DETERMINATION OF WATER SOLUBILITIES IN HIGH-VISCOSITY MELTS - AN EXPERIMENTAL-STUDY ON NAALSI3O8 AND KALSI3O8 MELTS

The solubility of water in melts of albitic (Ab: NaAlSi3O8) and orthoc lasic (Or: KAlSi3O8) compositions was re-investigated at T = 1100 degr ees C and at pressures between 0.5 and 5.0 kbar. The influence of star ting materials, sample preparation, run procedure and analytical metho ds on the solubility data was investigated systematically. Different a nalytical techniques (Karl-Fischer titration (KFT), near-infrared spec troscopy (NIR), 105 degrees C weight loss and weight loss on ignition) give consistent results under certain conditions, whereas water-speci fic methods (KFT and NIR) have a higher precision. The lowest solubili ties of water were found for samples of exact feldspathic composition. An excess of several wt% of SiO2 relative to a feldspar composition h as a negligible effect on water solubility, but an excess of Na2O or A l2O3 relative to NaAlSi3O8 increases the solubility by ca. 3 percent r elative per wt% of excess oxide. At P = 5 kbar, significantly higher s olubilities than reported in previous studies were obtained (11.60 wt% H2O for Ab and 10.32 wt% H2O for Or). This is attributed to drying of run products at 105 degrees C in previous studies. At P = 2 kbar, exp eriments using single glass pieces, crystal plates or glass pieces emb edded in glass powder as starting materials yielded a water solubility of 5.95 wt% for Ab melts, which is less than solubilities determined in previous studies. The reliability of the new solubility data could be confirmed by reversals. The water solubility in Ab melts at P = 5 k bar increases apparently with the ratio of fluid to melt. This effect is probably caused by formation of strongly hydrated quench products o n the melt body and diffusional exchange between both during cooling.