Abstract

The chemistry and stability of pollucite (Pol, CsAlSi 2 O 6 )-leucite (Lct, KAlSi 2 O 6 )-analcime (Anl, NaAlSi 2 O 6 H 2 O) solid-solutions and the effects of added pollucite on phase relations in the haplogranite system were studied experimentally between 450 degrees and 850 degrees C at 200 MPa H 2 O. Addition of Cs via dissolution of pollucite lowers the haplogranite solidus by approximately 40 degrees C (to approximately 640 degrees C) and displaces the minimum melt composition slightly toward the Qtz apex. The Cs content of melt saturated in pollucite solid-solution near the minimum is approximately 5 wt.% Cs 2 O. There is substantial miscibility among Pol and Lct components at magmatic temperatures, but the Lct content of pollucite decreases linearly with decreasing temperature to zero Lct component at approximately 385 degrees C. The Lct and Anl components vary inversely in pollucite solid-solutions as a function 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 hydrothermal solutions (and coexisting alkali feldspars) down to temperatures below approximately 400 degrees C. The experiments do not unequivocally resolve a standing question of a miscibility gap in the Pol-Anl system 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 composition between the extremes.

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