The experiments do not unequivocally resolve a standing question of a miscibility gap in the Pol-Anl syste m at low temperature. The compositions of synthetic pollucite diverge by >15 mol.% in terms of Pol and Anl components at 450 degrees C, but as in nature, the bulk of the synthetic pollucite is intermediate in c omposition between the extremes.