THE BREAKDOWN OF POTASSIUM-FELDSPAR AT HIGH WATER PRESSURES

The equilibrium position of the reaction between sanidine and water to form ''sanidine hydrate'' has been determined by reversal experiments on well characterised synthetic starting materials in a piston cylind er apparatus. The reaction was found to lie between four reversed brac kets of 2.35 and 2.50 GPa at 450 degrees C, 2.40 and 2.59 GPa at 550 d egrees C, 2.67 and 2.74 GPa at 650 degrees C, and 2.70 and 2.72 GPa at 650 degrees C. Infrared spectroscopy showed that the dominant water s pecies in sanidine hydrate was structural H2O. The minimum quantity of this structural H2O, measured by thermogravimetric analysis, varied b etween 4.42 and 5.85 wt% over the pressure range of 2.7 to 3.2 GPa and the temperature range of 450 to 680 degrees C. Systematic variation i n water content with pressure and temperature was not clearly establis hed. The maximum value was below 6.07 wt%, the equivalent of 1 molecul e of H2O per formula unit. The water could be removed entirely by heat ing at atmospheric pressure to produce a metastable, anhydrous, hexago nal KAlSi3O8 phase (''hexasanidine'') implying that the structural H2O content of sanidine hydrate can vary. The unit cell parameters for sa nidine hydrate, measured by powder X-ray diffraction, were a = 0.53366 (+/-0.00022) nm and c = 0.77141 (+/-0.00052)nm, and those for hexasan idine were a = 0.52893 (+/-0.00016) nm and c = 0.78185 (+/-0.00036) nm . The behaviour and properties of sanidine hydrate appear to be analog ous to those of the hydrate phase cymrite in the equivalent barium sys tem. The occurrence of sanidine hydrate in the Earth would be limited to high pressure but very low temperature conditions and hence it coul d be a potential reservoir for water in cold subduction zones. However , sanidine hydrate would probably be constrained to granitic rock comp ositions at these pressures and temperatures.